Assessment of antibody titer and side effects after third doses of COVID-19 mRNA vaccination in healthy volunteers

Objectives: Third dose of SARS-CoV-2 vaccination was started from December 1, 2021 in Japan. However, data on the precise analysis of the side effects after third vaccination, remain scarce. Here, we examined the side effects and the levels of SARS-CoV-2 IgG antibody in healthy volunteers who underwent BNT162b2 vaccination. Methods: Forty-one healthy volunteers were assessed for the side effects of the vaccination for the third dose, and samples were used for the measurement of SARS-CoV-2 IgG antibody with chemiluminescent assays against the Receptor Binding Domain (RBD) of the virus. Results: We analyzed the humoral responses and found that the IgG levels showed clear declining trends with age. Commonly reported side effects in the participants after the third dose were similar to those in second dose, such as, generalized weakness/fatigue (65.9%), headache (58.5%), and sore arm/pain (87.8%). The frequency of the fever was slightly less (39.0%), compared to the second dose (57.5%), but localized symptoms, such as itching (14.6%) and lymphadenopathy (14.6%) were not negligible, which were not seen at the second dose. The number of side effects were tended to be decreased with age. Conclusions: The production of IgG after the third doses of BNT162b2 vaccination decreases age-dependently. The number of side effects were tended to be decreased with age. The high frequencies of generalized weakness/fatigue, fever, and sore arm/pain were not negligible, after the third dose.