The information collected into the VAERS database comes from a wide array of sources, including patients and parents, state health agencies, pharmacies, health care providers, and the makers of the vaccines. Although complete information greatly enhances the analysis of vaccine safety, in order to capture as many reports as possible, reports with less complete information will be included in the database.
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.
A balance must be struck in reducing the burden on voluntary reporters while providing enough data to produce necessary "signal flags" to trigger analysis of potential significant events and trends.
The incidence rate (how often events occur) cannot be calculated from VAERS data, because not all events are reported and the exact number of administered doses cannot be known.
VAERS data are from a passive surveillance system and represent unverified reports of adverse events temporally associated with one or more vaccines. 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.