Real-world evidence for effectiveness of vaccination against 4 SARS-CoV-2 VOCs in Canada

By Susha Cheriyedath, M.Sc.Jul 7 2021

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants that are more transmissible, can cause more severe disease, and decrease vaccine effectiveness are classified as variants of concern (VOC).

Studies have previously reported the effectiveness of currently available COVID-19 vaccines against infection or severe disease outcomes caused by VOCs. All four VOCs - Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), and Delta (B.1.617.2) - have been circulating at different times in Canada, where vaccine supply constraints forced a delayed second-dose strategy.

Evaluating the effectiveness of Pfizer-BioNTech, Moderna, and AstraZeneca vaccines against symptomatic infection caused by the Alpha, Beta, Gamma, and Delta variants

Toronto-based researchers recently conducted a study to determine the effectiveness of BNT162b2 (Pfizer-BioNTech), mRNA-1273 (Moderna), and ChAdOx1 (AstraZeneca) vaccines against symptomatic COVID-19 infection caused by the Alpha, Beta, Gamma, and Delta variants between December 2020 to May 2021. They also assessed severe clinical outcomes, including hospitalization or death caused by the 4 VOCs. They conducted a test-negative design study with the help of linked population-wide vaccination, lab testing, and health administrative databases available in Ontario, Canada. This study is available on the medRxiv* preprint server.

Study: Effectiveness of COVID-19 vaccines against variants of concern, Canada. Image Credit: Corona Borealis Studio / Shutterstock

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

Results show lower effectiveness for first dose BNT162b2 and mRNA-1273 against the Delta variant compared to the Alpha variant

The results showed that vaccine effectiveness with partial vaccination or ≥14 days after dose 1 was higher for mRNA-1273 compared to BNT162b2 and ChAdOx1 against symptomatic infection caused by the Alpha variant.

Full vaccination ≥7 days after dose two increased the effectiveness of vaccines for BNT162b2 and mRNA-1273 against the Alpha variant. Protection against symptomatic infection by Beta or Gamma variants was lower with partial vaccination for ChAdOx1 compared to mRNA-1273.

Against the Delta variant, vaccine effectiveness after partial vaccination was lower than that against the Alpha variant for BNT162b2 and mRNA-1273 but was similar to the Alpha variant for ChAdOx1.

Full vaccination with BNT162b2 increased protection against the Delta variant to levels comparable to the Alpha and Beta/Gamma variants. Vaccine effectiveness against hospitalization/death caused by all four VOCs was higher than that for symptomatic infection after partial vaccination with all three vaccines.

Findings offer real-world evidence for good to excellent vaccine effectiveness after partial vaccination with all 3 vaccines against all 4 VOCs

Overall, the study estimated that partial vaccination with BNT162b2 and mRNA-1273 vaccines were >55% and >70% effective, respectively, against symptomatic infection by VOCs currently circulating in Canada.

Full vaccination with mRNA vaccines significantly improves vaccine effectiveness against all variants. Partial vaccination substantially increases effectiveness against hospitalization or death than symptomatic infection caused by all VOCs. Full vaccination further increased effectiveness against severe clinical outcomes.

“After full vaccination, our estimates against both outcomes for all VOCs were comparable with the effectiveness reported in previous studies.”

Vaccine effectiveness estimates agree with that from some studies and differ from others

The vaccine effectiveness estimates from this study against symptomatic infection with Alpha and Beta/Gamma variants after partial vaccination with mRNA or ChAdOx1 vaccines agree with that in British Columbia. Still, they are higher than the estimates reported recently from Qatar, Scotland, and England. The estimates after partial vaccination are also higher than that for severe, critical, or fatal disease in Qatar but are comparable with the estimates for hospitalization caused by the Alpha variant in England.

According to the authors, the findings show that even partial vaccination by all three vaccines provides good to excellent protection against symptomatic infection and severe disease caused by all four currently circulating VOCs, and that full vaccination or two doses of all three vaccines are likely to offer even higher protection against all VOCs.

“Our real-world vaccine effectiveness estimates suggest that even a single dose of these three vaccine products provide good to excellent protection against symptomatic infection and severe outcomes caused by the 4 currently circulating VOCs and that two doses are likely to provide even higher protection.”

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources