The Vaccine Adverse Event Reporting System (VAERS) is a valuable tool for post-marketing safety surveillance (monitoring after a product has been approved and is on the market). Although extensive studies are required for licensure of new vaccines, post-marketing research and surveillance are necessary to identify safety issues that may only be detected following vaccination of a much larger and more diverse population. Rare events may not come to light before licensure. Sometimes an event is noted, but the evidence may not be adequate to conclude that a noted event is due to the vaccine.
VAERS is a program created as an outgrowth of the National Childhood Vaccine Injury Act of 1986 (NCVIA) and is administered by the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC). VAERS accepts reports of adverse events that may be associated with U.S. licensed vaccines from health care providers, manufacturers, and the public. The FDA continually monitors VAERS reports for any unexpected patterns or changes in rates of adverse events.
The report of an adverse event to VAERS is not proof that a vaccine caused the event. More than 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 risk for a variety of medical events and serious childhood illnesses. These naturally occurring events include fevers, seizures, sudden infant death syndrome (SIDS), cancer, congenital heart disease, asthma, and other conditions. Some infants coincidentally experience an adverse event shortly after a vaccination. In such situations an infection, congenital abnormality, injury, or some other provocation may cause the event. Because of such coincidences, it is usually not possible from VAERS data alone to determine whether a particular adverse event resulted from a concurrent condition or from a vaccination - even when the event occurs soon after vaccination. Doctors and other vaccine providers are encouraged to report adverse events, whether or not they believe that the vaccination was the cause. If the VAERS data suggest a possible link between an adverse event and vaccination, the relationship may be further studied in a controlled fashion.
Analyzing VAERS reports is a complex task. Children are often administered more than one vaccine at a time, making it difficult to know which vaccine, if any, may have contributed to any subsequent adverse events. While about 85% of adverse events reported to VAERS are minor (such as mild fevers or redness and swelling at the injection site), the remaining 15% describe more serious events. The more serious events include hospitalizations, life-threatening events, and deaths. As part of the VAERS program, FDA reviews the deaths and serious reports weekly, and conducts follow up. In some cases, certain vaccines and potentially associated symptoms will receive more intense follow up.
In addition to analyzing individual VAERS reports, the FDA also analyzes patterns of reporting associated with vaccine lots. Many complex factors must be considered in comparing reports between different vaccine lots. More reports may be received for a large lot than for 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 for a lot that has been in use for a short time. Even among lots of similar size and time in use, some lots will receive more reports than others will simply due to chance. The FDA continually looks for lots that have received more death reports than would be expected on the basis of such factors as time in use and chance variation as well as any unusual patterns in other serious reports within a lot. If such a lot is detected, further review is conducted to determine if the lot continues to be safe for use, or if additional FDA actions are needed.