Distribution and Characteristics of Vaccine Adverse Events: Analysis of Over 2 Million Individual Case Safety Reports

Objectives: To describe the distribution and characteristics of adverse events following immunization (AEFI) using a large US pharmacovigilance database. Methods: A retrospective descriptive analysis of Individual Case Safety Reports (ICSRs) from the FDA AEMS database (January 2017-March 2026) was conducted. Reports were analyzed by vaccine type, adverse event terms, demographics, reporting year, and reporter category. Results: A total of 2 169 603 ICSRs were identified, with 44.0% classified as serious. COVID-19 vaccines accounted for 78.4% of reports, reflecting mass vaccination campaigns. The most frequently reported events were headache (12.8%), pyrexia (11.8%), and fatigue (11.1%), predominantly non-serious and consistent with known reactogenicity profiles. Reporting peaked in 2021 (48.8%). Females accounted for 60.6% of reports. Non-COVID vaccines contributed substantially fewer reports. Conclusions: Most vaccine-associated adverse events were mild and expected. Findings support a favorable benefit–risk profile and highlight the importance of continuous pharmacovigilance systems for vaccine safety monitoring.