Vaccine Adverse Event Reporting System (VAERS)

Technical Notes



(revised Oct 2003)


1. Important Information About the Vaccine Adverse Event Reporting System (VAERS)

VAERS was created by the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) to receive reports about adverse events that may be associated with vaccines. No prescription drug or biological product, such as a vaccine, is completely free from side effects. Vaccines protect many people from dangerous illnesses, but vaccines, like drugs, can cause side effects, a small percentage of which may be serious. VAERS is used to continually monitor reports to determine whether any vaccine or vaccine lot has a higher than expected rate of events.

Doctors and other vaccine providers are encouraged to report adverse events, even if they are not certain that the vaccination was the cause. Since it is difficult to distinguish a coincidental event from one truly caused by a vaccine, the VAERS database will contain events of both types.

In addition, it is often the case that more than one vaccine was administered, making it difficult to know to which of the vaccines the event might be attributed. In analyzing individual reports, researchers examine the medical information about the event, and obtain more specific information from the reporters whenever necessary. Patterns of reporting associated with vaccines and vaccine lots are also analyzed.

About 85% of vaccine adverse event reports concern relatively minor events, such as fevers or redness and swelling at the injection site. The remaining 15% describe serious events, such as hospitalizations, life-threatening illnesses, or deaths. The reports of serious events are of greatest concern and receive the most careful scrutiny by VAERS staff.

VAERS researchers apply procedures and methods of analysis to help us closely monitor the safety of vaccines. When a concern arises, action is taken. We hope that this brief explanation of the factors associated with vaccines and adverse events will assist you in understanding the data you are viewing.

Requests for additional information should be addressed to:

Freedom of Information Staff, HFI-35
Food and Drug Administration
5600 Fishers Lane
Rockville, MD 20857

2. Brief Description of VAERS

The U.S. Department of Health and Human Services (DHHS) established the Vaccine Adverse Event Reporting System (VAERS), which is co-administered by the Food and Drug Administration (FDA) and the Centers for Disease Control (CDC), to accept all reports of suspected adverse events, in all age groups, after the administration of any U.S. licensed vaccine. On November 1, 1990 VAERS replaced CDC's Monitoring System for Adverse Events Following Immunization (MSAEFI) for public sector reporting and FDA's Spontaneous Reporting System for private sector and manufacturer reporting. The primary purpose for maintaining the database is to serve as an early warning or signaling system for adverse events not detected during pre-market testing. In addition, the National Childhood Vaccine Injury Act of 1986 (NCVIA) requires health care providers and vaccine manufacturers to report to the DHHS specific adverse events following the administration of those vaccines outlined in the Act.

All reports are coded and entered into the VAERS database. The adverse events described in each report are coded utilizing the FDA's Coding Symbols for a Thesaurus of Adverse Reaction Terms (COSTART) which are key words representing the medical condition(s) described in the case report. An individual report in these files may include up to a total of 8 vaccines administered and 20 COSTART terms describing the event.

3. VAERS Data Should be Interpreted with Caution

  • VAERS data are from a passive surveillance system and represent unverified reports of health events that occur after vaccination. Such data are subject to limitations of under-reporting, simultaneous administration of multiple vaccine antigens, reporting bias, and lack of incidence rates in unvaccinated comparison groups.
  • When reporting and evaluating data from VAERS, it is important to note that for any reported event, no cause and effect relationship has been established. The event may have been related to an underlying disease or condition, to drugs being taken concurrently, or may have occurred by chance shortly after a vaccine was administered.
  • A report often involves more than one vaccine and may involve more than one reported adverse event.
  • A given report may meet more than one criterion for classification as "serious."
  • Accumulations of events reported to a passive surveillance system do not allow incidence rate calculations due to the generally unknown extent of under-reporting as well as lack of information on the number, age, and gender of people being vaccinated.

Source: accessed on April 25, 2006.

This page last reviewed: Tuesday, November 19, 2019
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