Protection by Vaccines and Previous Infection Against the Omicron Variant of Severe Acute Respiratory Syndrome Coronavirus 2

Effectiveness of vaccines and postinfection immunity against the Omicron variant of SARS-CoV-2 is significantly lower than against the Delta variant. The probability of a severe outcome is substantially lower for the Omicron variant compared to Delta. 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.