The race to vaccinate the world became more urgent with the emergence of variants of concern (VOCs). In a recent pre-print, not yet peer-reviewed, researchers from the Canadian Immunization Research Network (CIRN), including CITF-funded researchers Dr. Deshayne Fell, Dr. Jeff Kwong and Dr. Kumanan Wilson, estimated the effectiveness of vaccines given between December 2020 and May 2021 in protecting against symptomatic SARS-CoV-2 infection caused by the Alpha, Beta, Gamma, and Delta variants.

Key Points:

  • A single dose of Pfizer-BioNTech, Moderna or Oxford-AstraZeneca vaccine provided some protection against symptomatic COVID-19 infection and severe outcomes caused by VOCs.
  • Two doses of a vaccine, as recommended, provided even greater protection.

In this large Canadian study evaluating the effectiveness of available vaccines, the authors analyzed samples from 421,073 people in Ontario who had undergone a PCR test for SARS-CoV-2. All samples were from people who were symptomatic and who lived in the community (not in long-term care). Of these samples, 28,705 (6.8%) tested positive for non-VOC SARS-CoV-2 and 40,828 (9.7%) tested positive for a VOC (Alpha, Beta/Gamma, Delta). Among them, 14,168 individuals were identified as having severe COVID-19 resulting in hospitalization or death. The authors then compared vaccination status between individuals with symptomatic or a severe infection and those who were symptomatic but tested negative.

Linking with vaccination records, samples were tested against Alpha, Beta/Gamma, and Delta VOCs. A single vaccination with Pfizer-BioNTech and Moderna was shown to be >55% and >70% effective, respectively, against symptomatic SARS-CoV-2. One dose of the Oxford-AstraZeneca vaccine was observed to prevent approximately 50% of symptomatic infections against Beta and Gamma variants and was deemed to be >60% effective against the Alpha and Delta variants. Vaccine effectiveness estimates against severe outcomes were higher than estimates against symptomatic infection.

In accordance with other studies, the authors suggest that a single dose of any of the available vaccines is beneficial, but that full vaccination is recommended for all vaccines to provide greater effectiveness. However, in settings with vaccine supply constraints, delaying the second dose to provide first doses to twice as many people may yield greater overall benefits to the population, especially with more contagious and deadly variants now circulating.


Nasreen, S., He, S., Chung, H., Brown, K.A., Gubbay, J.B., Buchan, S.A., Wilson, S.E., Sundaram, M.E., Fell, D.B., Chen, B., Calzavara, A., Austin, P.C., Schwartz, K.L., Tadrous, M., Wilson, K., Kwong, J.C., 2021. Effectiveness of COVID-19 vaccines against variants of concern, Canada. doi:10.1101/2021.06.28.21259420