Effectiveness of Messenger RNA-based Vaccines During the Emergence of the Severe Acute Respiratory Syndrome Coronavirus 2 Omicron Variant.

BACKGROUND: We evaluated the effectiveness of mRNA-based vaccines following emergence of SARS-CoV-2 Omicron variant. METHODS: Recipients of a third dose of BNT162b2 or mRNA-1273 ≥180 days after the primary series were matched to primary-series recipients and unvaccinated persons. Participants were followed from 1 December 2021 to 12 March 2022. Outcomes were documented SARS-CoV-2 infection, COVID-19 hospitalization, and COVID-19 death. Effectiveness was calculated from 100-day risks estimated with the Kaplan-Meier estimator. RESULTS: BNT162b2 and mRNA-1273 groups included 221 267 and 187 507 third-dose recipients, respectively, matched to equal numbers of primary-series recipients and unvaccinated persons. Compared with no vaccination, effectiveness of a third dose of BNT162b2 was 47.8% (95% confidence interval [CI], 45.2-50.3), 81.8% (95% CI, 79.2-84.2), and 89.6% (95% CI, 85.0-93.6) against infection, hospitalization, and death, respectively. Effectiveness of a third dose of BNT162b2 compared with the primary series was 30.1% (95% CI, 26.2-33.7), 61.4% (95% CI, 55.0-67.1), and 78.8% (95% CI, 67.9-87.5) against infection, hospitalization, and death, respectively. Effectiveness of a third dose of mRNA-1273 compared with no vaccination was 61.9% (95% CI, 59.4-64.4), 87.9% (95% CI, 85.3-90.2), and 91.4% (95% CI, 86.4-95.6) against infection, hospitalization, and death, respectively. Effectiveness of a third dose of mRNA-1273 compared with the primary series was 37.1% (95% CI, 32.2-41.7), 63.5% (95% CI, 53.7-71.6), and 75.0% (95% CI, 55.4-88.0) against infection, hospitalization, and death, respectively. CONCLUSIONS: BNT162b2 and mRNA-1273 were effective against COVID-19 following emergence of Omicron variant. A third dose provided additional protection over the primary series.