Abstract
BACKGROUND: Vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are crucial for ending the pandemic of coronavirus disease 2019 (COVID-19). Currently, the cumulative effect of booster shots of mRNA vaccines on adverse events is not sufficiently characterized. METHODS: A survey-based study on vaccine adverse events was conducted in a Japanese medical institute after the third dose of Pfizer BNT162b2. Adverse events were grouped using network analysis, and a heteroscedastic probit model was built to analyse adverse events. RESULTS: There were two main clusters of adverse events, systemic and local injection site-associated events. Subject background and the experience of previous vaccine-related adverse events were variably associated with the occurrence and intensity of adverse events following the third dose. Among adverse events, only lymphadenopathy increased prominently following the third dose, while the largest increase in other systemic adverse events occurred generally following the second dose. CONCLUSIONS: The effect of repeated booster vaccines on the frequency and intensity of adverse events differs depending on the kind of adverse event.