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• Antibody response after BNT162b2 vaccination over 600 Japanese cohorts were investigated. • Sufficient antibody titer was observed among all the analyzed participants. • Younger age, female sex and adverse reactions were related to higher titer. • Adverse reactions after the second dose might reflect acquisition of the immunity. SARS-CoV-2 vaccination has started worldwide, including Japan. Although high rates of vaccine response and adverse reactions of BNT162b2 vaccine have been reported, knowledge about the relationship between sex differences and antibody response is limited. Furthermore, it is uncertain whether adverse reactions are associated with the vaccine response. This prospective observational study included 673 Japanese participants working in a medical school and its affiliated hospital in Tokyo, Japan (UMIN000043340). Serum samples were collected before the first dose and three weeks after the second dose of BNT162b2 vaccine, and antibody titers against the receptor-binding domain of the spike protein of SARS-CoV-2 were measured. Answers to questionnaires about background characteristics and adverse reactions were obtained at the time of sample collection, and the relationship between antibody titers was analyzed. After excluding participants who did not complete receiving two doses of vaccination or two series of serum sample collection, 646 participants were analyzed. Although all participants became sero-positive after vaccination, antibody titers were highly variable among individuals (260.9–57,399.7A U/mL), with a median titer of 13478.0AU/mL. Mean titer was higher in females than in males and higher in young (≤45 years old) participants than in aged (>45 years old) participants. Participants who experienced adverse reactions demonstrated a higher antibody titer after vaccination than those without adverse reactions. Multivariable analysis demonstrated that young age, female sex, and adverse reactions after the second dose were independently related to higher antibody titers after the second dose. A favorable antibody response was observed after two doses of BNT162b2 vaccination among mostly healthy Japanese participants, especially among female and young participants. Although further investigation is essential, our results imply that the systemic adverse reactions (i.e., fever and general fatigue) are associated with a higher antibody response that indicates the acquisition of humoral immunity. [ABSTRACT FROM AUTHOR]

Background: The antibody titer is known to wane within months after receiving two doses of the Pfizer-BioNTech BNT162b2 mRNA SARS-CoV-2 vaccine. However, knowledge of the cellular immune response dynamics following vaccination is limited. This study to aimed to determine antibody and cellular immune responses following vaccination, and the incidence and determinants of breakthrough infection., Methods: This prospective cohort study a 6-month follow-up period was conducted among Japanese healthcare workers. All participants received two doses of BNT162b2 vaccine. Anti-SARS-CoV-2 antibody titers and T-cell immune responses were measured in serum samples collected at several timepoints before and after vaccination., Results: A total of 608 participants were included in the analysis. Antibody titers were elevated 3 weeks after vaccination and waned over the remainder of the study period. T-cell immune responses showed similar dynamics. Six participants without predisposing medical conditions seroconverted from negative to positive on the IgG assay for nucleocapsid proteins, indicating breakthrough SARS-CoV-2 infection. Five of the six breakthrough infections were asymptomatic., Conclusions: Both humoral and cellular immunity waned within 6 months after BNT162b2 vaccination. The incidence of asymptomatic breakthrough infection within 6 months after vaccination was approximately one percent., Umin Clinical Trials Registry Id: UMIN000043340., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: [Naoki Hasegawa reports financial support was provided by the Japan Agency for Medical Research and Development (AMED). Yoshifumi Uwamino reports financial support was provided by the Public Foundation of the Vaccination Research Center, Japan. Mitsuru Murata reports equipment, drugs, or supplies was provided by MBL Corporation. Mitsuru Murata reports equipment, drugs, or supplies was provided by Sysmex Corp. Mitsuru Murata reports equipment, drugs, or supplies was provided by Qiagen. Yoshifumi Uwamino, Mitsuru Murata, Masatoshi Wakui has patent pending to Keio University]., (Copyright © 2022 Elsevier Ltd. All rights reserved.)