EMP
Causality assessment of an adverse
event following immunization (AEFI)
User manual for the revised WHO classification
Department of Essential Medicines and Health Products (EMP)
Health Systems and Innovation (HIS)
World Health Organization
20, Avenue Appia CH-1211 Geneva 27, Switzerland
E-mail: [email address]
Web site: http://www.who.int/vaccine_safety/en/
CAUSALITY ASSESSMENT
OF
AN ADVERSE EVENT FOLLOWING
IMMUNIZATION
(AEFI)
User manual for the revised WHO
classification
WHO/HIS/EMP/QSS. MARCH 2013
WHO Library Cataloguing-in-Publication Data
Causality assessment of adverse event following immunization (AEFI): user manual for the revised WHO
classification.
1.Adverse drug reaction reporting systems. 2.Immunization programs. 3.Vaccines – adverse effects. I.World
Health Organization.
ISBN 978 92 4 150533 8
(NLM classification: QW 805)
© World Health Organization 2013
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This publication contains the collective views of an international group of experts and does not necessarily
represent the decisions or the policies of the World Health Organization.
ACKNOWLEDGEMENT: The definitions and the concepts of this revised WHO AEFI
Causality Assessment scheme have been largely adapted from
Definition and application of
terms for vaccine pharmacovigilance. Report of CIOMS/WHO Working Group on Vaccine
Pharmacovigilance
(http://www.cioms.ch/index.php/component/booklibrary/?task=view&Itemid=&id=45&cati
d=58).
The protocol for the causality assessment of an adverse event following immunization
(AEFI) has been developed by a subgroup of the Global Advisory Committee on Vaccine
Safety (GACVS) that included Alberto Tozzi, Barbara Law, Brigitte Keller-Stanislawski,
Edwin Asturias, Michael Gold, Jerry Labadie and Neal Halsey with inputs provided by
Stephen Evans and Ananda Amarasinghe. The WHO secretariat supporting the work
included Madhava Ram Balakrishnan and Patrick Zuber
iii
PURPOSE: This user manual serves as a guide to a systematic, standardized global
causality assessment process for serious adverse events following immunization (AEFI). It
is intended to be used by staff at national level (such as members of national AEFI
committees) and at subnational level, as well as immunization programme managers and
others. It also serves as an educational tool for trainers and researchers and as a ready
reference guide on AEFI causality assessment.
iv
Contents
Glossary ................................................................................................................................ viii
Acronyms ................................................................................................................................ x
Introduction and rationale ......................................................................................................1
Definitions of adverse events following immunization ................................................................... 2
A number of key terms used in this document are defined here for the sake of clarity. ............... 2
The need for causality assessment of adverse events ...........................................................3
AEFI causality assessment in practice ............................................................................................. 3
Levels of causality assessment and their scientific basis ........................................................5
1.
The population level ................................................................................................................ 5
2.
The individual level .................................................................................................................. 6
3.
Investigation of signals ............................................................................................................ 7
Case selection for causality assessment .................................................................................8
Prerequisites for causality assessment ........................................................................................... 8
Who should do causality assessment? ............................................................................................ 8
Steps for causality assessment of an individual adverse event ............................................10
Step 1: Eligibility ............................................................................................................................ 10
Step 2: Checklist ............................................................................................................................ 11
I. Is there strong evidence for other causes? ............................................................................ 13
II. Is there a known causal association with the vaccine or vaccination? ................................. 13
II (time). If “yes” to any question in II, was the event within the time window of increased
risk? ........................................................................................................................................... 16
III. Is there strong evidence against a causal association? ........................................................ 16
IV. Other qualifying factors for classification ............................................................................ 16
Step 3: Algorithm ........................................................................................................................... 19
Step 4: Classification ...................................................................................................................... 20
I. Case with adequate information for causality conclusion ..................................................... 21
II. Case without adequate information for causality conclusion ............................................... 22
Summarizing the logic of causality .......................................................................................23
Underlying mechanisms for the classification of adverse events ........................................24
A. Consistent causal association to immunization ........................................................................ 24
A1 and A2. Vaccine product-related and vaccine quality defect-related reactions .................. 24
A3. Immunization error-related reaction .................................................................................. 25
A4. Immunization anxiety-related reaction............................................................................... 26
v
B. Indeterminate............................................................................................................................ 26
B1. Consistent temporal relationship but insufficient evidence for causality .......................... 26
B2. Conflicting trends of consistency and inconsistency with causality ................................... 27
C. Inconsistent causal association to immunization (coincidental) ............................................... 27
Underlying or emerging condition(s) in the vaccine ................................................................. 27
Conditions caused by exposure to external factors .................................................................. 27
Initiating action after causality assessment ..........................................................................28
A. Consistent causal association to immunization ........................................................................ 28
A1. Vaccine product-related reaction ....................................................................................... 28
A2. Vaccine quality defect-related reaction .............................................................................. 28
A3. Immunization error-related reaction .................................................................................. 28
A4. Immunization anxiety-related reaction............................................................................... 28
B. Indeterminate............................................................................................................................ 28
B1. Consistent temporal relationship but insufficient evidence for causality .......................... 28
B2. Conflicting trends of consistency and inconsistency with causality ................................... 28
C. Inconsistent causal association to immunization (coincidental) ............................................... 29
Conclusion .............................................................................................................................30
Step 1: Eligibility ............................................................................................................................ 31
Annex 1. Worksheet for causality assessment .....................................................................31
Step 2: Event checklist ................................................................................................................... 31
Step 3: Algorithm ........................................................................................................................... 32
Step 4: Classification ...................................................................................................................... 32
ANNEX 2. Examples ...............................................................................................................33
Example 1: Meningococcal conjugate vaccine and seizures ......................................................... 33
Step 1: Eligibility ........................................................................................................................ 34
Step 2: Event checklist ............................................................................................................... 34
Step 3: Algorithm ....................................................................................................................... 35
Step 4: Classification .................................................................................................................. 35
Example 2: OPV and acute flaccid paralysis .................................................................................. 36
Step 1: Eligibility ........................................................................................................................ 37
Step 2: Event checklist ............................................................................................................... 37
Step 3: Algorithm ....................................................................................................................... 38
Step 4: Classification .................................................................................................................. 38
Example 3: AEFI after MMR vaccine .............................................................................................. 39
Option 1 − MMR and thrombocytopenia .................................................................................. 40
vi
Step 1: Eligibility ........................................................................................................................ 40
Step 2: Event checklist ............................................................................................................... 40
Step 3: Algorithm ....................................................................................................................... 41
Step 4: Classification .................................................................................................................. 41
EXAMPLE 3: Option 2 − MMR and sepsis ...................................................................................... 42
Step 1: Eligibility ........................................................................................................................ 42
Step 2: Event checklist ............................................................................................................... 42
Step 3: Algorithm ....................................................................................................................... 43
Step 4: Classification .................................................................................................................. 43
vii
Glossary
Adverse event
Any untoward medical occurrence which follows immunization
following
and which does not necessarily have a causal relationship with
immunization (AEFI)
the usage of the vaccine. The adverse event may be any
unfavourable or unintended sign, abnormal laboratory finding,
symptom or disease.
Causal association
A cause-and-effect relationship between a causative factor and a
disease with no other factors intervening in the process.
CISA
Clinical Immunization Safety Assessment Network.
Coincidental event
An AEFI that is caused by something other than the vaccine
product, immunization error or immunization anxiety.
Cluster
Two or more cases of the same event or similar events related in
time, geography, and/or the vaccine administered. National
programme managers may decide upon a more precise
definition.
Data mining
A field at the intersection of computer science and statistics that
attempts to discover inapparent patterns in large data sets. Data
mining utilizes methods at the intersection of artificial
intelligence, machine learning, statistics and database systems.
The overall goal of the data mining process is to extract
information from a data set and transform it into an
understandable structure for further use.
Immunization anxiety-
An AEFI arising from anxiety about the immunization.
related reaction
Immunization error-
An AEFI that is caused by inappropriate vaccine handling,
related reaction
prescribing or administration and thus, by its nature, is
(formerly
preventable.
programmatic error)
Immunization safety
The public health practices and policies dealing with the various
aspects of the correct administration of vaccines, focusing on
minimizing the risk of transmission of disease with the injection
and maximizing the effectiveness of the vaccine. The term
encompasses the spectrum of events from proper manufacture to
correct administration
.
An event that is not “serious” and that has no potential risk to the
Minor AEFI
health of the recipient of the vaccine.
Signal (safety signal)
Information (from one or multiple sources) which suggests a
new and potentially causal association, or a new aspect of a
known association, between an intervention and an event or set
viii
of related events, either adverse or beneficial, that is judged to be
of sufficient likelihood to justify verificatory action.
Surveillance
The continuing, systematic collection of data that is analysed and
disseminated to enable decision-making and action to protect the
health of populations.
Vaccine
A biological substance that is administered to individuals to
elicit immunity (protection) against a specific disease.
Vaccination failure
Vaccination failure is based on clinical endpoints or
immunological criteria, where correlates or surrogate markers
for disease protection exist. Vaccination failure can be due to
vaccine failure (either “primary” when immune response is
inadequate or “secondary” when the immune response wanes) or
failure to vaccinate (i.e. when an indicated vaccine was not
administered appropriately for any reason).
Vaccine
The science and activities relating to the detection, assessment,
pharmacovigilance
understanding and communication of AEFI and other vaccine- or
immunization-related issues, and to the prevention of untoward
effects of the vaccine or immunization.
Vaccine product
All components of a given vaccine formulation, including the
immunogen (part of the vaccine that stimulates an immune
response) and others that may be present such as the adjuvant,
preservative and other additives used during the manufacturing
process to confirm product quality/stability (e.g. potassium or
sodium salts, albumin, gelatin), support growth and purification
of specific immunogens (e.g. egg or yeast proteins, antibiotic) or
inactivate toxins (e.g. formaldehyde).
Vaccine product-
An AEFI that is caused or precipitated by a vaccine due to one
related reaction
or more of the inherent properties of the vaccine product,
whether the active component or one of the other components of
the vaccine (e.g. adjuvant, preservative or stabilizer).
Vaccine quality defect-
An AEFI that is caused or precipitated by a vaccine that is due to
related reaction
one or more quality defects of the vaccine product, including its
administration device as provided by the manufacturer.
ix
Acronyms
ADR
Adverse drug reaction
AEFI
Adverse event(s) following immunization
AFP
Acute flaccid paralysis
AIDS
Acquired immune deficiency syndrome
BCG
Bacille Calmette–Guérin
CIOMS
Council for International Organizations of Medical Sciences
DTP
Diphtheria, tetanus and pertussis (vaccine)
GACVS
Global Advisory Committee on Vaccine Safety
GBS
Guillain-Barré syndrome
HIV
Human immunodeficiency virus
IPV
Inactivated poliovirus vaccine
ITP
Idiopathic thrombocytopenic purpura
LMIC
Low- and middle-income countries
MMR
Measles, mumps and rubella vaccine
OPV
Oral polio vaccine
SAE
Severe adverse event
VAPP
Vaccine-associated paralytic polio
WHO
World Health Organization
x
Introduction and rationale
Immunization is among the most successful and cost-effective public health interventions.
It has led to the global eradication of smallpox as well as the elimination of poliomyelitis in
regions of the world. Immunization currently averts an estimated 2 to 3 million deaths from
diphtheria, tetanus, pertussis (whooping cough), and measles every year in all age groups.1
More people than ever before are being reached with immunization. In 2011, in children
under the age of one year, about 83% (an estimated 109 million infants) were vaccinated
with three doses of diphtheria-tetanus-pertussis (DTP3) vaccine, about 84% (an estimated
110 million) with measles vaccine, and about 88% (an estimated 114 million) with the
BCG vaccine.
Immunization safety has become as important as the efficacy of the national vaccine-
preventable disease control programmes. Unlike drugs, the expectations from vaccinations
are much higher and problems arising from the vaccine or vaccination are less acceptable to
the general public. Vaccines are usually administered to healthy people, including entire
birth cohorts of infants and in vast numbers. The settings in which they are administered
vary from sophisticated tertiary care hospitals to primitive settings in remote, inhospitable
and inaccessible terrain. In many countries, specific vaccinations are mandatory for school
admission as well as international travel. The assessment, licensure, control and
surveillance of biological medicinal products, including vaccines, are major challenges for
national regulatory authorities confronted by a steadily increasing number of novel
products, complex quality concerns, and new technical issues arising from rapid scientific
advances.
The benefits of immunization are often not visible, particularly if the target disease
incidence is low. In contrast, adverse effects that follow immunization are promptly
noticeable, especially when the vaccinee was apparently healthy at the time of
immunization. Although other factors may have contributed to or even been totally
responsible for the event, they may not be considered or investigated. Fear of vaccine
reactions, real or perceived, deters many people from undergoing vaccination. The
problems of vaccine reaction and reluctance to be vaccinated have been known for many
years in industrialized countries and are often raised after most of the benefits from
immunization have been obtained. As immunization programmes have expanded in low-
and middle-income countries (LMICs) in recent decades, the problems have become
familiar there too.
Allegations that vaccines/vaccination cause adverse events must be dealt with rapidly and
effectively. Failure to do so can undermine confidence in a vaccine and ultimately have
dramatic consequences for immunization coverage and disease incidence long after proof is
generated that the adverse event was not caused by vaccine (e.g. autism and MMR,
encephalopathy and pertussis). On the other hand it must always be remembered that
vaccines are not 100% safe and harm can result from errors in immunization practice. Thus
vaccine-associated adverse reactions and error-related immunization events may affect
healthy individuals and should be promptly identified for further response. Appropriate
action(s) must be taken to respond promptly, efficiently, and with scientific rigour to
vaccine safety issues. This will minimize adverse effects to the health of individuals and
1 World Immunization Week 2012. Geneva, World Health Organization, 2012. See:
http://www.who.int/immunization/newsroom/events/immunization_week/2012/further_information/en/index1
.html (accessed 28 February 2013).
1
entire populations and in turn help to maximize the benefits of immunization programmes.
Causality assessment of AEFI is thus a vital component of AEFI risk assessment, decision-
making and the initiation of action.
Definitions of adverse events following immunization
A number of key terms used in this document are defined here for the sake of clarity.
General definition
Adverse event following immunization (AEFI): This is defined as any untoward medical
occurrence which follows immunization and which does not necessarily have a causal
relationship with the use of the vaccine. The adverse event may be any unfavourable or
unintended sign, an abnormal laboratory finding, a symptom or a disease.
Cause-specific definitions
Vaccine product-related reaction:
An AEFI that is caused or precipitated by a vaccine due
to one or more of the inherent properties of the vaccine product.
Vaccine quality defect-related reaction:
An AEFI that is caused or precipitated by a vaccine
due to one or more quality defects of the vaccine product, including the administration
device, as provided by the manufacturer.
Immunization error-related reaction:
An AEFI that is caused by inappropriate vaccine
handling, prescribing or administration and that thus, by its nature, is preventable.
Immunization anxiety-related reaction:
An AEFI arising from anxiety about the
immunization.
Coincidental event:
An AEFI that is caused by something other than the vaccine product,
immunization error or immunization anxiety.
2
The need for causality assessment of adverse events
Causality is the relationship between two events (the cause and the effect), where the
second event is a consequence of the first. A direct cause is a factor in absence of which the
effect would not occur (necessary cause). Sometimes there are multiple factors that may
precipitate the effect (event) or may function as co-factors so that the effect (event) occurs.
Many challenges are involved in deciding whether an adverse event is actually caused by a
vaccine. Vaccines are often administered to children at ages when many underlying
diseases become evident. Vaccines administered to adults can also coincide with an entirely
different risk factor for an event. The fact that a vaccine was administered within a
reasonable time period of the occurrence of an event does not automatically suggest that the
vaccine caused or contributed to the event.
The evidence of a link between a vaccine as a potential cause and a specific event is
derived from epidemiological studies that follow the scientific method and try to avoid
biases and confounders. An example is a patient who is a smoker but also has a family
history of breast cancer: is tobacco the cause of the cancer or only a co-factor? In the same
way, to perform causality assessment in individual cases after vaccination, even where
evidence for a causal link exists for some vaccines and AEFI (e.g. measles vaccine and
thrombocytopenia), it is important to consider all possible explanations for the event and
the degree of likelihood of each before attributing the event to the vaccine product, a
vaccine quality defect, an error in the immunization process, immunization anxiety or
coincidence.
AEFI causality assessment in practice
Causality assessment is the systematic review of
Causality assessment usually
data about an AEFI case; it aims to determine the
will not prove or disprove an
likelihood of a causal association between the
association between an event
event and the vaccine(s) received. For individual
and the immunization. It is
cases, one tries to apply the evidence available on
meant to assist in determining
the basis of the history and time frame of the event
the level of certainty of such
to arrive at a causal likelihood. The quality of the
an association. A definite
causality assessment depends upon:
causal association or absence
− the performance of the AEFI reporting
of association often cannot be
system in terms of responsiveness,
established for an individual
effectiveness
and
quality
of
event.
investigation and reports;
− the availability of adequate medical and
laboratory services and access to
background information;
− the quality of the causality review process.
With inadequate or incomplete data, an AEFI can be deemed unclassifiable. However, it
should also be noted that AEFI causality may be indeterminate due to lack of clear
evidence for a causal link, or conflicting trends, or inconsistency with causal association to
immunization. It is nevertheless important not to disregard the above reports of AEFI
because at some point they may be considered a signal and may lead to hypotheses
regarding a link between a vaccine and the event in question, with specific studies designed
to test for a causal association. Pooling of data on individual cases is very helpful in
3
generating hypotheses. The case of rotavirus vaccine and intussusception is a good example.
In 1998 a rotavirus vaccine (RotaShield®) was licensed for use in the USA. Initial clinical
trials with the vaccine showed that it had been effective in preventing severe diarrhoea
caused by rotavirus A, and researchers had detected no statistically significant serious
adverse effects. After RotaShield® was licensed, however, some infants vaccinated
developed intussusception. At first it was not clear if the vaccine or some other factor was
causing the bowel obstructions. The results of investigations showed that RotaShield®
vaccine caused intussusception in some healthy infants younger than 12 months of age who
normally would be at low risk for this condition. The United States Advisory Committee on
Immunization Practices (ACIP) voted on 22 October 1999 to no longer recommend use of
the RotaShield® vaccine in infants because of an association between the vaccine and
intussusception.1
1 See: http://www.cdc.gov/vaccines/vpd-vac/rotavirus/vac-rotashield-historical.htm (accessed 20 January
2013).
4
Levels of causality assessment and their scientific basis
Causality assessment of AEFI should be performed at several different levels. The first is
the population level, where it is necessary to test if there is a causal association between the
use of a vaccine and a particular AEFI in the population. Secondly, at the level of the
individual AEFI case report, one should review previous evidence and make a logical
deduction to determine if an AEFI in a specific individual is causally related to the use of
the vaccine. The third level of assessment is in the context of the investigation of signals.
1.
The population level
At the population level the aim is to answer the question “Can the given vaccine cause a
particular adverse event?” (i.e. “Can it?”)
Several criteria are relevant to establishing
causality but only the first criterion is absolutely essential:
• Temporal relationship:
The vaccine exposure must precede the occurrence of the
event.
• Strength of association:
The association should meet statistical significance to
demonstrate that it was not simply a chance occurrence.
• Dose−response relationship: Evidence that increasing exposure increases the risk of
the event supports the suggestion of a causal relationship. However, one should
keep in mind that, in the case of vaccines, dose and frequency tend to be fixed.
• Consistency of evidence:
Similar or the same results generated by studies using
different methods in different settings support a causal relationship.
• Specificity:
The vaccine is the only cause of the event that can be shown.
• Biological plausibility and coherence:
The association between the vaccine and the
adverse event should be plausible and should be consistent with current knowledge
of the biology of the vaccine and the adverse event.
One should also consider the presence of systematic bias (analytic bias) in study methods
as this weakens conclusions that a causal association exists.
The United States Institute of Medicine (IOM) has applied these criteria and has published
literature that addresses (in detail) two critical questions in the revised WHO causality
algorithm, namely: “Is there evidence in literature that this vaccine(s) may cause the
reported event even if administered correctly?” and “Did the event occur within an
appropriate time window after vaccine administration?”1
WHO information sheets on observed rates of vaccine reactions that summarize known
reactions to existing single antigen vaccines or combination products are available
online.2
1 The detailed document can be downloaded free at:
http://www.iom.edu/Reports/2011/Adverse-Effects-of-Vaccines-Evidence-and-Causality.aspx (accessed 23
February 2013).
2 See: http://www.who.int/vaccine_safety/initiative/tools/Guide_Vaccine_rates_information_sheet_.pdf
(accessed 23 February 2013).
5
2.
The individual level
At the individual level it is usually not possible to establish a definite causal relationship
between a particular AEFI and a particular vaccine on the basis of a single AEFI case
report. However, it is important to try in order to identify a possible new vaccine product-
related AEFI, as well as to determine if the event is preventable or remedial – such as a
product-related quality defect or immunization error. Identifying a coincidental AEFI that
is falsely attributed to a vaccine product is vital as otherwise the coincidence may result in
loss of public confidence in the vaccine, with the consequent return of vaccine-preventable
disease.
The aim of causality assessment at the individual level is to address the question “Did the
vaccine given to a particular individual cause the particular event reported?” (i.e. “Did it?”).
As noted, it is seldom possible to achieve a straightforward answer to this question, so in
most cases the assessment involves systematic consideration of all possible causes of an
AEFI in order to arrive at a conclusion that the evidence is consistent with the vaccine
being a cause, or is inconsistent with this conclusion, or is indeterminate.
The scientific basis for the criteria which are assessed in the process include:
• Temporal relationship: The vaccine exposure must precede the occurrence of the
event.
• Definitive proof that the vaccine caused the event: Clinical or laboratory proof that
the vaccine caused the event is most often found for live attenuated vaccines. (For
instance, in a case of aseptic meningitis after immunization with Urabe mumps
vaccine virus, isolation of the Urabe virus from the cerebrospinal fluid is definitive
proof that it caused the meningitis. Another example is isolation of the BCG agent
from a focus of osteomyelitis.)
• Population-based evidence for causality – i.e. what is known about “Can it?
”
o A definitive “yes” at the population level is consistent with causality at the
individual level.
o A strong “no” at the population level is inconsistent with causality at the
individual level.
o If there is no clear answer to the question at the population level, this will
often lead to an indeterminate conclusion at the individual level. If there are
significant numbers of individual cases, however, this clearly points to the
need to try to answer the question at the population level.
• Biological plausibility: In situations where the “Can it?” question has no clear “yes”
or “no” answer, biological plausibility may provide support for or against vaccine
causality. In other words, the association should be compatible with existing theory
and knowledge related to how the vaccine works.
• Consideration of alternative explanations:
In doing causality assessment on an
individual case report, it must be remembered that in essence one is conducting a
differential diagnosis. Thus it is important to consider “coincidental AEFI” – i.e. an
AEFI due to something other than the vaccine product, immunization error or
immunization anxiety. All reasonable alternative etiological explanations should be
considered, including:
− preexisting illness;
− newly acquired illness;
− spontaneous occurrence of an event without known risk factors;
− emergence of a genetically programmed disease;
6
− other exposures to drugs or toxins prior to the event;
− surgical or other trauma that leads to a complication;
− a manifestation of, or complication of, a coincidental infection that was
present before or at the time of immunization, or was incubating, but was
not apparent at the time of immunization.
• Prior evidence that the vaccine in question could cause a similar event. The concept
of “rechallenge”, which is more commonly used in the assessment of causality in
medicines, has been helpful for certain vaccine event considerations (e.g. Guillain-
Barré syndrome (GBS) after tetanus toxoid vaccination, where GBS occurred on
three separate occasions in the same individual within weeks of administration of
tetanus toxoid).
3.
Investigation of signals
The assessment of whether a particular vaccine is likely to cause a particular AEFI takes
into account all evidence from individual cases of AEFI, as well as surveillance data and,
where applicable, cluster investigations and nonclinical data.
7
Case selection for causality assessment
The selection of cases for causality assessment should focus on:
− serious AEFI1 that results in death, is life-threatening, requires inpatient
hospitalization or prolongation of existing hospitalization, results in
persistent or significant disability/incapacity, or is a congenital
anomaly/birth defect;
− the occurrence of events above the expected rate or of unusual severity;
− signals generated as a result of individual or clustered cases as these could
signify a potential for large public health impact.
WHO recommends that other AEFI should also be assessed if the reviewing team or review
committee decides that causality needs to be determined as a special case or in order to
conduct special studies. Such AEFI could include:
− AEFI that may have been caused by immunization error (e.g. bacterial
abscess, severe local reaction, high fever or sepsis, BCG lymphadenitis,
toxic shock syndrome);
− significant events of unexplained cause occurring up to 30 days after a
vaccination (and that are not listed on the product label);
− events causing significant parental or community concern (e.g. hypotonic
hyporesponsive episode (HHE), febrile seizures).
Prerequisites for causality assessment
AEFI are usually reported through passive or stimulated passive surveillance, and less
frequently from active surveillance systems. Timely reporting of AEFI followed by
appropriate and detailed investigation is the key to successful causality assessment and
signal detection. An AEFI report should fulfill three prerequisites before causality
assessment, namely:
• The AEFI case investigation should have been completed. Premature assessments
with inadequate information could mislead the classification of the event.
• All details of the case should be available at the time of assessment. Details should
include documents pertaining to the investigation as well as laboratory and autopsy
findings as appropriate.
• There must be a “valid diagnosis” (as explained below) for the unfavourable or
unintended sign, abnormal laboratory finding, symptom or disease in question.
Who should do causality assessment?
To ensure that the prerequisite criteria described above are met and to ensure broader
acceptance of the findings, causality assessment of AEFI should ideally be performed by a
reviewing team or committee of reviewers from relevant specialties. However, in many
countries and situations this broad level of expertise may not be available and existing
human resources need to be used for the causality assessment of AEFI.
1
Definition and application of terms for vaccine pharmacovigilance. Report of CIOMS/WHO Working Group
on Vaccine Pharmacovigilance. Geneva, Council for International Organizations of Medical Sciences, 2012.
See: http://www.cioms.ch/index.php/component/booklibrary/?task=view&Itemid=&id=45&catid=58
(accessed 4 March 2013).
8
Countries requiring special technical expertise (such as special laboratory tests or training)
in causality assessment should contact the respective WHO country office or WHO
regional office. Assistance is also available from WHO at global level.1
1 Contact: Immunization, Vaccines and Biologicals, World Health Organization, 20 avenue Appia, 1211
Geneva 27, Switzerland. Tel: +41 22 791 4468; Fax: +41 22 791 4227; e-mail: [email address]. The web
link is http://www.who.int/immunization_safety/en/.
9
Steps for causality assessment of an individual adverse
event
The revised process envisages the causality assessment of an individual AEFI case in
relation to a particular vaccine. If multiple vaccines are given simultaneously, the reviewers
will have to assess causality separately for each suspected vaccine.
Causality assessment has four steps, as follows:
• Step 1: Eligibility. The first step aims to determine if the AEFI case satisfies the
minimum criteria for causality assessment as outlined below.
• Step 2: Checklist. The second step involves systematically reviewing the relevant
and available information to address possible causal aspects of the AEFI.
• Step 3: Algorithm. The third step obtains a trend as to the causality with the
information gathered in the checklist.
• Step 4: Classification. The fourth step categorizes the AEFI’s association to the
vaccine or vaccination on the basis of the trend determined in the algorithm.
The worksheet used for the causality assessment of an individual AEFI case is presented in
Annex 1. This can be used by the reviewers to arrive at a decision on causality.
Step 1: Eligibility
Before proceeding with causality assessment, it is necessary first to confirm that the
vaccine was administered
before the event occurred (Fig. 1). This can be ascertained by
eliciting from the relevant informants a very detailed and careful history and physical
findings. It is also essential to have a valid diagnosis for the reported AEFI, which could be
an unfavourable or unintended sign, an abnormal laboratory finding, a symptom or a
disease.
Fig. 1. Causality assessment −
Eligibility
10
The valid diagnosis refers to the extent to which the unfavourable or unintended sign,
abnormal laboratory finding, symptom or disease is defined, and whether it is well founded
and corresponds accurately to the event being assessed. Validity can help determine what
types of tests and tools to use, and can help to make sure that the methods used are not only
correct but that they also truly measure the event in question.
For instance, a diagnosis of “altered consciousness” can be defined by a spectrum of terms
by various observers. Among such terms are: clouding of consciousness, confusional state,
delirium, lethargy, stupor, dementia, hypersomnia, vegetative state, coma and brain death.
Many of these terms mean different things to different people.1 For a reliable and objective
means of recording a person’s conscious status, the clinician uses a standard tool such as
the Glasgow Coma Scale.
The valid diagnosis should meet a standard case definition (or it could also be a syndromic
case definition). If available, it is best to adopt the Brighton Collaboration case definition
which can be accessed online.2 However, when a valid diagnosis exists but a case definition
does not, case definitions can be adopted from standard medical literature or national
guidelines, or may also be adopted locally by the reviewers. If the reported event does not
have a valid diagnosis, the AEFI cannot be classified and additional information should be
collected to arrive at a valid diagnosis.
At this stage it is also essential for the reviewers to define the “causality question” (Fig. 2).
Examples of causality questions are:
• “Has the vaccine A caused hepatomegaly?” (an example of an unfavourable or
unintended sign).
• “Has the vaccine B caused thrombocytopenia?” (an example of a laboratory
finding).
• “Has the patient complained that the vaccine C caused itching and redness?” (an
example of a symptom).
• “Has the vaccine D caused meningitis?” (an example of a disease).
Fig 2. Causality question
It is important that, if an AEFI is reported and does not meet the eligibility criteria, attempts
should be made to collect additional information to ensure that the criteria are met.
Additionally, all cases reported (including ineligible cases) should be stored in a repository
(preferably electronic) so that they can be accessed when additional information becomes
available through reports of similar cases or through periodic data mining.
Step 2: Checklist
The checklist (Table 1) contains elements to guide the reviewers as they collate the
evidence needed for a case review.
1 Tindall SC. Level of consciousness. Chapter 57 in: Walker HK, Hall WD, Hurst JW.
Clinical methods. The
history, physical, and laboratory examinations, 3rd edition. Boston, MA, Butterworths, 1990.
2 See: https://brightoncollaboration.org/public (accessed 23 February 2013).
11
Table 1. The causality assessment checklist
I. Is there strong evidence for other causes? Y N UK NA Remarks
Does a clinical examination, or laboratory tests on the patient, confirm
another cause?
II. Is there a known causal association with the vaccine or vaccination?
Vaccine product(s) Is there evidence in the literature that this vaccine(s) may cause the reported
event even if administered correctly?
Did a specific test demonstrate the causal role of the vaccine or any of the
ingredients?
Immunization error Was there an error in prescribing or non-adherence to recommendations for
use of the vaccine (e.g. use beyond the expiry date, wrong recipient etc.)?
Was the vaccine (or any of its ingredients) administered unsterile?
Was the vaccine's physical condition (e.g. colour, turbidity, presence of
foreign substances etc.) abnormal at the time of administration?
Was there an error in vaccine constitution/preparation by the vaccinator (e.g.
wrong product, wrong diluent, improper mixing, improper syringe filling
etc.)?
Was there an error in vaccine handling (e.g. a break in the cold chain during
transport, storage and/or immunization session etc.)?
Was the vaccine administered incorrectly (e.g. wrong dose, site or route of
administration; wrong needle size etc.)?
Immunization anxiety
Could the event have been caused by anxiety about the immunization (e.g.
vasovagal, hyperventilation or stress-related disorder)?
II (time). If “yes” to any question in II, was the event within the time window of increased risk?
Did the event occur within an appropriate time window after vaccine
administration?
III. Is there strong evidence against a causal association?
Is there strong evidence against a causal association?
IV. Other qualifying factors for classification
Could the event occur independently of vaccination (background rate)?
Could the event be a manifestation of another health condition?
Did a comparable event occur after a previous dose of a similar vaccine?
Was there exposure to a potential risk factor or toxin prior to the event?
Was there acute illness prior to the event?
Did the event occur in the past independently of vaccination?
Was the patient taking any medication prior to vaccination?
Is there a biological plausibility that the vaccine could cause the event?
Note: Y: Yes; N: No; UK: Unknown; NA: Not applicable.
The checklist is designed to assemble information on the patient-immunization-AEFI
relationship in the following key areas:
− evidence for other causes;
− association of the event and the vaccine/vaccination
with the vaccine
product(s), immunization error or immunization anxiety (if there is an
association, it is important to find out if the event occurred within an
appropriate time window);
− evidence against a causal association;
12
− other qualifying factors for classification such as the background rate of the
event, present and past health condition, potential risk factors, medication,
biological plausibility etc.
The checklist and the questions are described in detail in Table 1, with some illustrative
examples. It will be observed that sometimes the responses could be applicable to multiple
questions; therefore it is essential that the “Remarks” column is used to explain the reasons.
Please note that the list of examples and illustrations provided are not exhaustive.
I. Is there strong evidence for other causes?
In judging whether a reported association is causal, it is necessary to determine the extent
to which researchers have taken other possible explanations into account and have
effectively ruled out such alternative explanations.
Does a clinical examination or laboratory tests on the patient confirm another cause?
Clinical examination and laboratory tests may help to identify other conditions such as
other diseases and congenital anomalies that could have caused the event. For example:
The death of a teenage girl in the United Kingdom following vaccination with the
human papilloma virus (HPV) vaccine was initially attributed to the vaccine. A
post-mortem found it to be due to a malignant mediastinal tumour.1
• Japanese encephalitis vaccine was blamed for a viral encephalitis outbreak in Uttar
Pradesh, India in 2007. Investigations (into the seasonality as well as the
epidemiological, clinical and laboratory profile of cases) later proved that accidental
consumption of the
Cassia occidentalis beans by the children concerned was
responsible for the disease which was not encephalitis as initially believed but a
syndrome of acute hepato-myoencephalopathy.2
II. Is there a known causal association with the vaccine or vaccination?
Most vaccine adverse events are minor and temporary, such as a sore arm or mild fever.
More serious adverse events occur rarely (of the order of one case per thousands of doses to
one case per millions of doses), and some are so rare that risk cannot be accurately assessed.
Most AEFI, and particularly those related to vaccines that have been used for several
decades, are available in the literature. WHO information sheets on rates of vaccine
reactions are available online.3 It is important to be alert in order to detect new events
(signals). AEFI may be related to the vaccine product, immunization error or immunization
anxiety.
Vaccine product(s)
Is there evidence in the literature that this vaccine(s) may cause the reported event even if
administered correctly?
It is rare for vaccines to produce adverse events due to the vaccine’s inherent properties
when administered correctly. However, such cases have occurred. For example:
1 See
HPV vaccine in the clear at: http://www.nhs.uk/news/2009/09September/Pages/Cervical-cancer-
vaccine-QA.aspx (accessed 23 February 2013).
2 Cassia occidentalis toxicity causes recurrent outbreaks of brain disease in children in Saharanpur
. Indian
Journal of Medical Research, 2008, 127:413−414. See: http://icmr.nic.in/ijmr/2008/april/0421.pdf (accessed
23 February 2013).
3 See: http://www.who.int/vaccine_safety/initiative/tools/vaccinfosheets/en/index.html (accessed 23 February
2013).
13
• An extremely rare adverse event associated with OPV use is the vaccine-associated
paralytic poliomyelitis (VAPP), which may occur in vaccine recipients or their
contacts. The overall risk of VAPP is estimated at between 1 and 2.9 cases per
million doses of trivalent OPV administered.
A causal association between measles−mumps–rubella (MMR) vaccine and
idiopathic
thrombocytopenic
purpura
(ITP)
was
confirmed
using
immunization/hospital admission record linkage. The absolute risk within six weeks
of immunization was 1 in 22,300 doses.1
Did a specific test demonstrate the causal role of the vaccine or any of the ingredients?
• As an example, aseptic meningitis has been known to be a complication of mumps
vaccination. Among 630 157 recipients of trivalent MMR vaccine containing the
Urabe Am9 mumps vaccine, there were at least 311 meningitis cases suspected to
be vaccine-related. In 96 of these 311 cases, Urabe Am9 mumps vaccine virus was
isolated from cerebrospinal fluid.2
Immunization error
Immunization error describes an AEFI that is caused by inappropriate vaccine handling,
prescribing or administration and that therefore, by its nature, is preventable. In many
countries most AEFI are precipitated by immunization error. In such situations,
immunization error has to be ruled out first during an AEFI investigation. An immunization
error-related reaction may lead to a solitary event or a cluster of events associated with
immunization.
Was there an error in prescribing or non-adherence to recommendations for use of the
vaccine (e.g. use beyond the expiry date, wrong recipient, wrong diluent etc.)?
It is essential that vaccines are used in accordance with the indications, contraindications,
dosage, storage conditions, reconstitution procedures etc. outlined in the package insert.
Each vaccine from a different manufacturer may have different specifications and failure to
comply with them can result in AEFI. For example:
− systemic and/or local reactions following administration of an incorrect dose;
− systemic and/or local reactions following administration of the wrong
product or administration to an individual in an incorrect age group;
− vaccine failure if a live attenuated product is given too soon after blood
products or at an age when maternally transferred antibody could interfere
with the replication required to induce an immune response
Was the vaccine (or any of its ingredients) administered unsterile?
Children immunized with contaminated vaccine (usually the bacterium
Staphylococcus
aureus) become sick within a few hours. Local tenderness and tissue infiltration, vomiting,
diarrhoea, cyanosis and high temperature are the most frequent symptoms (toxic shock
syndrome). Bacteriological examination of the vial, if still available, or of local tissue can
confirm the source of the infection.3
1 Miller E et al. Idiopathic thrombocytopenic purpura and MMR vaccine.
Archives of Disease in
Childhood, 2001, 84:227−229 (doi:10.1136/adc.84.3.227)
2 Sugiura A. Aseptic meningitis as a complication of mumps vaccination.
Pediatric Infectious Disease
Journal, 1991, 10(3):209−213. See: http://www.ncbi.nlm.nih.gov/pubmed/2041668 (accessed 23 February
2013).
3 Simon PA et al. Outbreak of pyogenic abscesses after diphtheria and tetanus toxoids and pertussis
vaccination.
Pediatric Infectious Disease Journal, 1993, 12(5):368−371. See:
http://www.ncbi.nlm.nih.gov/pubmed/8327295 (accessed 23 February 2013).
14
Was the vaccine's physical condition (e.g. colour, turbidity, presence of foreign substances
etc.) abnormal at the time of administration?
Abnormal colour, turbidity or presence of visible contaminants may be the first indication
that the vaccine contents are abnormal and may have caused the AEFI.
Was there an error in vaccine constitution/preparation by the vaccinator (e.g. wrong
product, wrong diluent, improper mixing, improper syringe filling etc.)?
AEFI have resulted because of accidental use of the wrong product or the wrong diluent.
This may occur because of improper storage and/or improper selection.1 Vaccine failure
can result if the entire content is not dissolved when freeze-dried vaccines are used or if the
cold chain is not maintained properly. Errors in drawing up vaccine into syringes may
result in AEFI due to excess filling or vaccine failure due to inadequate filling.
Was there an error in vaccine handling (e.g. a break in cold chain during transport,
storage and/or immunization session etc.)?
Exposure to excess heat or cold as a result of inappropriate transport, storage or handling of
the vaccine (and its diluent where applicable) may result in:
− vaccine failure as a result of inactivation of the active vaccine components;
− systemic or local reactions due to changes in the physical nature of the
vaccine, such as agglutination of aluminium-based excipients in freeze-
sensitive vaccines.
Reconstituted vaccines used beyond the prescribed time and recommended maintenance
conditions can result in vaccine failure and/or disease in the recipient (e.g. toxic shock
syndrome).
Was the vaccine administered incorrectly (e.g. wrong dose, site or route of administration;
wrong needle size etc.)?
A variety of AEFI may result from incorrect administration of a vaccine. For example:
− neurological, muscular, vascular or bone injury from the use of an incorrect
injection site, equipment or technique;
− systemic and/or local reactions following administration of an incorrect dose;
− sterile abscess following subcutaneous instead of intramuscular injection of
alum adjuvanted vaccines – usually a result of using a needle that is too
short to reach the muscle layer.
Immunization anxiety
Could the event have been caused by anxiety about the immunization (e.g. vasovagal,
hyperventilation or stress-related disorder)?
The types of reactions caused by immunization anxiety include, but are not limited to,
vasovagal-mediated reactions, hyperventilation-mediated reactions or stress-related
psychiatric disorders. For example:
• In September 1998, more than 800 young people in Jordan believed they had
suffered from the side-effects of tetanus-diphtheria toxoid vaccine administered at
school; 122 of them were admitted to hospital. For the vast majority of the young
people, the symptoms did not result from the vaccine but arose from mass
psychogenic illness. A review of the literature showed, however, that this mass
1 Bundy DG et al. Pediatric vaccination errors: application of the "5 rights" framework to a national error
reporting database.
Vaccine, 2009, 27(29):3890−3896 (Epub 2009 Apr 23). See:
http://www.ncbi.nlm.nih.gov/pubmed/19442422 (accessed 23 February 2013).
15
reaction was similar in many ways to previous outbreaks, even though the
underlying causes varied.1
Adolescents, especially if immunized in mass clinical settings, are more prone to have
anxiety-related vasovagal reactions resulting in fainting, sometimes accompanied by
tonic−clonic seizure-like movements (not a seizure).2
II (time). If “yes” to any question in II, was the event within the time window of
increased risk?
Did the event occur within an appropriate time window after vaccine administration?
It is important to confirm if the event took place in an “appropriate” time window of
increased risk. This is applicable to all questions under II. For example:
• The “appropriate” time window for VAPP is between 4 and 40 days. A case
classified as a recipient VAPP is a person who has onset of acute flaccid paralysis
(AFP) 4−40 days after receiving OPV, isolating Sabin virus and with neurological
sequelae compatible with polio 60 days after the onset of paralysis.3 Thus cases
with AFP onset less than 4 days or over 40 days after receiving OPV and isolating
Sabin virus in the stool are not classified as recipient VAPP.
III. Is there strong evidence against a causal association?
Is there strong evidence against a causal association?
An AEFI that is initially thought to be due to a vaccine may, after investigation, be found to
be explained by a similar manifestation caused by another factor. For example:
• In recent years, some researchers hypothesized that measles vaccine may be
associated with autism. A series of studies were reviewed by the GACVS and also
the IOM Committee to review adverse effects of vaccines. Both groups concluded
that no evidence exists of a causal association between MMR vaccine and autism or
autistic disorders.4 5
IV. Other qualifying factors for classification
Sections I to III outline the strong evidence for or against causality for most cases of AEFI.
Below are some additional factors that support the above observations. If the AEFI is still
unclassified, these qualifying factors provide reviewers with indications on causality.
Could the event occur independently of vaccination (background rate)?
1 Kharabsheh S et al. Mass psychogenic illness following tetanus-diphtheria toxoid vaccination in Jordan.
Bulletin of the World Health Organization, 2001, 79(8):764−770. See:
http://www.who.int/docstore/bulletin/pdf/2001/issue8/vol79.no.8.764-770.pdf (accessed 23 February 2013).
2 Syncope after vaccination − United States, January 2005−July 2007. Morbidity and Mortality Weekly
Report, 2008, 57(17):457−460. See: http://www.ncbi.nlm.nih.gov/pubmed/18451756 (accessed 23 February
2013).
3
Case definition: poliomyelitis. Pan American Health Organization. See:
http://www.paho.org/english/ad/fch/im/PolioCaseDefinition.htm (accessed 23 February 2013).
4
MMR and autism. World Health Organization. See:
http://www.who.int/vaccine_safety/topics/mmr/mmr_autism/en/ (accessed 23 February 2013).
5
Adverse effects of vaccines: evidence and causality. United States Institute of Medicine. See:
http://www.iom.edu/Reports/2011/Adverse-Effects-of-Vaccines-Evidence-and-Causality.aspx (accessed 23
February 2013).
16
Knowledge of the background incidence of events which may occur in temporal
relationship with a vaccine is essential for assessing a cluster of events in terms of the
strength of the signal it may provide. For example:
• In Israel, during the early phases of the annual influenza immunization campaign in
October 2006, four deaths occurred among elderly vaccinees and the campaign was
temporarily halted for an investigation. It was determined that the expected death
rate among similarly aged vaccinees within seven days of a vaccine exposure was
0.01 to 0.02% and this rate had been constant for several years prior to the apparent
signal. The background rate for death in the population was relatively high as a
result of age (>75 years) and comorbid conditions (e.g. diabetes, cardiovascular
disease, homebound status).1
• The importance of understanding background rates of disease prior to a mass
campaign were pointed out in a study published in October 2009. This provides
many examples of how background rates can impact on temporally associated AEFI
observed during immunization campaigns.2
Could the event be a manifestation of another health condition? It is important to consider other health conditions that may have precipitated the AEFI. For
example:
• Glutaric aciduria type I, a rare inborn error of metabolism, was identified after
magnetic resonance imaging in monozygotic twin females with glutaric aciduria
type I who were admitted with acute encephalopathic crisis symptoms three days
after immunization for poliovirus.3
Did a comparable event occur after a previous dose of a similar vaccine?
The occurrence of an AEFI after a previous dose of a similar vaccine should be handled
cautiously. In specialized settings, vaccination schedules can continue taking appropriate
precautions. For example:
•
Revaccinations have to be avoided in patients with a history of anaphylaxis after
vaccine injection because of the potential risk of recurrent anaphylaxis. However,
without diagnostic work-up, vaccine allergy remains a presumption and necessary
vaccinations may be unjustifiably withheld. Diagnostic testing should be performed
after suspected vaccination-induced anaphylaxis in order to rule out IgE-mediated
allergy to the incriminated vaccine and its constituents and to enable future
vaccinations with the tested compounds. Therefore, a history of anaphylaxis after
vaccination may not be an absolute contraindication for revaccination.4
1 Kokia ES et al. Deaths following influenza vaccination − background mortality or causal connection?
Vaccine, 2007, 25(51):8557−8561. See: http://www.ncbi.nlm.nih.gov/pubmed/18006121 (accessed 23
February 2013).
2 Black et al. Importance of background rates of disease in assessment of vaccine safety during mass
immunisation with pandemic H1N1 influenza vaccines.
Lancet, 2009, 374:2115−2122. See:
http://download.thelancet.com/flatcontentassets/H1N1-flu/vaccination/vaccination-86.pdf (accessed 23
February 2013).
3 Alkan A et al. Glutaric aciduria type I diagnosed after poliovirus immunization: magnetic resonance
findings.
Pediatric Neurology, 2002, 26(5):405−407. See: http://www.ncbi.nlm.nih.gov/pubmed/12057806
(accessed 23 February 2013).
4 Seitz CS et al. Vaccination-associated anaphylaxis in adults: diagnostic testing ruling out IgE-mediated
vaccine allergy.
Vaccine, 2009, 27(29):3885–3889. See: http://www.ncbi.nlm.nih.gov/pubmed/19428162
(accessed 23 February 2013).
17
•
Revaccination of children who have a past history of an AEFI appears safe (with the
exception of anaphylaxis and encephalopathy). A special immunization service
should be part of a comprehensive immunization programme.1
Was there exposure to a potential risk factor or toxin prior to the event?
Prior exposure to risk factors/toxins may be a clue to the possibility that an AEFI is a
coincidental event. One should also consider the possibility of an interaction between a risk
factor/toxin and vaccine in causing the AEFI. For example:
• A patient who undergoes a surgical procedure a week prior to vaccination (with an
apparently normal post-operative period), may present with fever the day after
immunization. One needs to determine if the fever (which is an AEFI) is a
coincidental event (to vaccination) that occurred as a late complication of surgery or
if it is due to the vaccine or vaccination (product-related, quality defect-related, or
immunization error-related).
• An AEFI involving hair loss in a patient on chemotherapy who was given HBV
vaccine may be a coincidental event due to the chemotherapy or may be a vaccine
product-related reaction following immunization with HBV vaccine.
• Accidental ingestion and drug interaction are known causes of carbamazepine
toxicity. Less well recognized is the possibility that influenza vaccination may
significantly increase carbamazepine blood levels.2
Was there acute illness prior to the event?
About a quarter of patients with Guillain-Barré syndrome have had a recent Campylobacter
jejuni infection.3 A prior history of a diarrhoeal illness a week or two before vaccination
may be a clue that the GBS is a coincidental event relative to immunization because it was
due to the same agent that caused the diarrhoeal illness prior to vaccination.
Did the event occur in the past independently of vaccination?
It is important to verify if a similar event occurred in the vaccinee and family in the past
independent of immunization. For example:
• A 24-month-old child receives MMR immunization. Seven days later her
pediatrician poses a diagnosis of atopic dermatitis. A careful clinical history,
however, reveals that the child had signs of atopic dermatitis since five months of
age.4
Was the patient taking any medication prior to vaccination?
1 Gold M et al. Re-vaccination of 421 children with a past history of an adverse vaccine reaction in a special
immunization service.
Archives of Disease in Childhood, 2000, 83:128−131 (doi:10.1136/adc.83.2.128). See:
http://www.ncbi.nlm.nih.gov/pubmed/10906018 (accessed 23 February 2013).
2 Robertson WC Jr. Carbamazepine toxicity after influenza vaccination.
Pediatric Neurology, 2002, 26(1):61–
63. See: http://www.sciencedirect.com/science/article/pii/S0887899401003320 (accessed 23 February 2013).
3 Hughes RAC et al. Guillain-Barré syndrome.
Lancet, 2005, 366(9497):1653–1666.
4 Hennino A et al. Influence of measles vaccination on the progression of atopic dermatitis in infants.
Pediatric Allergy and Immunology, 2007, 18:385−390. See: http://www.ncbi.nlm.nih.gov/pubmed/17617807
(accessed 23 February 2013).
18
Medications are known to cause adverse reactions and, when given concurrently with
vaccine(s), must be considered as possible causes of an observed AEFI. For example
• Stevens-Johnson syndrome that occurs nine days after vaccination in an individual
on a sulfa antibiotic could be a coincidental event (due to the sulfa drug) or a
vaccine product-related reaction (due to the vaccine)
Is there a biological plausibility that the vaccine could cause the event?
Biological plausibility – or biological mechanisms as an additional qualifying factor − can
be invoked only when a laboratory finding or a symptom/sign are similar and consistent
with the natural history and physiopathology of the infection or antigen. Evidence
regarding biological plausibility, however, can never prove causality. At best, biological
plausibility adds an additional piece of supportive evidence. For example:
• Acute cerebellar ataxia is a proven complication of wild type varicella zoster virus
(VZV) infection with an estimated incidence of five per 100 000 infections among
children aged five years and under.1 Since the wild virus causes acute cerebellar
ataxia, it is biologically plausible that the attenuated vaccine virus could also result
in this complication of VZV infection in certain vaccinees. However, existing
evidence is still not sufficient to confirm or reject this hypothesis so it remains a
theoretical possibility based on biological plausibility.2
• Some attenuated mumps vaccines, like mumps disease, are associated with aseptic
meningitis. The lack of a standardized clinical case definition of aseptic meningitis
and criteria for CSF evaluation complicates the interpretation of available data and
may increase the probability of higher “case” ascertainment influenced by factors
other than the vaccine strain.3
Step 3: Algorithm
After the checklist is completed, the AEFI case is ready to be applied to the algorithm. The
algorithm aims to be a roadmap for the decision-making of the reviewers but it does not,
and should not, take away the expert and deductive logical process inherent in linking a
diagnosis to its potential cause. The stepwise approach of the algorithm helps to determine
if the AEFI could be consistent or inconsistent with an association to immunization, an
indeterminate outcome or unclassifiable (Fig. 3).
The algorithm allows the reviewers to focus logically and document their observations to
the appropriate conclusions. “Yes” responses in the checklist should have corresponding
conclusions in the algorithm. The boxes on the mandatory path correspond to the four
major sections in the checklist (I to IV). It is essential that the reviewers evaluate all four
boxes using the responses in the checklist. The conclusions are colour-coded green if the
conclusion is inconsistent with a causal association to immunization; red if it is consistent
with a causal association to immunization; yellow if it is indeterminate; and blue if the
event is unclassifiable.
1 Van der Maas NA et al. Acute cerebellar ataxia in the Netherlands: a study on the association with
vaccinations and varicella zoster infection.
Vaccine, 2009, 27(13):1970−1973 (Epub 2009 Jan 30). See:
http://www.ncbi.nlm.nih.gov/pubmed/19186201 (accessed 23 February 2013).
2 Institute of Medicine.
Adverse effects of vaccines: evidence and causality. Washington, DC, National
Academies Press, 2012. See: http://www.nap.edu/catalog.php?record_id=13164 (accessed 23 February 2013).
3 Bonnet M-C et al. Mumps vaccine virus strains and aseptic meningitis.
Vaccine, 2006, 24(49–50):7037–
7045. See: http://dx.doi.org/10.1016/j.vaccine.2006.06.049 (accessed 23 February 2013).
19
Summarizing the responses in the checklist adjacent to the corresponding conclusion or as a
summary note at this point will enable the reviewers to have a transparent “dashboard view”
of their conclusions and the logic for arriving at them.
Fig. 3. Causality assessment algorithm
Responses IA, IIA and IIIA have greater strength and these conclusions have greater
weight. When the conclusion is “unclassifiable”, the reviewers should determine the
reasons why classification was not possible and all attempts should be made to obtain the
necessary supporting evidence for classification.
Step 4: Classification
The final classification has been adapted from
Definition and application of terms for
vaccine pharmacovigilance. Report of the CIOMS/WHO Working Group on Vaccine
Pharmacovigilance. 1 The cause-specific definitions provide clarity on “A. Consistent
causal association to immunization” and “C. Inconsistent causal association to
immunization” (coincidental). The association is considered “B. indeterminate” when
adequate information on the AEFI is available but it is not possible to assign it to either of
the above categories. The details are presented in Fig. 4.
Fig 4. Causality assessment classification
1
Definition and application of terms for vaccine pharmacovigilance. Report of the CIOMS/WHO Working
Group on Vaccine Pharmacovigilance. Geneva, Council for International Organizations of Medical Sciences,
2012.
20
The final classification is based on the availability of adequate information.
I. Case with adequate information for causality conclusion
A case with adequate information for causality conclusion can be classified as follows:
A. Consistent causal association to immunization
A1. Vaccine product-related reaction; or
A2. Vaccine quality defect-related reaction; or
A3. Immunization error-related reaction; or
A4. Immunization anxiety-related reaction.
B. Indeterminate
B1. Temporal relationship is consistent but there is insufficient definitive evidence
that vaccine caused the event (it may be a new vaccine-linked event). This is a
potential signal and needs to be considered for further investigation.
B2. Reviewing factors result in conflicting trends of consistency and inconsistency
with causal association to immunization (i.e. it may be vaccine-associated as well as
coincidental and it is not possible clearly to favour one or the other).
C. Inconsistent causal association to immunization (coincidental): This could be due to
underlying or emerging condition(s) or conditions caused by exposure to something other
than vaccine.
21
II. Case without adequate information for causality conclusion
This case is categorized as “unclassifiable” and requires additional information for further
review of causality. The available information on unclassifiable cases should be placed in a
repository or an electronic database which should be periodically reviewed to see if
additional information is available for classification and to perform analyses for identifying
signals.
22
Summarizing the logic of causality
Causality assessment is performed with the available information and resources that are at
the reviewers’ disposal at a given point in time. The information and resources may be
adequate or inadequate. If the information is inadequate, causality assessment is not
possible. Even with adequate information, the precision of causality is largely determined
by the expertise, experience and skill of the reviewers (Fig. 5). Different cases, when
systematically reviewed, may reveal conflicting findings that have to be debated by a group
of experts before a clearer picture of causality emerges. It is possible that there may be
more than one conclusion on causality by the same reviewers.
Fig. 5. Summary of classification logic
The categories “Consistent causal association to immunization” and “Inconsistent causal
association to immunization” (coincidental) are clearly outlined in the
Report of
CIOMS/WHO Working Group on Vaccine Pharmacovigilance1 and are described below
.
With available evidence, several cases would be classified as “indeterminate”. This must be
discussed by the assessment team to determine if there is a signal or if additional
investigation or special tests are needed.
Causality can change when additional information becomes available either about the same
case or about similar cases. Resource constraints such as non-availability of autopsy
facilities and special laboratory tests (such as the tryptase test as an indicator of mast cell
activation in anaphylaxis) can modify interpretations.
1
Definition and application of terms for vaccine pharmacovigilance. Report of the CIOMS/WHO Working
Group on Vaccine Pharmacovigilance. Geneva, Council for International Organizations of Medical Sciences,
2012.
23
Underlying mechanisms for the classification of adverse
events
A. Consistent causal association to immunization
A1 and A2. Vaccine product-related and vaccine quality defect-related reactions
Vaccines are designed to induce a response by the immune system which involves a
complex interaction between the vaccine antigens, the adjuvant (if present), antigen-
presenting cells, lymphocytes and multiple immune mediators (cytokines). This interaction
is important to the development of the desired immunity against the specific vaccine-
preventable disease. However, the immune response in a vaccinee may manifest as
relatively common and mild adverse reactions to the vaccine(s), such as redness and
swelling at the injection site, or fever. Homeostatic mechanisms usually limit the
inflammatory response so that such reactions are short-lived and have no lasting
consequence. Uncommonly, the immune response to one or more vaccine components may
result in a longer-lasting and more severe adverse reaction. Rarely, the immune response
may cause a life-threatening allergic reaction.
It is important to note that vaccine product-related reactions may unmask a predisposition
in certain high-risk individuals to other adverse events that would not occur in the majority
of vaccinees. For example, fever is a relatively common inflammatory response following
vaccination. For most vaccinees the fever is of short duration and there are no associated
adverse reactions. However, in children with an underlying seizure disorder, or in infants
and toddlers with a tendency to have febrile seizures, the fever may trigger a seizure. Other
events that cause fever, such as respiratory infection, could also trigger a seizure. In such
cases, the seizures result from a combination of an inherent property of the vaccine that
caused fever and underlying factors in the vaccinee that lowered the threshold for seizure
associated with fever.
Vaccine product-related and vaccine quality defect-related reactions are as follows:
• Reactions associated with the route and/or site of administration of the vaccine
product or vaccinee-specific characteristics:
− Bell’s palsy following intranasal administration of a specific influenza
vaccine where the causative mechanism was attributed to the vaccine
composition combined with the mode of administration;
− pain at the time of injection and associated physiological responses.
• Immune-mediated vaccine reaction:
− local reactions, with involvement of the injection site, due to one or more
vaccine components, i.e.
o non-granulomatous inflammation with or without regional lymphadenitis
extensive limb swelling e.g. post-DTP vaccination,
mild, moderate or severe local inflammation, manifest as one or
more of swelling, redness, pain, local tenderness and induration
(examples of the mechanisms underlying more severe reactions
include
- subcutaneous injection of a vaccine [e.g. alum adsorbed]
recommended for intramuscular administration,
- localized antigen-antibody reaction [antibody excess],
- aluminium adjuvant hypersensitivity, and
24
- infection),
o granulomatous inflammation at the injection site with or without
regional lymphadenitis (most commonly related to BCG vaccine);
− multisystem (generalized) reactions due to one or more vaccine components,
i.e.
o systemic inflammatory response (e.g. fever or lethargy)
o mast cell degranulation
IgE mediated hypersensitivity (anaphylaxis),
non-IgE mediated hypersensitivity (reactions in this group are
commonly referred to as anaphylactoid reactions),
o disseminated granulomatous reaction (e.g. disseminated BCG in
immunodeficient hosts)
o immune complex mediated reaction (serum sickness reaction);
− organ-specific reactions due to one or more vaccine components, i.e.
o auto-immune or undefined mechanism
central nervous system (e.g. demyelinating conditions such as GBS
post-influenza vaccination),
blood (e.g. thrombocytopenia post-MMR vaccination),
skin (e.g. rashes after vaccination, including urticarial).
28
• Reactions as a consequence of replication of vaccine-associated microbial agent(s)
in the vaccinee or in a close contact of the vaccinee. The microbial agent(s) could
be:
− an attenuated vaccine agent;
− a wild-type vaccine agent due to insufficient inactivation during the
manufacturing process;
− a contaminant introduced into vaccine during the manufacturing process.
• Direct toxic effect of a vaccine component or contaminant (e.g. quality defect).
A3. Immunization error-related reaction
The emphasis for AEFI in this category is their preventable nature. Thus the classification
mechanism focuses on the nature of the error rather than on the biological process(es)
giving rise to the specific AEFI. Nevertheless, many of the AEFI in this category result
from the same or similar processes as those that underlie vaccine product-related or vaccine
quality defect-related reactions. Immunization error-related reactions are described below.
• Error in vaccine handling:
− exposure to excess heat or cold as a result of inappropriate transport, storage
or handling of the vaccine (and its diluent where applicable), resulting in:
o failure to vaccinate as a result of inactivation of the active vaccine
components
o systemic or local reactions due to changes in the physical nature of the
vaccine, such as agglutination of aluminium-based excipients in freeze-
sensitive vaccines;
− Use of a product after the expiry date, resulting in:
o failure to vaccinate as a result of loss of potency or non-viability of an
attenuated product.
• Error in vaccine prescribing or non-adherence to recommendations for use:
− failure to adhere to a contraindication, resulting in:
25
o anaphylaxis following administration of a vaccine to an individual
known to have an immune-mediated hypersensitivity to one or more
components
o disseminated infection with an attenuated live vaccine agent following
administration to an individual with a known immunodeficiency that
contraindicated use of any live vaccines
o vaccine-associated paralytic polio in an immunocompromised household
contact of a child given oral polio vaccine;
− failure to consider appropriately warnings or precautions for vaccine use;
− failure to adhere to vaccine indications or prescription (dose or schedule),
resulting in:
o systemic and/or local reactions following administration of an incorrect
dose
o systemic and/or local reactions following administration of the wrong
product or administration to an individual in an incorrect age group
o vaccine failure if a live attenuated product is given too soon after blood
products or at an age when maternally transferred antibody could
interfere with the replication required to induce an immune response
o neurological, muscular, vascular or bone injury due to incorrect injection
site, equipment or technique.
• Error in administration:
− use of an incorrect diluent or injection of a product other than the intended
vaccine, resulting in:
o failure to vaccinate due to incorrect diluent
o reaction due to the inherent properties of whatever was administered
other than the intended vaccine or diluent;
− incorrect sterile technique or inappropriate procedure with a multidose vial,
resulting in:
o infection at the site of injection due to a microbial contaminant
introduced during administration of the vaccine
o infection beyond the site of injection due to a microbial contaminant
introduced during administration of the vaccine;
− failure to ensure a safe environment during and immediately following
immunization, resulting in:
o
head injury during a syncopal episode post-immunization;
− inadvertent administration of vaccine to someone for whom it was not
intended (e.g. via a needlestick injury or splash to the eye depending, on the
vaccinee characteristics).
A4. Immunization anxiety-related reaction
The types of reactions caused by immunization anxiety include, but are not limited to:
− vasovagal mediated reactions;
− hyperventilation mediated reactions;
− stress-related psychiatric disorders.
B. Indeterminate
B1. Consistent temporal relationship but insufficient evidence for causality
In this case, the temporal relationship is consistent but there is insufficient definitive
evidence for vaccine causing the event (it may be a new vaccine-linked event). The details
of such AEFI cases should be maintained in a national database. Over time, as more similar
26
vaccines are administered and if similar events are reported from one or multiple sources,
the recorded cases will help to identify a signal suggesting a new potential causal
association, or a new aspect of a known association, between a vaccine and an event or a
set of related events.
B2. Conflicting trends of consistency and inconsistency with causality
Reviewing factors may result in conflicting trends of consistency and inconsistency with
causal association to immunization. Even with adequate information, these AEFI cases
cannot be clearly categorized because the outcomes of investigation may give contradictory
conclusions. There could be clear pointers indicating that the event is related to the vaccine
or the vaccination and at the same time there could also be clear evidence that the vaccine
cannot be responsible.
C. Inconsistent causal association to immunization (coincidental)
AEFI can result from underlying or emerging conditions of the vaccine as well as from
external exposures that can cause harm independent of immunization. These include, but
are not limited to, the following:
Underlying or emerging condition(s) in the vaccine
Such underlying or emerging conditions could include:
− manifestation or complication of a congenital or inherited underlying
disease condition or birth injury;
− manifestation or complication of an underlying acquired disease condition
that may or may not have been diagnosed prior to immunization;
− psychogenic illness.
Conditions caused by exposure to external factors
Conditions caused by factors other than vaccine could include:
− infection due to agents such as bacteria, viruses, fungi or parasites;
− adverse reaction due to recent or concomitant medication or use of illicit
substances;
− allergic and other hypersensitivity reactions due to exposure to allergens
other than those present in the vaccine;;
− injury due to exposure to environmental toxins;
− injury due to trauma, including surgery.
27
Initiating action after causality assessment
Determining causality is not an end in itself. The lessons learned from the assessment
should provide insights for the technical, immunization programme and administrative
managers on the causes and the next steps to take – including training, research, modifying
systems, refining tools and so on – to avoid and/or minimize recurrences.
A. Consistent causal association to immunization
National immunization programmes need to establish standard protocols for responding to
AEFI. These have to be decided by a national committee and approved by the existing
decision-making system in the country.
A1. Vaccine product-related reaction
• It will be necessary to follow protocols adopted by each country when such
cases are confirmed.
A2. Vaccine quality defect-related reaction
• If this reaction is related to a particular lot or batch, the distribution of the lot or
batch has to be ascertained and specific instructions must be provided on the
utilization or non-utilization of the lot or batch.
• It is important to inform the national regulatory authority and the marketing
authorization holder about the AEFI. The event should be communicated to the
manufacturer through these bodies.
• WHO should be contacted through the Organization’s local country office or the
WHO Uppsala Monitoring Centre (http://www.who-umc.org/) and the
information communicated to ensure that other countries using the vaccine are
alerted.
A3. Immunization error-related reaction
• Training and capacity-building are critical to avoid recurrences of such reactions.
A4. Immunization anxiety-related reaction
• Vaccination should take place in an ambient and safe environment.
B. Indeterminate
B1. Consistent temporal relationship but insufficient evidence for causality
• The details of such AEFI cases should be maintained in a national database.
Later this can help to identify a signal suggesting a new potential causal
association, or a new aspect of a known association, between a vaccine and an
event or set of related events.
B2. Conflicting trends of consistency and inconsistency with causality
• These cases are classified on the basis of available evidence. If additional
information becomes available, the classification can move into a more
definitive category. During the assessment, the reviewers should clarify what
additional information would be helpful to finalize the causality assessment and
should seek information and expertise from national or international resources.
28
The GACVS can be approached for guidance through WHO, particularly when
an event is likely to impact the immunization programme significantly.
C. Inconsistent causal association to immunization (coincidental)
• The information and confirmation should be provided to patients, their relatives,
the care provider and the community.
29
Conclusion
It is important to recognize that causality assessment of an AEFI in an individual patient is
an exercise in medical differential diagnosis. A good clinician does not diagnose diabetes
or coronary artery disease on the basis of conflicting or vague information. In the same way,
an AEFI should not be causally linked to a vaccine without adequate information.
In WHO’s revised AEFI causality assessment process, end-users are encouraged to
determine if the minimum criteria for causality assessment are reached, use a checklist to
identify factors that could have caused the event, recognize a pattern through an algorithm
and finally apply the human element in ascertaining causality.
In assessing causality of an AEFI, the human elements of experience, proficiency,
resources and teamwork clearly play an important role. Tools like the one described above
empower investigators to think about the rationale of an assessment, collect relevant data
and help to improve consistency in assessments.
There are several models, algorithms and tools (including software) available for causality
assessment, each with its own merits and with varying sensitivity and specificity. After a
thorough review of the existing methodologies for assessing causality in adverse drug
reactions and AEFI, and after pilot-testing of several approaches (including scoring scales,
algorithms, questionnaires etc.), this revised scheme was developed by a GACVS working
group in consultation with experts from around the world.
There was consensus that it is difficult to create a perfect system that clearly pinpoints the
causality of AEFI. The basic steps in the algorithm developed by the Clinical Immunization
Safety Assessment (CISA) Network was used by the GACVS working group and was
developed into the present scheme to make it applicable in multiple settings.1
1 Halsey NA et al. Algorithm to assess causality after individual adverse events following immunizations.
Vaccine, 2012, 30(39):5791−5798 (Epub 2012 Apr 14). See: http://www.ncbi.nlm.nih.gov/pubmed/22507656
(accessed 23February 2013).
30
Annex 1. Worksheet for causality assessment
Step 1: Eligibility
Name of the
Name of one or more vaccines
What is the valid
Does the diagnosis meet
patient
administered before this event
diagnosis?
a case definition?
Create your question on causality here
Has the ________________ vaccine/vaccination caused ___________________? (The event is for review in step 2)
Step 2: Event checklist
Check all boxes that apply
Y: Yes N: No UK: Unknown NA: Not applicable
I. Is there strong evidence for other causes? Y N UK NA Remarks
Does a clinical examination, or laboratory tests on the patient, confirm
another cause?
II. Is there a known causal association with the vaccine or vaccination?
Vaccine product(s) Is there evidence in the literature that this vaccine(s) may cause the reported
event even if administered correctly?
Did a specific test demonstrate the causal role of the vaccine or any of the
ingredients?
Immunization error Was there an error in prescribing or non-adherence to recommendations for
use of the vaccine (e.g. use beyond the expiry date, wrong recipient etc.)?
Was the vaccine (or any of its ingredients) administered unsterile?
Was the vaccine's physical condition (e.g. colour, turbidity, presence of
foreign substances etc.) abnormal at the time of administration?
Was there an error in vaccine constitution/preparation by the vaccinator (e.g.
wrong product, wrong diluent, improper mixing, improper syringe filling
etc.)?
Was there an error in vaccine handling (e.g. a break in the cold chain during
transport, storage and/or immunization session etc.)?
Was the vaccine administered incorrectly (e.g. wrong dose, site or route of
administration; wrong needle size etc.)?
Immunization anxiety
Could the event have been caused by anxiety about the immunization (e.g.
vasovagal, hyperventilation or stress-related disorder)?
II (time). If “yes” to any question in II, was the event within the time window of increased risk?
Did the event occur within an appropriate time window after vaccine
administration?
III. Is there strong evidence against a causal association?
Is there strong evidence against a causal association?
IV. Other qualifying factors for classification
Could the event occur independently of vaccination (background rate)?
Could the event be a manifestation of another health condition?
Did a comparable event occur after a previous dose of a similar vaccine?
Was there exposure to a potential risk factor or toxin prior to the event?
Was there acute illness prior to the event?
Did the event occur in the past independently of vaccination?
Was the patient taking any medication prior to vaccination?
Is there a biological plausibility that the vaccine could cause the event?
31
Step 3: Algorithm
Review all steps and check all the appropriate boxes
Notes for step 3:
Step 4: Classification
Check all boxes that apply
S ummarize the classification logic:
W ith available evidence, we could conclude that the classification is ___________________________________because:
32
ANNEX 2. Examples
Example 1: Meningococcal conjugate vaccine and seizures
Presenting problem: A five-month-old male (name PQ), given a second dose of
Menjugate vaccine (first dose at age three months); two days post-immunization reported
onset of fever – not documented. Five days post-immunization the infant had a right focal
seizure and altered level of consciousness. The documented temperature was 39° C. The
patient was treated with anticonvulsants and was admitted to hospital. He had persistent
seizure activity on the third and fourth days in hospital. He was transferred to a tertiary-care
referral paediatric hospital and admitted to the intensive care unit with status epilepticus.
Seizures were controlled within 24 hours.
Past medical history: unremarkable good general health; no evidence of immune
deficiency
− no prior history of seizures.
Investigations:
− CSF: 61 RBC; 144 WBC; 57% PMN; and 26% lymphocytes;
− protein 1.2; glucose 3.1;
− culture of CSF, pharynx and stool all negative;
− PCR positive for herpes simplex virus;
− MRI showed extensive inflammation of right frontal, parietal and temporal
lobes, and a small amount of bleeding into the left temporal lobe;
− EEG showed paroxysmal lateral epileptiform discharges.
An investigation at the immunization session site confirmed the application of correct
procedures in vaccine administration.
Treatment and course of illness: Treated with antibiotics and antiviral (acyclovir). The
former was discontinued once PCR results were known; the latter was continued for 21
days. Good recovery in hospital on treatment. At discharge the infant was alert and active
with normal tone. Home on anticonvulsants.
Note: The case meets the Brighton Collaboration case definition for encephalitis - at a level 2 of diagnostic
certainty (evidence of encephalopathy with decreased level of consciousness and associated seizures; multiple
indicators of CNS inflammation [temp 39C; CSF pleocytosis; EEG findings consistent with encephalitis;
neuroimaging consistent with encephalitis]).
33
Step 1: Eligibility
Name of the
Name of one or more vaccines
What is the valid
Does the diagnosis meet
patient
administered before this event
diagnosis?
a case definition?
PQ
Menjugate (Meningococcal
Meningoencephalitis
Yes (level 2)
Group C conjugate vaccine)
Create your question on causality here
Has the Menjugate
vvaccine/vaccination caused meningoencephalitis
?
Step 2: Event checklist
Y: Yes N: No UK: Unknown NA: Not applicable
Check all boxes that apply
I. Is there strong evidence for other causes? Y N UK NA Remarks
Does a clinical examination, or laboratory tests on the patient, confirm another cause?
Yes −
CSF PCR positive
for herpes simplex virus
II. Is there a known causal association with the vaccine or vaccination?
Vaccine product(s) Is there evidence in the literature that this vaccine(s) may cause the reported event even if
administered correctly?
Did a specific test demonstrate the causal role of the vaccine or any of the ingredients?
Immunization error
Verified and found to be
Was there an error in prescribing or non-adherence to recommendations for use of the vaccine
correct
(e.g. use beyond the expiry date, wrong recipient etc.)?
Was the vaccine (or any of its ingredients) administered unsterile?
Verified and found to be
correct
Was the vaccine's physical condition (e.g. colour, turbidity, presence of foreign substances
Verified and found to be
correct
etc.) abnormal at the time of administration?
Was there an error in vaccine constitution/preparation by the vaccinator (e.g. wrong product,
Verified and found to be
correct
wrong diluent, improper mixing, improper syringe filling etc.)?
Was there an error in vaccine handling (e.g. a break in the cold chain during transport, storage
Verified and found to be
correct
and/or immunization session etc.)?
Was the vaccine administered incorrectly (e.g. wrong dose, site or route of administration;
Verified and found to be
correct
wrong needle size etc.)?
Immunization anxiety
Could the event have been caused by anxiety about the immunization (e.g. vasovagal,
Anxiety cannot cause
Meningoencephalitis
hyperventilation or stress-related disorder)?
II (time). If “yes” to any question in II, was the event within the time window of increased risk?
Did the event occur within an appropriate time window after vaccine administration?
III. Is there strong evidence against a causal association?
Unknown – hasn’t been
Is there strong evidence against a causal association?
studied
IV. Other qualifying factors for classification
Yes – several causes of
meningoencephalitis in
Could the event occur independently of vaccination (background rate)?
infants
Yes – could be one of
several different
Could the event be a manifestation of another health condition?
infections
Did a comparable event occur after a previous dose of a similar vaccine?
Was there exposure to a potential risk factor or toxin prior to the event?
Was there acute illness prior to the event?
Did the event occur in the past independently of vaccination?
Was the patient taking any medication prior to vaccination?
Unknown – not reported
Is there a biological plausibility that the vaccine could cause the event?
so far in literature
34
Step 3: Algorithm
Review all steps and check all the appropriate boxes
Notes for step 3: I A: Because PCR positive for herpes simplex virus. IV C: because several causes of
meningoencephalitis in infants. Could be one of several different infections.
Step 4: Classification
Check all boxes that apply
S ummarize the classification logic:
W ith available evidence, we could conclude that the classification is
inconsistent (coincidental) because:
There is a clear alternative explanation for the meningoencephalitis (herpes simplex virus confirmed).
35
Example 2: OPV and acute flaccid paralysis
MA, a male child, was born on 29 December 2006 to a farmer couple in a polio endemic
country. On 1 July 2009, he suddenly developed inability to use the left upper limb. This
was reported by the local health worker to the medical officer on the same day and was
investigated on 2 July 2009.
The medical officer obtained the details of the present illness from the parents. MA had a
sudden onset of flaccid paralysis in the left arm on 1 July 2009. On the day of paralysis,
there was no fever. The paralysis was static (neither ascending nor descending). There was
no sensory loss. He did not travel outside his locality for 35 days preceding his illness.
There was no history of trauma, no loss of consciousness and no convulsions. Within 30
days prior to the paralysis onset, he had injections in the gluteal region.
MA had a BCG scar. The health worker mentioned that MA had received three doses of
OPV through routine immunization and the parents mentioned that he had over 10 doses of
OPV through mass immunization campaigns. The last OPV before paralysis onset (and
stool sample collection) was administered on 24 May 2009.
On clinical examination the medical officer observed that the tone was markedly
diminished in the left upper limb. There was power of 0/5 in the muscles of the wrist,
forearm and upper arm. The biceps, triceps and supinator jerks were diminished.
Examination also showed that all other limbs were clinically within the normal range of
expected findings. Using a measuring tape, he determined and recorded the circumference
of all the limbs.
To test for the presence of enterovirus, two stool specimens were collected on 2 July 2009
and 4 July 2009. Both specimens were of adequate volume and were sent to a WHO-
accredited laboratory in good condition (i.e. without desiccation or leakage, with adequate
documentation, and with evidence that the cold chain was maintained). The second stool
sample isolated Sabin type 1 and Sabin type 2 strains of poliovirus.
The medical officer re-examined MA on 9 September 2009 and observed that that the tone
was diminished in the left upper limb compared to the right. There was improvement in the
power in the muscles of the wrist (4/5), forearm (2/5) and upper arm (2/5). The biceps,
triceps and supinator jerks were still diminished. Examination also showed that all other
limbs were clinically within the normal range of expected findings. On measuring the limbs,
the medical officer determined that there was wasting in the left upper arm.
36
Step 1: Eligibility
Name of the
Name of one or more vaccines
What is the valid
Does the diagnosis meet
patient
administered before this event
diagnosis?
a case definition?
MA
OPV
AFP
Yes
http://www.who.int/immunization_
monitoring/diseases/poliomyelitis_
Create your question on causality here
Has the OPV vaccine/vaccination caused AFP?
(The event is for review in step 2)
Step 2: Event checklist
Y: Yes N: No UK: Unknown NA: Not applicable
Check all boxes that apply
I. Is there strong evidence for other causes? Y N UK NA Remarks
Does a clinical examination, or laboratory tests on the patient, confirm another cause?
No details available on
the other tests conducted
on this child
II. Is there a known causal association with the vaccine or vaccination?
Vaccine product(s) Is there evidence in the literature that this vaccine(s) may cause the reported event even
VAPP is a recognized
if administered correctly?
event
Did a specific test demonstrate the causal role of the vaccine or any of the ingredients?
Sabin 1 and 2 isolated
from stool
Immunization error
Was there an error in prescribing or non-adherence to recommendations for use of the vaccine
(e.g. use beyond the expiry date, wrong recipient etc)?
Was the vaccine (or any of its ingredients) administered unsterile?
Was the vaccine's physical condition (e.g. colour, turbidity, presence of foreign substances etc)
abnormal at the time of administration?
Was there an error in vaccine constitution/preparation by the vaccinator (e.g. wrong product,
wrong diluent, improper mixing, improper syringe filling etc)?
Was there an error in vaccine handling (e.g. a break in the cold chain during transport, storage
and/or immunization session etc)?
Was the vaccine administered incorrectly (e.g. wrong dose, site or route of administration;
wrong needle size etc)?
Immunization anxiety Could the event have been caused by anxiety about the immunization (e.g. vasovagal,
hyperventilation or stress-related disorder)?
II (time). If “yes” to any question in II, was the event within the time window of increased risk?
Did the event occur within an appropriate time window after vaccine administration?
Within 4–40 days
III. Is there strong evidence against a causal association?
Is there strong evidence against a causal association?
IV. Other qualifying factors for classification
Could the event occur independently of vaccination (background rate)?
There are many causes for AFP
Child received injection 30 days prior; there must
Could the event be a manifestation of another health condition?
have been an illness
Did a comparable event occur after a previous dose of a similar vaccine?
Intramuscular injections within 30 days of
immunization with oral poliovirus vaccine – a risk
factor for vaccine-associated paralytic
Was there exposure to a potential risk factor or toxin prior to the event?
poliomyelitis
Was there acute illness prior to the event?
Inadequate information
Did the event occur in the past independently of vaccination?
Was the patient taking any medication prior to vaccination?
Is there a biological plausibility that the vaccine could cause the event?
OPV can cause AFP
37
Step 3: Algorithm
Review all steps and check all the appropriate boxes
Notes for step 3: II A: With available information, it seems likely that the vaccine caused the event. This is because OPV
is known to cause the event and the time window is suitable. IV C: There are other causes of flaccid paralysis and the
child was treated for an illness 30 days prior to paralysis; however, this information is inadequate.
Step 4: Classification
Check all boxes that apply
Summarize the classification logic:
With available evidence, we could conclude that the classification is consistent because:
With available information, it seems likely that the vaccine caused the event. (But we need to keep in mind that VAPP
is more likely to occur after the first dose than after later doses.)
However, even though the trend is consistent we cannot completely rule out inconsistent, since information
available on other causes is inadequate.
38
Example 3: AEFI after MMR vaccine
XX, a South Asian girl child was born on 1December 2010 through LSCS (gestational age 38 weeks + 2
days). She was the first child to the parents. Birth weight was 3200g and Apgar at birth was 10.
On 22 May 2012 (at 18 months) between 9.30 and 10 a.m. she received 0.5ml MMR vaccine in the left arm
with a 25nm 23G needle. She died 10 days after immunization.
She was not on any simultaneous medication. She had no antenatal complications, she had no food allergies,
and her feeding and activities were normal. She had no history of hospitalization, no underlying congenital or
acquired diseases or disorders, and no evidence of abuse, harm, neglect, accidental injury or previous need for
child protection.
Previously she had the following immunizations: Penta (DTP Hep B and Hib) 1/OPV 1 on 9 August 2011,
Penta 2/OPV 2 on 25 October 2011, and JE on 10 January 2012.
Prior to immunization, her feeding and activity were normal. She had an attack of fever one week prior which
resolved. She was not receiving any medication at the time of vaccination.
After immunization with MMR, she developed mild fever on the same day (22 May 2012). On the third day
after immunization (25 May 2012), she developed cough, high fever, vomiting and flushed face. On day 8
after immunization (30 May 2012), she was admitted to the local district hospital where tentative diagnosis of
lower respiratory tract infection was made. Full blood examination showed that the initial WBC count was
3800 and platelets 152 000. The prescribed medications included Paracetamol, chlorpheneramine maleate,
Cefaloxine, Salbutamol, Theophyllin, and Diclofenac sodium suppository.
She was later transferred to the district general hospital on 30 May 2012. The next day she developed fever,
right hypochondrial tenderness and tenderness of the liver (1cm). Although she was haemodynamically stable,
her WBC was 1300 and platelets 112 000. The condition was diagnosed as probable dengue illness. She
further developed watery diarrhoea and convulsions and was treated for acute gastroenteritis with IV
antibiotics and IV fluids. In the evening, the platelet count dropped from 112 000 (at 5:00 a.m.) to 77 000 (at
5:00 p.m.). Clinicians considered probable entry into the critical phase of dengue haemorrhagic fever, even
though evidence of haemorrhages was not detected. At 8.00 p.m., there was a further drop in platelet count to
54 000 which clinicians considered as entry into the critical phase with haemodynamic instability (HR- >200;
systolic BP – 60mmHg). She was then placed on IV fluids over six hours, exceeding the fluid quota (1330 ml
given – 90.5%).
On day 10 following immunization, she was transferred to the intensive care unit. Her heart rate remained
high and she continued to be haemodynamically unstable, with pupils wide, tachypnoea, peripheral cyanosis
and fluid overload. She died at 9 a.m. on 1 June 2012.
Diagnosis of dengue illness was considered but no objective confirmation of dengue haemorrhagic fever was
made (ultrasound, chest X-ray or virological examination). The primary cause of death was considered to be
both prolonged shock and fluid overload. Her body was sent for autopsy.
No written autopsy report was available. The case (at the time of writing this report) was awaiting the
pathological report. The medical officer who performed the autopsy unofficially communicated to the
immunization programme manager that the appearance was compatible with a viral infection; however, there
was no macroscopic evidence of bleeding or fluid leakage.
Field investigation by the immunization programme
Investigation on vaccine cold chain and vaccination technique at the Ministry of Health showed that the
MMR vaccine, Batch number 065004 and expiry date February 2014 was given. It was manufactured by the
manufacturer xyz. There was no breakdown in the cold chain after receipt of the stocks of vaccine at national
level according to the daily temperature record. The VVM status was stage 1.
Further investigation showed that, of the 30 other children vaccinated on the same day at the same clinic,
three were vaccinated with the same vaccine and there were no similar events.
39
Option 1 − MMR and thrombocytopenia
Step 1: Eligibility
Name of the
Name of one or more vaccines
What is the valid
Does the diagnosis meet
patient
administered before this event
diagnosis?
a case definition?
XX
MMR
Thrombocytopenia
Yes (Brighton level 2)
Create your question on causality here
Has the ___MMR____ vaccine / vaccination caused Thrombocytopenia resulting in death?
Step 2: Event checklist
Check (check) all boxes that apply
Y: Yes N: No UK: Unknown NA: Not applicable
I. Is there strong evidence for other causes? Y N UK NA Remarks
Does a clinical examination, or laboratory tests on the patient, confirm another
With platelet count Liver enlarged,
cause?
Dengue can affect liver. TWBC the tests
may support dengue as a dx but don’t
confirm it
II. Is there a known causal association with the vaccine or vaccination?
Vaccine Product(s) Is there evidence in the literature that this vaccine(s) may cause the reported event even if
Measles vaccine can cause
administered correctly?
thrombocytopenia but not
severe enough for death
Thrombocytopenia is a sign
Did a specific test demonstrate the causal role of the vaccine or any of the ingredients?
but not proof of causality
Immunization error Was there an error in prescribing or non-adherence to recommendations for use of the vaccine
Verified and found correct
(e.g. use beyond the expiry date, wrong recipient etc.)?
Was the vaccine (or any of its ingredients) administered unsterile?
Verified and found correct
Was the vaccine's physical condition (e.g. colour, turbidity, presence of foreign substances
etc.) abnormal at the time of administration?
Was there an error in vaccine constitution or preparation by the vaccinator (e.g. wrong product,
Verified and found correct
wrong diluent, improper mixing, improper syringe filling etc.)?
Was there an error in vaccine handling (e.g. a break in the cold chain during transport, storage
Verified and found correct
and/or immunization session etc.)?
Was the vaccine administered incorrectly (e.g. wrong dose, site or route of administration;
Verified and found correct
wrong needle size etc.)?
Immunization anxiety Could the event have been caused by anxiety about the immunization (e.g. vasovagal,
hyperventilation or stress-related disorder)?
II (time). If “yes” to any question in II, was the event within the time window of increased risk?
Did the event occur within an appropriate time window after vaccine
Within time (0−
42 days) for measles
administration?
vaccine to cause thrombocytopenia
III. Is there strong evidence against a causal association?
Is there strong evidence against a causal association?
Autopsy report compatible with viral
infection. No evidence of bleeding
IV. Other qualifying factors for classification
Could the event occur independently of vaccination (background rate)?
Dengue endemic country
Other viral infection (H/o prior febrile
Could the event be a manifestation of another health condition?
illness +, present illness - fever, flushing
cough and vomiting)
Did a comparable event occur after a previous dose of a similar vaccine?
Was there exposure to a potential risk factor or toxin prior to the event?
H/o fever, unsure if there was an exposure.
Fever week prior to event – unclear
Was there acute illness prior to the event?
etiology
Did the event occur in the past independently of vaccination?
Was the patient taking any medication prior to vaccination?
Measles vaccine and thrombocytopenia but
Is there a biological plausibility that the vaccine could cause the event?
no evidence of bleeding
40
Step 3: Algorithm
Review all steps and check all the appropriate boxes
Notes for step 3: II A: Because measles vaccine can cause thrombocytopenia. The time window fits. Measles vaccine is associated
with thrombocytopenia, but is not severe enough to cause death by bleeding. There is no evidence for bleeding on autopsy.
IVB: Because other viral infection (H/o prior febrile illness +,present illness - fever, flushing cough and vomiting). IVC: Because we
need to consider other viral infections (dengue cannot be ruled out).
Step 4: Classification
Check all boxes that apply
S ummarize the classification logic:
W ith available evidence, we could conclude that the classification could be
indeterminate / inconsistent because:
With
available evidence it is not possible to come to a conclusion as to whether the thrombocytopenia was caused
by the vaccine, by dengue or by another viral disease. However, there is no evidence of bleeding on autopsy.
Therefore, even if the MMR contributed to thromb 4
o 1
cy topenia, it did not contribute to death.
EXAMPLE 3: Option 2 − MMR and sepsis
Step 1: Eligibility
Name of the
Name of one or more vaccines
What is the valid
Does the diagnosis meet
patient
administered before this event
diagnosis?
a case definition?
Yes weblink:
XX
MMR
Sepsis
http://www.bmj.com/content/
335/7625/879
Create your question on causality here
Has the ___ MMR ____ vaccine / vaccination caused ___sepsis resulting in death ___?
(The event is for review in step 2)
Step 2: Event checklist
Y: Yes N: No UK: Unknown NA: Not applicable
Check all boxes that apply
I. Is there strong evidence for other causes? Y N UK NA Remarks
Does a clinical examination, or laboratory tests on the patient, confirm
Dengue is suspected but not confirmed.
another cause?
(Platelets , TWBC , liver ++) IgM
and virus isolation not done
II. Is there a known causal association with the vaccine or vaccination?
Vaccine Product(s) Is there evidence in the literature that this vaccine(s) may cause the reported event even if
administered correctly?
Did a specific test demonstrate the causal role of the vaccine or any of the ingredients?
Immunization error
Verified and found
Was there an error in prescribing or non-adherence to recommendations for use of the vaccine
correct
(e.g. use beyond the expiry date, wrong recipient etc.)?
Verified and found
Was the vaccine (or any of its ingredients) administered unsterile?
correct
Was the vaccine's physical condition (e.g. colour, turbidity, presence of foreign substances
etc.) abnormal at the time of administration?
Was there an error in vaccine constitution or preparation by the vaccinator (e.g. wrong product,
Verified and found
correct
wrong diluent, improper mixing, improper syringe filling etc.)?
Was there an error in vaccine handling (e.g. a break in the cold chain during transport, storage
Verified and found
correct
and/or immunization session etc.)?
Was the vaccine administered incorrectly (e.g. wrong dose, site or route of administration;
Verified and found
correct
wrong needle size etc.)?
Immunization anxiety Could the event have been caused by anxiety about the immunization (e.g. vasovagal,
hyperventilation or stress-related disorder)?
II (time). If “yes” to any question in II, was the event within the time window of increased risk?
Did the event occur within an appropriate time window after vaccine administration?
III. Is there strong evidence against a causal association?
Is there strong evidence against a causal association?
No evidence of bacterial
sepsis
IV. Other qualifying factors for classification
In this situation, it is possible that sepsis could be a
Could the event occur independently of vaccination (background rate)?
complication of the respiratory tract infection
Could the event be a manifestation of another health condition?
Other infections, probably dengue (unconfirmed)
Did a comparable event occur after a previous dose of a similar vaccine?
Was there exposure to a potential risk factor or toxin prior to the event?
Was there acute illness prior to the event?
Fever one week prior to immunization
Did the event occur in the past independently of vaccination?
Was the patient taking any medication prior to vaccination?
Is there a biological plausibility that the vaccine could cause the event?
42
Step 3: Algorithm
Review all steps and check all the appropriate boxes
Notes for step 3:
IV C: In this situation, it is possible that sepsis causing death is a complication of the respiratory tract infection. IV B: Other infections,
probably dengue (unconfirmed). Fever one week prior to immunization.
Step 4: Classification
Check all boxes that apply
S ummarize the classification logic:
W ith available evidence, we could conclude that the classification is
indeterminate / inconsistent because:
The sepsis
th at caused the chain of events lea
ding to the death of the child could have been due to a complication of
respiratory tract infection or other viral disease (dengue suspected). The autopsy findings will give a better
picture. We cannot rule out that the viraemia which is of low level with MMR may have made the viral
infection already present worse. MMR is not the c 4
a 3
us e of death
Technical assistance for AEFI causality assessment is available from the World Health
Organization through the Essential Medicines and Health Products (EMP) Department.
Additional information on AEFI surveillance, investigation, management and causality
assessment, as well as on vaccine safety communication, can be found online at
http://www.who.int/immunization_safety/en/.
You can also contact us at
Quality, Safety and Standards (QSS)
Essential Medicines and Health Products (EMP) Department
World Health Organization
20 avenue Appia
1211 Geneva 27
Switzerland
Tel: +41 22 791 4468
Fax: +41 22 791 4227
E-mail: [email address]
44
Causality assessment of an adverse
event following immunization (AEFI)
User manual for the revised WHO classifi cation
Department of Essential Medicines and Health Products (EMP)
Health Systems and Innovation (HIS)
World Health Organization
20, Avenue Appia CH-1211 Geneva 27, Switzerland
E-mail: [email address]
Web site: http://www.who.int/vaccine_safety/en/
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