Causality · individual cases

Was this event caused by the vaccine? The WHO AEFI causality method

Population studies ask whether a vaccine can cause an event. The WHO method asks whether it did, in one person, using a fixed sequence of questions so that different reviewers reach the same answer.

This tool teaches the method; it does not adjudicate real cases. A real assessment is done by a committee with the full medical record, laboratory results and the latest evidence. Answer the questions for a hypothetical or published case to see how the classification follows.

Step 1 — Eligibility

Has the vaccine (product, dose and date) been identified? Yes No

Is there a valid diagnosis that meets a case definition (e.g. Brighton Level 1–3)? Yes No

Step 2 — Checklist

I. Is there strong evidence for another cause (another illness, drug, injury or condition) that could fully explain the event? Yes No Unknown

II. Is there a known causal association with the vaccine or vaccination? Yes — with the vaccine product itself (published evidence, e.g. febrile seizure after MMRV)
Yes — with a quality defect of this lot (contamination, failed potency)
Yes — with an immunization error (wrong route, dose, storage, recipient)
Yes — with immunization anxiety (e.g. fainting, hyperventilation)
No known association Not established / unknown

III. Did the event occur within the time window of increased risk for that association? Yes No Unknown / no window defined

IV. Is there strong evidence against a causal association (e.g. high-quality studies showing no increased risk)? Yes No

Do the available facts point in conflicting directions (some consistent with the vaccine, some with another cause)? Yes No

Answer the questions to see the classification.

The classification

A Consistent causal association with immunization

A1 vaccine product–related reaction
A2 vaccine quality defect–related
A3 immunization error–related
A4 immunization anxiety–related

B Indeterminate

B1 timing consistent, but insufficient definitive evidence that the vaccine caused it (may be a new signal)
B2 qualifying factors point to conflicting trends of consistency and inconsistency

C Inconsistent causal association / coincidental

An underlying or emerging condition, or a condition caused by exposure to something other than the vaccine.

U Unclassifiable

Not enough information to classify; the specific missing information is listed so the case can be reassessed.

How this fits with the rest of the site

Case definition first: eligibility needs a valid diagnosis — see how many VAERS reports document one on the Brighton analysis.
Checklist item II ("known causal association") is answered by controlled studies — see vaccine safety studies — not by report counts.
Population vs individual: the Bradford Hill assessments ask whether a product can cause an event at all; this algorithm asks whether it did in a given case. How a signal becomes a finding shows the whole chain.

Source: World Health Organization. Causality assessment of an adverse event following immunization (AEFI): user manual for the revised WHO classification, 2nd edition. Geneva: WHO; 2018 (ISBN 978-92-4-151365-4). This page simplifies the published algorithm; consult the manual for the full checklist. Reviewed 2026-10-09. Not medical advice.