Objectives: Many countries are administering a third dose of some coronavirus disease 2019 (COVID-19) vaccines, but the evaluation of vaccine-induced immunity after boosting in East Asia is insufficient. This study aimed to evaluate anti-spike immunoglobulin G [IgG(S)] titers after the third BNT162b2 vaccination. Methods: The dynamics of IgG(S) titers were assessed two months following the third BNT162b2 vaccination in 52 participants. All participants received the primary series of vaccination with BNT162b2 and received the third dose eight months after the second vaccination. Associations among the IgG(S) titer, baseline characteristics, and adverse reactions were also evaluated. Results: The geometric mean titer of IgG(S) one month after the third vaccination was 17,400 AU/ml, which increased by approximately 30 times that immediately before the third vaccination. The rate of IgG(S) titer decline was significantly slower after the third vaccination (35.7%) than after the second vaccination (59.1%). The IgG(S) titer was significantly negatively associated with age (r = −0.31). Participants who had a headache at the time of vaccination showed significantly higher IgG(S) titers than those without a headache. Conclusions: The IgG(S) titer induced by primary immunization with BNT162b2 waned over time. The third dose of BNT162b2 substantially increased the IgG(S) titer, with a slower rate of decline.
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