Vaccine Adverse Event Report System (VAERS)

What is the Vaccine Adverse Event Reporting System (VAERS)?

The Vaccine Adverse Event Reporting System (VAERS) was created by the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) to receive reports about adverse events which 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. The FDA continually monitors reports to determine whether any vaccine or vaccine lot has a higher than expected rate of events.

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

The report of an adverse event to VAERS is not a documentation that a vaccine caused the event. With regard to childhood vaccines, over ten million vaccinations per year are given to children less than one year old, usually between 2 months and 6 months of age. At this stage of development, infants are at greatest risk for certain medical events, including high fevers, seizures, and sudden infant death syndrome (SIDS). Some infants will by coincidence experience such an event shortly after a vaccination. In such situations, the event may be caused by an infection, congenital abnormality, injury, or some other provocation. Because of such coincidences, it is usually not possible to be sure whether a particular adverse event resulted from a concurrent condition or from a vaccination, even when it occurred soon afterward. Therefore, doctors and other vaccine providers are encouraged to report adverse events, whether or not they believe 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, we examine the medical information about the event, and obtain more specific information from the reporting doctors whenever necessary.

We also analyze patterns of reporting associated with vaccines and vaccine lots. In analyzing patterns of adverse event reporting, the FDA considers more than just the number of reports for a lot. More reports will be received for a large lot than a small one, simply because more doses of vaccine from the large lot will be given to more children. Some lots contain as many as 700,000 doses, while others as few as 20,000 doses. Similarly, more reports will be received for a lot that has been in use for a long time than a lot in use for a short time. Even among lots of similar size and time in use, some lots will receive more reports than others simply due to chance. The FDA continually looks for lots that have received more serious reports that should be expected on the basis of such factors as size, time in use, and chance variation. When such a lot is detected, further investigations are initiated to determine if the lot continues to be safe for use, or if a recall may be needed.

Many complex factors must be considered in order to decide whether a lot of vaccine is unsafe. At the FDA, we apply procedures and methods of analysis to help us to understand these complex factors and closely monitor the safety of vaccines. We continually analyze these many factors to help ensure that all lots in use are safe. We hope that this brief explanation of the factors associated with vaccines and adverse events will assist you in understanding the data you have requested.

The National Childhood Vaccine Injury Act of 1986 (NCVIA) requires health care providers and vaccine manufacturers to report to the Department of Health and Human Services (DHHS) specific adverse events following the administration of those vaccines outlined in the Act. 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 and Prevention (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 data base is to serve as an early warning or signaling system for adverse events not detected during pre-market testing.