Adverse Event Reporting and Pharmacovigilance

Vaccine pharmacovigilance is the science of detecting, assessing, and responding to adverse events following immunization once a vaccine is in use. Because pre-licensure trials cannot detect very rare events or effects in subgroups not studied, post-marketing surveillance — through spontaneous reports and active monitoring — is essential to maintaining vaccine safety across whole populations.

Definition

Vaccine pharmacovigilance is the set of activities for detecting, reporting, assessing, and preventing adverse events following immunization, combining spontaneous reporting, active surveillance, standardized case definitions, signal detection, and causality assessment.

Scope

This entry covers how adverse events following immunization (AEFI) are reported and analyzed: passive (spontaneous) reporting systems, active surveillance in linked databases, standardized case definitions that make reports comparable, signal detection, and the causality assessment that interprets them. It treats reporting and surveillance as a methodological topic and not as clinical guidance.

Core questions

• How are adverse events following immunization reported and captured after licensure?
• What is the difference between passive and active surveillance?
• How do standardized case definitions improve the comparability of reports?
• How is a safety signal detected and then investigated?
• What are the strengths and limitations of spontaneous reporting data?

Key concepts

• Adverse event following immunization (AEFI)
• Spontaneous (passive) reporting
• Active surveillance
• Safety signal
• Brighton Collaboration case definitions
• Causality assessment
• Background (expected) incidence rate
• Underreporting and reporting bias

Mechanisms

Pharmacovigilance combines complementary methods. Passive systems such as spontaneous reporting collect reports from clinicians and the public; they are inexpensive and broad but subject to underreporting, stimulated reporting, and incomplete data, so they generate hypotheses rather than rates. Active surveillance in linked health databases provides denominators and comparison groups, allowing observed event rates to be set against expected background rates. Standardized case definitions, developed by the Brighton Collaboration, make events comparable across systems and studies. When a disproportionate pattern suggests a signal, it is investigated with controlled epidemiologic designs and formal causality assessment.

Clinical relevance

Pharmacovigilance is the mechanism by which rare or delayed vaccine risks are identified after a product reaches the public, informing labeling, contraindications, and program decisions. Clinicians contribute by reporting suspected events. This entry describes how surveillance evidence is generated and interpreted at the population level and is not a basis for individual clinical decisions.

Epidemiology

Spontaneous reporting captures large numbers of events but cannot by itself establish incidence because the number of doses administered and the level of underreporting are uncertain. Reliable risk estimates therefore depend on active surveillance with defined populations and on comparison with background rates. Standardized definitions and pooled international data improve the power to detect rare events.

Evidence & guidelines

International frameworks, including the CIOMS/WHO working-group terminology for vaccine pharmacovigilance and the Brighton Collaboration case definitions, provide the standardized vocabulary and criteria used in surveillance. Large evidence reviews, such as the Institute of Medicine's causality assessments, illustrate how accumulated surveillance and epidemiologic data are synthesized to judge specific vaccine-event relationships.

History

Spontaneous reporting systems for vaccines were established as mass immunization expanded and the need to catch post-licensure events became clear. The Brighton Collaboration, founded in 2000, introduced standardized AEFI case definitions, and CIOMS/WHO later harmonized the terminology of vaccine pharmacovigilance, moving the field from heterogeneous national reporting toward comparable international surveillance.

Debates

Can spontaneous reporting estimate risk?

Spontaneous systems are powerful for hypothesis generation but cannot reliably estimate incidence because of underreporting and unknown denominators; whether and how to quantify risk from such data, versus relying on active surveillance, is a recurring methodological question.

Key figures

• Robert Chen
• Jan Bonhoeffer
• Tom Shimabukuro

Seminal works

• bonhoeffer-2002
• shimabukuro-2015

Frequently asked questions

Why can't spontaneous report counts tell us how risky a vaccine is?

Spontaneous reporting captures suspected events but not the total number of people vaccinated or the proportion of events reported, so it lacks the denominator and completeness needed to calculate true rates. It is best used to detect possible signals that are then studied with controlled designs.

What is a safety signal?

A safety signal is information suggesting a new or changing possible association between a vaccine and an event that warrants further investigation. A signal is a hypothesis to be tested, not proof of causation.