ABSTRACT
Objectives: This study investigated the reporting rates of adverse events following immunization (AEFIs) to the spontaneous reporting system (SRS) and its predictors among individuals with AEFIs after coronavirus disease 2019 (COVID-19) vaccination. Methods: A cross-sectional, web-based survey was conducted from December 2 to 20, 2021, recruiting participants >14 days after completion of a primary COVID-19 vaccination series. Reporting rates were calculated by dividing the number of participants who reported AEFIs to the SRS by the total number of participants who experienced AEFIs. We estimated adjusted odds ratios (AORs) using multivariate logistic regression to determine factors associated with spontaneous AEFI reporting. Results: Among 2,993 participants, 90.9% and 88.7% experienced AEFIs after the first and second vaccine doses, respectively (reporting rates, 11.6% and 12.7%). Furthermore, 3.3% and 4.2% suffered moderate to severe AEFIs, respectively (reporting rates, 50.5% and 50.0%). Spontaneous reporting was more prevalent in women (AOR=1.54; 95% CI, 1.31-1.81); those with moderate to severe AEFIs (AOR=5.47; 95% CI, 4.45-6.73), comorbidities (AOR=1.31; 95% CI, 1.09-1.57), a history of severe allergic reactions (AOR=2.02; 95% CI, 1.47-2.77); and those who had received mRNA-1273 (AOR=1.25; 95% CI, 1.05-1.49) or ChAdOx1 (AOR=1.62; 95% CI, 1.15-2.30) vaccines versus BNT162b2. Reporting was less likely in older individuals (AOR=0.98; 95% CI, 0.98-0.99 per 1-year age increment). Conclusion: Spontaneous reporting of AEFIs after COVID-19 vaccination was associated with younger age, female sex, moderate to severe AEFIs, comorbidities, history of allergic reactions, and vaccine type. AEFI under-reporting should be considered when delivering information to the community and in public health decision-making.