COVID-19(353) SARS-CoV-2(278) Patient(182) coronavirus(124) coronavirus disease(121) acute respiratory syndrome(116) Japan(104) SARS-CoV-2.(94) positive(80) conducted(76) Treatment(72) acute respiratory syndrome coronavirus(71) Coronavirus disease 2019(70) severe acute respiratory syndrome Coronavirus(68) vaccination(67) Infection(66) age(65) acute respiratory syndrome coronavirus 2(63) hospital(62) dose(60) evaluated(59) variant(59) Japanese(59) reported(58) antibody(55) investigated(54) analyzed(54) respiratory(53) IgG(52) collected(51)