Effectiveness of COVID-19 vaccines against VOCs in Canada

The race to vaccinate the world against SARS-CoV-2 became more urgent as variants of concern (VOCs) began to emerge. In a CITF-funded study originally released as a preprint and now published in Nature Microbiology, researchers from the Canadian Immunization Research Network (CIRN), including Drs. Deshayne Fell, Jeff Kwong, and Kumanan Wilson, estimated the effectiveness of vaccines given between December 2020 and August 2021 in protecting against symptomatic SARS-CoV-2 infections caused by the Alpha, Beta, Gamma, and Delta variants. This study affirmed that two doses of vaccine are necessary to provide adequate protection against these VOCs.

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, including Alpha, Beta, Gamma, and Delta.
• Effectiveness against symptomatic infection ≥7 days after two doses was 89–92% against Alpha, 87% against Beta, 88% against Gamma, 82–89% against Beta/Gamma and 87–95% against Delta across vaccine products.
• 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 682,071 people in Ontario who had undergone PCR testing for SARS-CoV-2. All samples were from people who were symptomatic and who lived in the community (i.e. not in long-term care). Of these samples, 31,440 (5%) tested positive for non-VOC SARS-CoV-2 and 51,440 (8%) tested positive for a VOC (Alpha, Beta/Gamma, Delta). Among them, 15,269 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 with 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 or Moderna was shown to be >50% and >70% effective, respectively, against symptomatic SARS-CoV-2. One dose of the Oxford-AstraZeneca vaccine was observed to prevent approximately 41% of symptomatic infections against Gamma variants and was deemed to be >60% effective against the Alpha and Delta variants.

Effectiveness against symptomatic infection ≥7 d after two doses was 89–92% against Alpha, 87% against Beta, 88% against Gamma, 82–89% against Beta/Gamma and 87–95% against Delta across all approved vaccine products in Canada.

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.