How vaccine safety monitoring works
Vaccine safety is not assessed once and then forgotten. Monitoring starts during development and continues after a vaccine enters routine use.
1. Clinical trials
Before authorisation, vaccines pass through multiple stages of clinical testing. Regulators evaluate safety, immune responses and effectiveness data before deciding whether a vaccine can be supplied.
2. Post-authorisation surveillance
Clinical trials cannot realistically detect every extremely rare event. Once millions of people receive a vaccine, national and international surveillance systems can identify unusual patterns that were too uncommon to appear during trials.
3. Signal detection
A safety signal is a reason to investigate, not a conclusion. Signals can arise from spontaneous reports, healthcare databases, research studies or international monitoring. Investigators then ask whether the event occurs more often than expected and whether other explanations fit the data.
4. Regulatory action
If evidence supports a new risk, regulators may change patient information, alter recommendations, restrict use, issue warnings or take other action. Monitoring therefore continues throughout a vaccine's use.
Primary sources
Overview of testing, monitoring and investigation of vaccine safety.
The UK system for reporting suspected adverse reactions to medicines and vaccines.