The Vaccine Adverse Event Reporting System (VAERS) is a cooperative program for vaccine safety of the Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA). VAERS allows coordinated and centralized monitoring of possible adverse events (possible side effects) that occur after the administration of U.S.-licensed vaccines.
Health professionals are required to report all reactions that occur after a vaccination, regardless of whether symptoms seem more likely to be related to another condition or illness. Anyone can submit a report.
Just because a reaction was reported to VAERS does not mean the vaccine caused the reaction. Careful studies are conducted if a pattern emerges that suggests potential problems with a vaccine.