A comprehensive overview of how vaccine safety is monitored before and after regulatory approval.
Vaccine safety doesn't end at approval. Before any vaccine reaches the public, it undergoes rigorous clinical trials involving tens of thousands of participants. But even the largest trials can't detect every possible adverse event—some rare side effects only appear when a vaccine is administered to millions of people.
That's why governments around the world maintain robust post-market surveillance systems that continuously monitor vaccine safety after rollout. These systems collect reports from healthcare providers, patients, and manufacturers, analyze data for unusual patterns, and investigate potential safety signals.
This page provides an overview of the major surveillance systems, what each does, their strengths and limitations, and how safety signals lead to regulatory action.
A national passive surveillance system co-managed by CDC and FDA. Anyone—patients, caregivers, healthcare providers—can submit a report of adverse events following vaccination. VAERS is a signal detection system, not a causation determination.
Strengths: Early detection, large population coverage, publicly accessible.
Limitations: Cannot determine causality, underreporting, inconsistent data quality.
An active surveillance system using electronic health records from participating healthcare organizations covering ~10% of the US population. Enables rapid analysis of vaccine safety using controlled comparisons.
Strengths: Active surveillance, controlled comparisons, detailed medical data.
Limitations: Limited population coverage, participating sites only.
FDA's active surveillance system using claims data and electronic health records from ~100 million patients. Focuses on regulatory decision support and comprehensive safety assessments.
Strengths: Large population coverage, rapid safety assessments, regulatory integration.
Limitations: Data may lack clinical context, claims-based.
WHO coordinates global vaccine safety monitoring through its Global AEFI Surveillance System. Countries report adverse events to WHO, enabling cross-border signal detection. WHO provides guidelines, training, and support for national surveillance systems.
The UK's MHRA runs this voluntary reporting system for adverse drug reactions, including vaccines. Healthcare professionals and patients can submit reports. Data is publicly searchable and regularly published in drug safety updates.
The European Medicines Agency operates this centralized database for adverse drug reactions across the EU. All member states report vaccine adverse events, enabling Europe-wide signal detection and coordinated regulatory response.
The CDC's active surveillance system linking vaccination records to health outcomes across 10% of the US population. Enables controlled comparisons and incidence rate calculations that passive systems cannot provide.
The FDA's Biologics Effectiveness and Safety system uses large claims databases and electronic health records from ~150 million patients to conduct rapid active surveillance of vaccines and biologics.
Surveillance systems identify unusual patterns in reported adverse events.
FDA/CDC scientists review the signal, assess biological plausibility, and check for confounding factors.
Additional studies using VSD, BEST, or other active surveillance systems to assess causation.
Label updates, warnings, contraindication changes, or in rare cases, vaccine withdrawal.
CDC. "Vaccine Adverse Event Reporting System (VAERS)." Centers for Disease Control and Prevention.
https://vaers.hhs.govCDC. "Vaccine Safety Datalink (VSD)."
https://www.cdc.gov/vaccinesafety/ensuringsafety/monitoring/vsd/index.htmlFDA. "Biologics Effectiveness and Safety (BEST) System."
https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/best-systemWHO. "Global Vaccine Safety Initiative."
https://www.who.int/vaccine_safety/initiative/en/UK MHRA. "Yellow Card Scheme."
https://www.gov.uk/report-reaction-medicineEMA. "EudraVigilance."
https://www.ema.europa.eu/en/human-regulatory/overview/pharmacovigilance/eudravigilance