What is a vaccine safety signal?
A safety signal is information suggesting a possible new risk, or a change in a known risk, that warrants further investigation.
Short answer
Where can a signal come from?
Possible signals may arise from spontaneous reports such as Yellow Cards, clinical trials, healthcare databases, observational studies, published case reports or safety information from other countries.
What happens next?
Investigators examine the timing and pattern of reports, how many people were exposed, expected background rates, alternative explanations, biological plausibility and findings from other studies. A regulator may seek more data, commission analyses, update product information or conclude that the available evidence does not support a causal relationship.
Signal, association and confirmed risk
A signal identifies something to examine. An association means two things occur together more or less often than expected. A confirmed risk requires a stronger assessment of whether the evidence supports causation.
Sources and further reading
WHO — causality assessment of adverse events following immunisation
Educational scope
General information only. This page does not assess an individual report, diagnose symptoms or replace advice from a suitably qualified healthcare professional.