Changes in serum antibody titers after vaccination for COVID-19 and evaluation of post-vaccination health conditions
Introduction: The coronavirus disease 2019 (COVID-19) vaccine has preventive effects and high immunogenicity, but the outcomes of vaccination have not been widely reported. The goal of this study was to examine serum antibody titers before and after vaccination and to evaluate post-vaccination health conditions. Methods: The subjects were 2,304 medical workers (mean age 41 years) at Kawasaki Gakuen who agreed to participate in the study and underwent COVID-19 vaccination, beginning in March 2021. Serum IgG antibody titers for SARS-CoV-2 spike protein were measured before the first vaccination and 4 weeks after the second vaccination. Health conditions were observed for 4 weeks after the second vaccination. Results: The rates of seroconversion, seroprotection, and change in geometric mean antibody titer at 4 weeks after the second vaccination were 99.9%, 99.9%, and 2,685.5 (95% CI 587.8-5,319.2), respectively, suggesting high immunogenicity. After the first vaccination, pain, enlargement, and reddening occurred at the local injection site, and systemic side effects included fatigue, headache, physical pain, chill, nausea, and fever. After the second vaccination, the incidence of pain decreased, but those of other events increased. There were no serious side effects requiring hospitalization. In logistic regression analysis, sex, age, fever, chill, and lymph node enlargement after the second vaccination were associated with a change in antibody titer. Conclusions: Serum antibody titers suggested high immunogenicity of the COVID-19 vaccine and a health condition survey confirmed the safety of the vaccine. Systemic side effects may serve as an index of immunization (acquisition of antibody) by the vaccine.