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

Results of the CanPath SUPPORT-Canada study “Real-world insights on COVID-19 vaccine effectiveness and risk factors for COVID-19 infection,” were presented during a hybrid event on May 30th.

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


Of the 62% of participants in a CanPath study who had infection-acquired antibodies, 86% had never suspected they had previously had COVID-19. These results of the CanPath SUPPORT-Canada study “Real-world insights on COVID-19 vaccine effectiveness and risk factors for COVID-19 infection” were made public during a hybrid event on May 30th. The CanPath study is led by Dr. Philip Awadalla (University of Toronto, National Scientific Director of CanPath and Executive Scientific Director, Ontario Health Study) and the presentation was made by Dr. Victoria Kirsh (Dalla Lana School of Public Health, University of Toronto and Scientific Associate, Ontario Health Study). The study aimed to understand the relationship between vaccination status and the risk of COVID-19 infections.

Key findings:

  1. 62% of participants were found to have had a SARS-CoV-2 infection based on anti-N measured antibody levels. Of those, 86% did not suspect they had ever had COVID-19.
  2. 95% of the infections were observed during the Omicron period. Participants who had been infected and had reported symptoms had symptoms that ranged from mild to severe, as very few were hospitalized (n=41).
  3. During the Omicron period (December 2021 to January 2023), vaccine effectiveness peaked at only 43% about 45 days after the last dose, and waned to zero by four months post-dose.
  4. When each dose number was analyzed separately, it was observed that the time since the last vaccination was the most important determinant of protection against infection – more important than the number of doses.
  5. Moderna consistently provided better protection compared to Pfizer: with every dose, the risk of infection was 14% lower among those who received the Moderna vaccine compared to those who received Pfizer.
  6. Bivalent boosters restored waning protection and may have broadened protection.
  7. Prior COVID-19 infection was found to be more protective than vaccination alone during the Omicron surge. However, the risk reduction after initial infection that was maintained at ~90% at six months had declined to 50% by eight months post-infection.
  8. During the Omicron era, the risk of infection was increased among younger individuals, men, essential workers, and those who travelled. There were no ethnic differences in risk in this study population.

The CanPath study collected dried blood spots at up to three time points from 22,300 participants from British Columbia, Alberta, Manitoba, Ontario, Quebec, and the Atlantic provinces. All study participants were over than 19 years old, and 66% were female. Blood collection began in February 2021 and ended in January 2023. By time point three, almost all study participants had been vaccinated

These findings suggest that booster campaigns could be strategically used to rapidly boost population immunity before upcoming waves of infections.