To evaluate reporting sensitivities for vaccine adverse events, reporting rates were estimated by dividing the number of events reported to the Monitoring System for Adverse Events Following Immunization and the Vaccine Adverse Event Reporting System in a given period by the number of doses administered or distributed during the same period. Reporting sensitivity was calculated as the ratio of the rates at which events were reported to each passive surveillance system (numerator) and occurred in controlled studies (denominator). Reporting sensitivities were generally better in the public sector than in the private sector. The significant underreporting of known outcomes, together with the nonspecific nature of most adverse event reports, highlights the limitations of passive surveillance systems in assessing the incidence of vaccine adverse events.