This is a summary, written by members of the CITF Secretariat, of:

Grewal R, Buchan SA, Nguyen L, Nasreen S, Austin PC, Brown KA, Gubbay J, Lee N, Schwartz KL, Tadrous M, Wilson K, Wilson SE, Kwong JC. Effectiveness of mRNA COVID-19 monovalent and bivalent vaccine booster doses against Omicron severe outcomes among adults aged ≥50 years in Ontario, Canada. medRxiv 2023.04.11.23288403; doi:

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A CITF-funded study, published in preprint and not-yet peer-reviewed, reported that third and fourth doses of monovalent and bivalent mRNA vaccines provided similar strong protection against severe outcomes from COVID-19 among community-dwelling adults over the age of 50. However, further follow-up is necessary to determine the long-term protection of bivalent vaccines and their effectiveness against newer Omicron variants.

The study was led by Dr. Jeffrey Kwong (ICES, University of Toronto) on behalf of the Canadian Immunization Research Network’s (CIRN) Provincial Collaborative Network.

Key findings:

  • For the Moderna and Pfizer-BioNTech monovalent vaccines, vaccine effectiveness (VE) was 85% and 88%, respectively, 7-29 days after vaccination and 82% for both vaccines 90-119 days post-vaccination.
  • For the Moderna BA.1 bivalent vaccine, VE was 86% 7-29 days after vaccination and 76% 90-119 days post-vaccination.
  • For the Pfizer-BioNTech BA.4/BA.5 bivalent vaccine, VE was 83% 7-29 days after vaccination and 81% 60-89 days after vaccination.
  • VE was slightly lower against the BQ subvariant than against the BA.4/BA.5 subvariant:
  • VE for the Moderna BA.1 bivalent vaccine and Pfizer-BioNTech BA.4/BA.5 bivalent vaccine was 93% and 87%, respectively, 30-59 days after vaccination during the BA.4/BA.5-predominant period.
  • VE was 82% for both vaccines during the BQ-predominant period.

The study included 3,755 Omicron cases and 14,338 test-negative controls. VE was evaluated with unvaccinated adults as the comparator group against hospitalization or death among adults aged ≥50 years tested for SARS-CoV-2 between June 19, 2022, and January 28, 2023. Approximately 57% of cases and 69% of controls had their fourth dose as their most recent dose.