A new study of Ontario COVID-19 data finds all vaccines approved for use in Canada are doing better than first expected at preventing symptomatic infection, hospitalization or death from coronavirus variants of concern, even after just one dose.

The pre-print study published on Saturday shows one dose of the Pfizer, Moderna or AstraZeneca COVID-19 vaccines had between a 48 to 83 per cent effectiveness against symptomatic infection by any of the four variants of concern detected in Ontario recipients this past winter in spring.

Each of the vaccines also had a 78 to 96 per cent efficacy against preventing hospitalization or death from infection by a variant of concern, two weeks after a first dose.

“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 four currently circulating (variants of concern), and that two doses are likely to provide even higher protection,” Scientists at Public Health Ontario, University of Toronto and several hospitals wrote.

The P.1 “Gamma” variant first discovered in Brazil showed the greatest ability to neutralize the positive effects of all vaccines studied.

Of more than 3,000 people infected with P.1 during the study period, a first dose of AstraZeneca showed only 48 per cent efficacy at preventing infection, while Pfizer was 60 per cent effective and Moderna was 77 per cent effective.

The study found just 0.4 per cent (26 people) of those who received two doses of Pfizer became infected with a B.1.1.7 Alpha variant case requiring hospitalization one week after their second dose.

For Moderna, it was 0.2 per cent who required intense intervention with B.1.1.7 one week or more after their second dose, and for AstraZeneca it was less than 0.1 per cent.

A parallel ongoing survey by Public Health Ontario found only 0.15 per cent of partially vaccinated people developed a symptomatic case of COVID-19 during the same period, and only 0.02 per cent of fully vaccinated people developed a detectable case.

For the Delta B.1.617+ variants of concern, the study’s authors had to rely on much smaller and less reliable sample sizes, as Delta cases are only confirmed through whole genomic sequencing which is too expensive and laborious to apply to all positive cases.

Even so, they found first shots of Pfizer, Moderna and AstraZeneca were 57, 72 and 67 per cent effective respectively in preventing symptomatic infection from Delta, roughly in line with studies conducted in the UK and B.C.

One week after a second dose, they found Pfizer was 87 per cent effective at preventing Delta infection, but there were not enough Moderna or AstraZeneca recipients to calculate efficacy.

Across all four variants of concern, the three vaccines showed strong efficacy in preventing symptomatic infection one week after dose 2, ranging from 92 per cent for Moderna against the Alpha variant to 87 per cent for Pfizer against the Delta variant.

The study used data generated by more than 420,000 people who were either infected with COVID-19 in Ontario or received a COVID-19 vaccine, or both, between December 2020 and the end of May 2021.

Of those, more than 40,000 tested positive for a variant of concern during the study period.

A previous letter published in the New England Journal of Medicine by researchers in Qatar found far lower rates of efficacy for Pfizer and AstraZeneca vaccines against the “Alpha” B.1.1.7 variant, but used data involving people infected sometimes only days after receiving a first dose.

Canadian officials embarked on a strategy in March of giving out as many first doses as possible, extending the interval between doses up to 16 weeks in the belief that more partially vaccinated people would suppress transmission better than fewer fully vaccinated people in the context of restricted vaccine supply.

Also contributing to the study were authors from the Institute for Clinical and Evaluative Sciences and The University of Ottawa.