This is a summary, written by members of the CITF Secretariat, of:
Nasreen S, Febriani Y, Velásquez García HA, Zhang G, Tadrous M, Buchan SA, Righolt CH, Mahmoud SM, Janjua NZ, Krajden M, De Serres G, Kwong JC. Effectiveness of COVID-19 vaccines against hospitalization and death in Canada: A multiprovincial test-negative design study. medRxiv.  2022.04.13.22273825; doi: results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

CITF-funded researchers Drs. Jeff Kwong from the University of Toronto, Gaston de Serres from l’Institut national de santé publique du Québec, and Mel Krajden of the University of British Columbia, found that two doses of mRNA or viral vector (Astra Zeneca) vaccines provide excellent protection against severe outcomes (hospitalization or death) from COVID-19. In a pre-print, not yet peer-reviewed, the study included more than 2.5 million community-dwelling adults in Ontario, Quebec, British Columbia, and Manitoba, and covered the period before the emergence of the Omicron variant.

During the study period (December 2020-September 2021), there were 31,776 COVID-19 hospitalizations and 5,842 deaths in the provinces for which data were included.

Key findings

  • Vaccine effectiveness for both types of vaccine was 83% (95% CI- 78%-87%) against hospitalization and 83% (95% CI- 72-90%) against death after a first dose, and 98% after a second dose, against both hospitalization and death.
  • Against severe outcomes (hospitalization or death), effectiveness was 87% at 84 days after the first dose of an mRNA vaccine, increasing to 98% 112 days after a second dose.
  • Vaccine effectiveness from the viral vector vaccine (ChAdOx1) against severe outcomes was 88% at 56 days after a first dose, increasing to 97% at 56 days after a second dose.
  • One dose was less effective against severe outcomes in adults aged ≥80 years and those with comorbidities, but effectiveness became comparable after a second dose.
  • Two doses of vaccines provided very high protection for both homologous (two doses of the same vaccine) and heterologous (mix-and-match vaccination) dosage, and against Alpha, Gamma, and Delta variants.
  • Vaccine effectiveness was maintained at ≥97% with longer dose intervals from 56 days through ≥112 days after receiving a second dose.

The results from this large multi-provincial study provide strong evidence of excellent protection against severe outcomes (hospitalizations and deaths) with two doses of COVID-19 vaccines during the pre-Omicron period. The findings further support the interchangeability of homologous and heterologous vaccine schedules. The results from the current study may not apply to outcomes caused by Omicron.