Vaccination

Adverse Events Monitoring in Vaccines

2025-08-26

Adverse Events Monitoring in Vaccines

Intro

Adverse events monitoring refers to the systems and processes used to track side effects or unexpected outcomes following vaccination. These surveillance programs are critical for ensuring vaccine safety in real-world use. While clinical trials identify common side effects, rare or very delayed effects may only appear once millions of people have been vaccinated.

Key Points

• Definition: An adverse event following immunisation (AEFI) is any medical occurrence after a vaccine, which may or may not be causally related.
• Purpose: To detect rare or unexpected reactions not seen in clinical trials.
• Global systems:
  • VAERS (US) and EudraVigilance (EU): passive reporting databases.
  • VSD (US), PRISM (US), and AusVaxSafety (Australia): active monitoring systems.
  • ATAGI (Australia): expert advisory group that incorporates safety findings into policy.
• Most reports are mild (fever, soreness), but monitoring ensures early detection of very rare but serious risks.

Background

Every licensed vaccine undergoes rigorous safety trials before approval. However, trials may include tens of thousands of people — not millions. Rare side effects (1 in 100,000 or 1 in 1,000,000) often can’t be detected until vaccines are widely used. To fill this gap, governments established post-marketing surveillance systems, some passive (relying on reports submitted voluntarily) and some active (proactively contacting recipients or mining health records).

Mechanisms

United States

• VAERS (Vaccine Adverse Event Reporting System):

  • Established 1990; open to anyone (patients, families, clinicians).
  • Strength: sensitive early warning system.
  • Limitation: reports are unverified and can’t prove causality.

• Vaccine Safety Datalink (VSD):

  • Partnership between CDC and large healthcare organizations.
  • Uses electronic health records to actively monitor outcomes.

• PRISM (Post-Licensure Rapid Immunization Safety Monitoring):

  • Part of FDA’s Sentinel Initiative.
  • Monitors health insurance databases covering tens of millions.

Europe

• EudraVigilance (EMA):
  • Collects adverse event reports for drugs and vaccines across the EU.
  • Reports come from healthcare professionals, patients, and manufacturers.
  • Used to detect safety signals and inform EMA regulatory actions.

Australia

• AusVaxSafety:

  • Active, real-time monitoring since 2014.
  • Sends SMS surveys after vaccination to track side effects.
  • Provides near-real-time safety updates to health authorities.

• ATAGI (Australian Technical Advisory Group on Immunisation):

  • Independent expert body that advises the Minister for Health.
  • Uses AusVaxSafety and other data to update vaccine recommendations.

Risks / Benefits

• Benefits: Detects very rare side effects (e.g., Guillain–Barré syndrome, myocarditis, thrombosis after adenoviral COVID vaccines). Provides transparency and builds trust.
• Risks/Limitations: Passive systems can be misinterpreted (anyone can submit a report). A report does not prove causation — it only raises a signal for investigation. Active systems are more robust but require infrastructure.

FAQ

Q: What’s the difference between an adverse event and a side effect?
A: A side effect is a known, expected reaction caused by the vaccine (e.g., fever). An adverse event is any health issue after vaccination, whether or not the vaccine caused it.

Q: Does a VAERS or AusVaxSafety report mean a vaccine caused harm?
A: No. These systems are signal detectors. Each report must be evaluated for causality.

Q: Why not rely only on clinical trials?
A: Trials are too small to detect extremely rare events. Post-marketing surveillance fills this gap.

Further Reading

• CDC: VAERS
• FDA: Sentinel Initiative
• EMA: EudraVigilance
• AusVaxSafety: Vaccine Safety Monitoring in Australia
• ATAGI – Australian Technical Advisory Group on Immunisation

Related Guides

• /guides/vaccines-and-autism
• /guides/aluminum-in-vaccines

• #safety
• #monitoring
• #policy