General information only. This Parent Resource summarizes WHO information on vaccine-safety testing and surveillance.
How is vaccine safety assessed before introduction?
WHO explains that vaccines undergo multiple phases of clinical testing and that health authorities review safety and efficacy evidence before a vaccine is considered suitable for use.
Does safety monitoring stop after introduction?
No. WHO states that national health authorities continue monitoring vaccine safety after vaccines are introduced so that possible safety concerns can be detected and reviewed.
What is an adverse event following immunization?
WHO defines an adverse event following immunization as an unexpected medical issue occurring after vaccination. The timing alone does not establish that the vaccine caused the event.
Who reviews possible safety signals?
WHO supports national safety systems and receives reports from countries. At global level, the independent Global Advisory Committee on Vaccine Safety evaluates evidence and provides scientific advice to WHO.
“After vaccination” does not automatically mean “caused by vaccination.” Causality assessment considers other possible explanations and available evidence.
References
- World Health Organization. Vaccines and immunization: Vaccine safety, 23 September 2025.