IntroductionIn addition to the original monovalent vaccines available for SARS-CoV-2, bivalent vaccines covering wild-type (WT) and Omicron BA.1 are also available. However, there is a lack of real-world data on the effectiveness of bivalent vaccines as second boosters on the dominant Omicron sublineages, including BA.2 and BA.5. MethodsThis prospective longitudinal cohort study was conducted at Toyama University Hospital, a tertiary medical center in Japan. Participants (n = 565) who received the first booster vaccination were followed up until 2 weeks after the second booster dose of the monovalent mRNA-1273 (WT group, n = 168) and bivalent BNT162b2 (WT+BA.1 group, n = 23) vaccines. Participants with previous SARS-CoV-2 infections were excluded from the study. Anti-receptor-binding domain (RBD) antibody levels and neutralizing activity were measured. Vaccine-related symptoms were also assessed using a questionnaire after the second booster dose. ResultsThe anti-RBD antibody levels after the second booster dose in the WT and WT+BA.1 group were similar (median [inter quartile], 26262.0 [16951.0-38137.0] U/mL vs. 24840.0 [14828.0-41460.0] U/mL, respectively). Although the neutralization activity of the pooled sera of the WT+BA.1 group was the lowest against BA.5, the activities against BA.2 and BA.5 were higher than those of the WT group in both pseudotyped and live virus assays. Vaccine-related symptoms, including systemic and local symptoms, were strongly correlated with anti-RBD antibody levels and neutralizing titers with significant differences. ConclusionThe second booster dose of the bivalent (WT/Omicron BA.1) vaccine induced higher neutralizing activity against BA.2 and BA.5 than that of the original monovalent vaccine.