Canada and other countries globally have authorized different SARS-CoV-2 vaccines, mostly based on findings from clinical trials which have demonstrated a high degree of efficacy against disease. However, the real-world effectiveness of these vaccines in an evolving pandemic with several variants needs to be better understood. In a recent pre-print (not yet peer-reviewed), the VSRG’s Vaccine Effectiveness Working Group member Dr. Danuta Skowronski, from the BC Centre for Disease Control, evaluated the single-dose effectiveness of vaccines authorized in Canada (Pfizer, Moderna, AstraZeneca) during spring in British Columbia.

Key points:

  • To assess ‘real-world’ vaccine effectiveness (VE), i.e. whether COVID-19 vaccines worked outside of a clinical trial, researchers used a test-negative designFor “test-negative design” vaccine effectiveness studies: there is no distinction between cases (vaccinated individuals) and controls (non-vaccinated individuals) during enrollment, and laboratory testing is subsequently used to distinguish which individuals were cases and which were controls.. They identified groups of otherwise matched individuals (having similar important characteristics to the study) and separated them into two groups: those who tested positive for SARS-CoV-2 and those who tested negative for the virus. They then looked retrospectively to compare vaccination rates to assess whether those who had received a dose of vaccine were less likely to test positive for the virus.
  • Among adults aged 50-69 years, a single dose of any given vaccine reduced the risk of infection (Pfizer-BioNTech’s Cominarty: 75%; Moderna’s Spikevax: 82%; Oxford-AstraZeneca’s Vaxzevria: 61%).
  • VE was greatest at or >21 days after the administration of the first dose for all three vaccines.
  • Vaxzevria showed slightly lower VE against infection compared to the mRNA vaccines (Cominarty, Spikevax).
  • However, while all three vaccines showed single-dose VE against hospitalization exceeding 80% more than 21 days post-vaccination: a single dose of Vaxzevria was 96% effective against hospitalization compared to the mRNA vaccines at 83%. The authors hypothesized that the better protection against severe outcomes following one dose of Vaxzevria may be linked to similar or better T cell responses compared to the mRNA vaccines.
  • Given the emergence of the Alpha and Gamma variants of concern (VOCs) in Canada during spring 2021, VE following a single dose was generally highest against non-VOCs (mRNA: 84%; Vaxzevria: 92%). While VE against infection dropped slightly against Alpha (mRNA: 77%; Vaxzevria: 66%) and Gamma (mRNA: 79%; Vaxzevria: 60%), effectiveness against hospitalization generally remained >80% for all vaccines. Given the study timeline, researchers also cautiously reported decreased VE against the Delta variant following a single dose of either mRNA vaccine (Cominarty: 62%; Spikevax: 74%) and vector vaccine (Vaxzevria: 47%).

In conclusion, even one dose of any of the Health Canada-approved vaccines protected against severe COVID-19 outcomes by reducing hospitalization risk by more than 80%. Further follow up studies examining VE upon completion of the full vaccination series (two doses), and the effectiveness against more concerning variants such as delta are needed to inform health policy decisions.

Skowronski DM, Setayeshgar S, Zou M, Prystajecky N, Tyson JR, Sbihi H, Fjell CD, Galanis E, Naus M, Patrick DM, El Adam S. Comparative single-dose mRNA and ChAdOx1 vaccine effectiveness against SARS-CoV-2, including early variants of concern: a test-negative design, British Columbia, Canada. medRxiv. 2021 Sept 22. doi : 10.1101/2021.09.20.21263875