evidence: Modern vaccines are safe and effective; however, adverse events have been reported after administration of all vaccines (3). More complete information about adverse reactions to a specific vaccine is available in the package insert for each vaccine and from CDC. An adverse event is an untoward event that occurs after a vaccination that might be caused by the vaccine product or vaccination process. These events range from common, minor, local reactions to rare, severe, allergic reactions (e.g., anaphylaxis). Reporting to VAERS helps establish trends, identify clusters of adverse events, or generate hypotheses. However, establishing evidence for cause and effect on the basis of case reports and case series alone is usually not possible, because health problems that have a temporal association with vaccination do not necessarily indicate causality.