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:
A VAERS report can be filed online or by mail, and anyone may submit one — the person who was vaccinated, a parent or caregiver, a healthcare provider, or a vaccine manufacturer. Healthcare providers are required by law to report certain events, such as those listed in the VAERS Table of Reportable Events Following Vaccination, and any adverse event listed in a vaccine's package insert as a contraindication. Manufacturers must report all adverse events brought to their attention.
Each report captures the person's age and sex, the vaccine(s) given, the date of vaccination and the date symptoms began, a free-text description of what happened, and a set of outcome checkboxes. The symptoms are then coded by trained staff using MedDRA, a standardized medical dictionary, so that similar events can be grouped and searched. Importantly, VAERS staff do not investigate or confirm reports before accepting them — the system is intentionally open so that potential signals are not filtered out at the door. This is why raw reports must always be read as unverified and why they cannot, on their own, establish that a vaccine caused an event. For more on this, see our methodology and glossary.
VAERS records several outcome flags that describe how serious a reported event was. A single report can carry more than one flag, so these categories overlap and should not be added together:
A report is classified as a serious adverse event if it involves any of death, a life-threatening condition, hospitalization, permanent disability, or a birth defect. The great majority of VAERS reports fall outside these categories and describe mild, self-limiting reactions. Even for serious reports, classification reflects what was reported — not a determination that the vaccine was responsible. Explore how outcomes differ across vaccines in our dashboard or by browsing specific symptoms.
A vaccine adverse event is any health problem that happens after vaccination. It may or may not be caused by the vaccine. VAERS collects reports of these events to look for potential safety signals.
VAERS receives approximately 30,000-60,000 reports per year in normal years. In 2021, during the COVID-19 vaccination campaign, VAERS received over 768,000 reports. The total database contains 1,983,260 reports from 1990 to present.
No. VAERS reports show correlation, not causation. Many reported events are coincidental — they would have happened regardless of vaccination. Only controlled studies can determine whether a vaccine actually causes a specific adverse event.
The most commonly reported adverse events in VAERS include headache, fever, pain at the injection site, fatigue, chills, nausea, and dizziness. These are typically mild and resolve within days.
A side effect is a health problem that has been shown to be caused by a vaccine. An adverse event is anything that happens after vaccination, whether or not the vaccine caused it. Because VAERS records adverse events by timing rather than by proven cause, most reports are not confirmed side effects.
Anyone can submit a VAERS report — patients, parents, caregivers, healthcare providers, and manufacturers. Healthcare providers are legally required to report certain serious events. This openness helps capture rare signals but means reports are unverified and vary in quality.
VAERS classifies an adverse event as serious if it results in death, a life-threatening condition, hospitalization or prolonged hospitalization, permanent disability, or a congenital anomaly/birth defect. Serious reports are a small share of the total, and being classified as serious does not confirm the vaccine as the cause.