COVID-19(6) investigated(4) outcome(4) male(4) Pfizer/BioNTech(4) median age(4) first vaccination(4) collected(4) vaccination(4) of BNT162b2(4) IgG antibody(4) Japan(4) Older(4) blood sample(4) decrease(4) BNT162b2 vaccine(4) peaked(4) immunization(4) female(4) IgG antibody titer(4) statistical significance(4) significantly lower(4) subsequent(4) hospital(4) SARS-CoV-2 spike protein(4) BNT162b2 vaccination(4) second vaccination(4) participant(4) performed(4) immune response(4)