Association of a third vaccination with antibody levels and side reactions in essential workers: A prospective cohort study

OBJECTIVE: To evaluate the relationship between the change of titer and adverse events after the third vaccination for COVID-19 among healthcare workers. Design and setting: This was a prospective cohort study, and the follow-up period was from December 2021 to November 2023. Participants: A total of 392 healthcare workers aged over 20 years who worked at the facility and wished to have vaccine antibody titers measured participated in this study. Exposures: A third dose of BNT162b2 COVID-19 vaccine was administered to healthcare workers working at the hospital, and we evaluated the changes in antibody titers before and after the vaccine, as well as adverse reactions after vaccination. Main Outcomes and Measures: The primary endpoints were adverse reactions within 7 days after the third dose of COVID-19 vaccine and the rate of increase in COVID-19 vaccine antibody titer at 4 weeks. RESULTS: A total of 392 people participated in the study, of whom 358 participants had their antibody titers measured before and after the booster vaccination. The overall IgG geometric mean was 609 U/mL (561–663) before booster vaccination and increased to 18,735 U/mL (17,509–20,049) at 4 weeks after vaccination (p < 0.001). Multivariate analysis showed no statistically significant relationship between the primary endpoints, such as a change in antibody titer due to the presence of fever after vaccination or a change in antibody titer due to swelling at the vaccination site. Factors affecting the rate of increase in antibody titer, evaluated as secondary endpoints, were suggested to be age (1.02 (95 % confidence interval (CI): 1.01–1.03)) and hypertension (0.66 (95 % CI: 0.47–0.93)). CONCLUSIONS AND RELEVANCE: In relation to the booster effect of the third dose of COVID-19 vaccination, there was no statistically significant difference in the presence of fever or use of antipyretic or other drugs.