BACKGROUND: The Omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) evades immunity conferred by vaccines and previous infections. METHODS: We used a Cox proportional hazards model and a logistic regression on individual-level population-wide data from the Czech Republic to estimate risks of infection and hospitalization, including severe states. RESULTS: A recent (≤2 months) full vaccination reached vaccine effectiveness (VE) of 43% (95% confidence interval [CI], 42%-44%) against infection by Omicron compared to 73% (95% CI, 72%-74%) against Delta. A recent booster increased VE to 56% (95% CI, 55%-56%) against Omicron infection compared to 90% (95% CI, 90%-91%) for Delta. The VE against Omicron hospitalization of a recent full vaccination was 45% (95% 95% CI, 29%-57%), with a recent booster 87% (95% CI, 84%-88%). The VE against the need for oxygen therapy due to Omicron was 57% (95% CI, 32%-72%) for recent vaccination, 90% (95% CI, 87%-92%) for a recent booster. Postinfection protection against Omicron hospitalization declined from 68% (95% CI, 68%-69%) at ≤6 months to 13% (95% CI, 11%-14%) at >6 months after a previous infection. The odds ratios for Omicron relative to Delta were 0.36 (95% CI, .34-.38) for hospitalization, 0.24 (95% CI, .22-.26) for oxygen, and 0.24 (95% CI, .21-.28) for intensive care unit admission. CONCLUSIONS: Recent vaccination still brings substantial protection against severe outcome for Omicron.

1st Faculty of Medicine Charles University Prague Czech Republic

Biology Centre Institute of Entomology Department of Ecology Czech Academy of Sciences Budjovice Czech Republic

Centre for Mathematical Biology Institute of Mathematics Faculty of Science University of South Bohemia Budjovice Czech Republic

Centre for Modelling of Biological and Social Processes Prague Czech Republic

Department of Biochemistry Cell and Molecular Biology 3rd Faculty of Medicine Charles University Prague Czech Republic

Department of Data Analysis Institute of Health Information and Statistics of the Czech Republic Prague Czech Republic

Department of Mathematics and Statistics Faculty of Science Masaryk University Brno Czech Republic

Department of Statistical Modelling Czech Academy of Sciences Institute of Computer Science Prague Czech Republic

Institute of Biostatistics and Analyses Faculty of Medicine Masaryk University Brno Czech Republic

Institute of Information Theory and Automation Czech Academy of Sciences Prague Czech Republic

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J Infect Dis. 2022 Sep 13;226(5):938-940. doi: 10.1093/infdis/jiac256. PubMed

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J Infect Dis. 2022 Sep 13;226(5):941. doi: 10.1093/infdis/jiac258. PubMed

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J Infect Dis. 2023 Apr 18;227(8):1021-1022. doi: 10.1093/infdis/jiac429. PubMed

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