Effectiveness of mRNA COVID-19 Monovalent and Bivalent Vaccine Booster Doses Against Omicron Severe Outcomes Among Adults Aged 50 Years in Ontario, Canada: A Canadian Immunization Research Network Study

We 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. Monovalent and bivalent mRNA COVID-19 booster doses provided similar strong initial protection against severe outcomes. Uncertainty remains around waning of protection from these vaccines. Shortly after bivalent mRNA COVID-19 vaccines were introduced, monovalent and bivalent booster doses provided similar strong initial protection against severe outcomes among Ontario community-dwelling adults aged >= 50 years. Uncertainty remains around waning of protection.