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The Vaccine Adverse Events Reporting

System: Understanding the Data



January 16, 2025

In general, vaccines undergo rigorous testing for safety through clinical trials before they are licensed
(i.e., approved) or authorized for use by the Food and Drug Administration (FDA). These trials, which
can involve thousands of patients, are designed to identify any major safety concerns with the vaccines.
FDA  licenses vaccines if they are determined safe for their intended use. Once a vaccine is on the market,
federal agencies monitor vaccine safety through complementary systems such as the Vaccine Adverse
Events Reporting System (VAERS) to help identify any rare or new safety issues that occur.
VAERS   is a monitoring system for reported adverse events following vaccination jointly operated by
FDA  and the Centers for Disease Control and Prevention (CDC). An adverse event is any health
problem that happens after a vaccination. Using the VAERS system, anyone-including clinicians and the
general public-can submit a report of an adverse event following vaccination. Under federal laws and
regulations, health care providers and vaccine manufacturers are required to report certain adverse events
that occur following vaccination. For Coronavirus Disease 2019 (COVID-19) vaccines, there have been
additional VAERS reporting requirements through FDA and CDC that changed over time.
CDC  makes data on initial VAERS reports publicly available through its WONDER database. This CRS
Insight provides an overview of how to understand publicly available VAERS data, its limitations, and the
evidence usually required to determine whether a vaccine caused or contributed to a certain adverse
health event.

Understanding VAERS Data
CDC  and FDA  monitor VAERS  reports and use the information to determine whether to conduct further
studies and investigations on the reported cases. VAERS serves as an early warning system to help detect
possible new safety issues with vaccines, providing some of the earliest information on potential safety
issues with newly introduced vaccines.
VAERS   is a passive reporting system, meaning it relies on individuals to send reports. VAERS is not
designed to determine if a vaccine caused a specific adverse health event. Instead, VAERS is meant for
detecting unusual or unexpected patterns of adverse event reporting that might indicate a potential safety
problem with a vaccine that warrants further investigation. CDC and FDA can follow up on reported

                                                                 Congressional Research Service
                                                                   https://crsreports.congress.gov
                                                                                       IN12486

CRS INSIGHT
Prepared for Members and
Committees of Congress