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

Skowronski DM, Setayeshgar S, Febriani Y, Ouakki M, Zou M, Talbot D, Prystajecky N, Tyson JR, Gilca R, Brousseau N, Deceuninck G. Two-dose SARS-CoV-2 vaccine effectiveness with mixed schedules and extended dosing intervals: test-negative design studies from British Columbia and Quebec, Canada. medRxiv. 2021 Jan 1. doi:

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

Dr. Danuta Skowronski and colleagues from the BC Centre for Disease Control evaluated the real-world single-dose effectiveness (as opposed to the findings of clinical trials) of the Pfizer, Moderna and AstraZeneca vaccines. They demonstrated that even a single dose of these vaccines reduced the risk of hospitalization from COVID-19 infection by over 80%. A recent follow-up on this initial report showed that two doses, whether of the same vaccine or a mix-and-match approach, were associated with more than 95% efficacy against risk of hospitalization. Moreover, vaccine effectiveness was maintained above 95% against hospitalization through at least seven months post-vaccination and the Delta variant did not compromise vaccine effectiveness. The findings are available in not-yet peer reviewed preprint.

Key points:

  • Two-dose ‘real-world’ vaccine effectiveness against both SARS-CoV-2 infection and hospitalizations was evaluated using data collected from PCR-based diagnostic testing centres administered to adults in British Columbia and Quebec between May and October 2021.
  • In both provinces, regimens using two doses of the same vaccine or a mix-and-match approach were both associated with more than 95% efficacy against risk of hospitalization (98% in BC and 97% in QC), and more than 90% efficacy against risk of infection (90% and 88% in BC and QC). Vaccine effectiveness was only lower post-two doses of Vaxzevria against infection at ~70% (71% and 73% in BC and QC), but this improved to 90% with a boost from an mRNA vaccine.
  • The Delta variant, which represented most cases in the fourth wave of COVID-19 infections in these provinces, did not compromise vaccine effectiveness.
  • The longevity of vaccine protection was also significantly improved when the spacing between doses was longer than the manufacturer recommended interval. Specifically, a 3–4-week interval between doses was associated with lower effectiveness (85% and 79% in BC and QC), compared with a longer interval of 7 to 8 weeks (91% and 89% n BC and QC).
  • In both provinces, vaccine effectiveness was maintained at >95% against hospitalization through at least 7-months post-vaccination. Two-dose effectiveness against infection peaked above 90% at 2-3 weeks post-vaccination and remained above 80% through at least 8-months post-vaccination.

The findings from this study strongly support vaccine interchangeability and mixed vaccine schedules to better protect Canadians against SARS-CoV-2. The results of this study are also consistent with a previous report demonstrating an extended dosing interval was associated with strong antibody responses in elderly individuals4. Furthermore, this data can help advise other countries about vaccine distribution, and help vaccinate as many people as possible