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, Mahmud SM, Janjua NZ, Krajden M, De Serres G, Kwong JC, Canadian Immunization Research Network (CIRN) Provincial Collaborative Network (PCN) Investigators. Effectiveness of COVID-19 vaccines against hospitalization and death in Canada: A multiprovincial test-negative design study, Clinical Infectious Diseases, 2022; ciac634, doi: https://doi.org/10.1093/cid/ciac634.

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

Published in Clinical Infectious Diseases, CITF-funded researchers Drs. Jeff Kwong (University of Toronto), Gaston de Serres (L’Institut national de santé publique du Québec), and Mel Krajden (University of British Columbia), found that two doses of mRNA or viral vector ChAdOx1 (AstraZeneca’s Vaxzevria or COVIDSHIELD) vaccines provided excellent protection against severe outcomes (hospitalization or death) from COVID-19 during the period before the emergence of the Omicron variant.

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. Both types of vaccine were 98% effective against both hospitalization and death after a second dose.
  • Against severe outcomes (hospitalization or death), the pooled vaccine 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 against severe outcomes from the viral vector vaccine (ChAdOx1) was 88% at 56 days after a first dose, increasing to 97% at 56 days after a second dose.
  • One dose of any vaccine was less effective against severe outcomes in adults aged ≥80 years and those with comorbidities, but effectiveness became comparable after a second dose.
  • The pooled vaccine effectiveness against severe outcomes in individuals with comorbidities was >80% after 14 days of the first dose of either Pfizer, Moderna or ChAdOx1 vaccine, which increased to >97% seven days after the second dose.
  • Two doses of vaccine 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 of concern (VOCs).
  • Vaccine effectiveness against severe outcomes caused by VOCs was lowest against Beta (61%) and highest against Delta at 89% 14 days after the first dose, which increased to >97% against Alpha, Gamma, and Delta seven days after the second dose.
  • 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 – including more than 2.5 million community-dwelling adults in Ontario, Quebec, British Columbia, and Manitoba -provided strong evidence of excellent protection against severe outcomes (hospitalizations and deaths) with two doses of COVID-19 vaccine during the pre-Omicron period. The findings further supported the interchangeability of homologous and heterologous vaccine schedules. 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. These results may not apply to outcomes caused by the Omicron variant.