Effectiveness of mRNA COVID-19 monovalent and bivalent vaccine booster doses against Omicron severe outcomes among adults aged >=50 years in Ontario, Canada

ObjectiveWe estimated the effectiveness of booster doses of monovalent and bivalent mRNA COVID-19 vaccines against Omicron-associated severe outcomes among adults aged [≥]50 years in Ontario, Canada. MethodsWe used a test-negative design to estimate vaccine effectiveness (VE), with unvaccinated adults as the comparator, against hospitalization or death among SARS-CoV-2-tested adults aged [≥]50 years between June 19, 2022 and January 28, 2023 stratified by time since vaccination. We explored VE by vaccine product (Moderna Spikevax(R) monovalent; Pfizer-BioNTech Comirnaty(R) monovalent; Moderna Spikevax(R) BA.1 bivalent; Pfizer-BioNTech Comirnaty(R) BA.4/BA.5 bivalent). ResultsWe included 3,755 Omicron cases and 14,338 test-negative controls. For the Moderna and Pfizer-BioNTech monovalent vaccines, VE 7-29 days after vaccination was 85% (95% confidence interval [CI], 72-92%) and 88% (95%CI, 82-92%), respectively, and was 82% (95%CI, 76-87%) and 82% (95%CI, 77-86%) 90-119 days after vaccination. For the Moderna BA.1 bivalent vaccine, VE was 86% (95%CI, 82-90%) 7-29 days after vaccination and was 76% (95%CI, 66-83%) 90-119 days after vaccination. For the Pfizer-BioNTech BA.4/BA.5 bivalent vaccine, VE 7-29 days after vaccination was 83% (95%CI, 77-88%) and was 81% (95%CI 72-87%) 60-89 days after vaccination. ConclusionsBooster doses of monovalent and bivalent mRNA COVID-19 vaccines provided similar, strong initial protection against severe outcomes in community-dwelling adults aged [≥]50 years in Ontario. Nonetheless, uncertainty remains around waning protection of these vaccines.