Vaccine Adverse Event Reporting System (VAERS)
The FDA and CDC support a voluntary reporting system for vaccine adverse events, named Vaccine Adverse Event Reporting System (VAERS). A subset of population adverse events are reported to VAERS. The number of adverse events in a population can be estimated by comparing the reports in VAERS to the reports collected in clinical trials. The proportion of reports is expressed as under-reporting factor (URF). Adverse events are expected to occur within a population at background frequency rates, usually expressed as number of cases per 100,000 individuals per year. Post immunization, the reports to VAERS are predicted to be skewed towards higher reports immediately following immunization with decreased likelihood of reporting as time increases – this is referred to as reporting bias.
Measles, Mumps, and Rubella vaccine
In 1998, Wakefield et al. [Lancet 1998;351:637-41] published with subsequent retractions a flawed article linking the measles, mumps, and rubella (MMR) vaccine may predispose to behavioral regression and developmental disorder in children.