• Ryu Kyung Kim, Young June Choe, Eun Jung Jang, Chungman Chae, Ji Hae Hwang, Kil Hun Lee, Ji Ae Shim, Geun-Yong Kwon, Jae Young Lee, Young-Joon Park, Sang Won Lee, and Donghyok Kwon.
• Korea Disease Control and Prevention Agency, Cheongju, Korea.
• J. Korean Med. Sci. 2023 Nov 27; 38 (46): e396e396.

BackgroundThis retrospective observational matched-cohort study of 2,151,216 individuals from the Korean coronavirus disease 2019 (COVID-19) vaccine effectiveness cohort aimed to evaluate the comparative effectiveness of the COVID-19 bivalent versus monovalent vaccines in providing additional protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, critical infection, and death in Korea.MethodsAmong individuals, those vaccinated with COVID-19 bivalent vaccines were matched in a 1:1 ratio with those who were vaccinated with monovalent vaccines (bivalent vaccines non-recipients) during the observation period. We fitted a time-dependent Cox proportional-hazards model to estimate hazard ratios (HRs) of COVID-19 outcomes for infection, critical infection, and death, and we defined vaccine effectiveness (VE) as 1-HR.ResultsCompared with the bivalent vaccination group, the incidence proportions in the monovalent vaccination group were approximately three times higher for infection, nine times higher for critical infection, and 11 times higher for death. In the early stage of bivalent vaccination, relative VE of bivalent vaccine against monovalent vaccine was 42.4% against SARS-CoV-2 infection, 81.3% against critical infection, and 85.3% against death. In addition, VE against critical infection and death according to the elapsed period after bivalent vaccination was maintained at > 70%.ConclusionThe bivalent booster dose provided additional protection against SARS-CoV-2 infections, critical infections, and deaths during the omicron variant phase of the COVID-19 pandemic.© 2023 The Korean Academy of Medical Sciences.

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