Vaccines protect many people from dangerous illnesses, but vaccines, like drugs, can cause side effects, a small percentage of which may be serious. Vaccine Adverse Event Reporting System (VAERS) is used to continually monitor reports to determine whether any vaccine or vaccine lot has a higher than expected rate of events.
Each year the VAERS receives 10,000–20,000 reports of adverse events following immunization by more than 10 million vaccines. Higher-priority uses of the data include reports of death and other serious adverse events, recognizing and detecting adverse effects, and finding unexpected adverse events involving new vaccines. The VAERS data are also used to monitor known reactions to vaccines and for vaccine lot surveillance. Data mining techniques such as empirical Bayes methods can be used to improve the quality of data analysis.