Important: VAERS reports alone cannot determine if a vaccine caused an adverse event. Reports may contain incomplete, inaccurate, or unverified information. Correlation does not equal causation.
A vaccine adverse event is any health problem that occurs after vaccination. The Vaccine Adverse Event Reporting System (VAERS) has collected 1,983,260 reports of adverse events across 104 vaccines since 1990. This page explains what adverse events are, how they're tracked, and how to explore the data.
A vaccine adverse event (also called an adverse event following immunization, or AEFI) is any health problem that happens after someone receives a vaccine. This includes everything from common reactions like soreness at the injection site to rare serious events like anaphylaxis.
Importantly, an adverse event is defined by timing, not causation. If you get a headache the day after a flu shot, that's an adverse event — even though millions of people get headaches every day regardless of vaccination. The key challenge in vaccine safety science is separating true vaccine-caused events from coincidental ones.
In the United States, vaccine adverse events are monitored through several complementary systems:
VAERS is the most publicly accessible of these systems, which is why it's frequently cited in public discourse. However, it has significant limitations — most importantly, reports don't prove causation.
Adverse events fall on a spectrum from common and mild to rare and serious:
Expected reactions showing immune response. Usually resolve in 1-3 days.
May require medical attention but typically resolve fully.
Well-documented but extremely rare. Often 1-5 per million doses.
The 1,983,260 reports in VAERS span 35 years of vaccination history. Key context: