AEFI Causality Assessment — The WHO Methodology

Module 6 built the WHO-UMC causality scale in depth, and it’s tempting to assume the same tool applies here — but AEFI causality assessment uses a genuinely distinct, purpose-built WHO methodology, developed specifically because vaccine causality questions have a different evidence structure than conventional drug reactions. A vaccine is typically a single, defined exposure at a known point in time, given to a generally healthy person, often compared against an established body of population-level background incidence data — a fundamentally different evidentiary situation than a drug taken repeatedly over an extended period, where dechallenge and rechallenge (Module 6’s central concepts) are genuinely meaningful.

The WHO AEFI methodology works through a structured sequence of questions rather than WHO-UMC’s four key criteria. First: does the reported case actually meet an established case definition — ideally a Brighton Collaboration definition from Lesson 14.3 — for the event in question? A case that doesn’t meet the diagnostic criteria for what it’s being called can’t proceed meaningfully through the rest of the assessment. Second: is there a consistent, established causal association between this vaccine (or vaccine class) and this specific event, based on existing evidence — biological mechanism, prior case series, published literature? Third: is there a temporal relationship plausible for the proposed biological mechanism — did the event occur within a timeframe consistent with how the vaccine would plausibly cause it, if it does? And fourth: can other, more likely causes be reasonably excluded through the available clinical information?

That sequence resolves into four final classification categories, deliberately worded differently from WHO-UMC’s drug-focused categories to avoid conflating the two systems: Consistent with causal association, where the evidence supports a genuine link; Indeterminate, where the evidence is insufficient to reach a conclusion either way; Inconsistent with causal association, where the evidence points away from a causal link; and Unclassifiable, reserved for cases where critical information needed for any of the above is simply missing. Notice the deliberate parallel to, and equally deliberate difference from, WHO-UMC’s six categories — the underlying discipline (structured, criteria-based reasoning rather than intuition) is the same principle Module 6 built, applied through a tool designed for this specific evidence structure.

Given the public-health stakes Lesson 14.1 introduced, AEFI causality assessment — particularly for serious or AESI-flagged events — typically isn’t performed by a single reviewer working alone the way an individual drug case might be. It commonly goes through a formally constituted causality assessment committee, often including clinicians, epidemiologists, and sometimes independent experts, precisely because the conclusion can carry consequences for an entire immunization programme’s public standing, not just for the individual case record.