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The first doses of the Pfizer-BioNTech COVID-19 vaccine are delivered to the Maimonides CHSLD, in Montreal on Dec. 14, 2020.

Ryan Remiorz/The Canadian Press

Quebec’s decision to vaccinate as many people as possible against COVID-19 with a first dose and delay second doses is paying off in the short-term, according to preliminary data crunched by the province’s public health institute.

The first dose of COVID-19 vaccine reached about 80-per-cent effectiveness among health care workers and long-term care residents, according to an analysis by the Institut national de santé publique du Québec (INSPQ) released Thursday. The researchers tracked 33,351 nursing home residents and 172,979 health care workers who received only first doses in January.

Nursing home residents, who are older and have less robust immune systems, needed 21 days to reach the 80-per-cent level of efficacy while health care workers reached the level after 14 days.

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The researchers warned the data, based on surveillance from public health units, are preliminary and not equivalent in quality to a clinical trial, which would follow a separate control group. The British Columbia Centre for Disease Control is expected to publish a similar community surveillance report Friday about first-dose efficacy.

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“It’s working well, and we will keep monitoring week by week to see if there is any change in efficacy. But as long as we see this strong immunity from one dose, obviously it is advantageous to give a first dose to others first,” said Dr. Gaston De Serres, a physician, epidemiologist and researcher at the INSPQ.

“The scientific results are very reassuring.”

Quebec decided early in its vaccination campaign to give first doses to vulnerable people as quickly as possible and delay second doses up to 90 days – three-to-four times longer than manufacturer recommendations. The pharmaceutical companies, several families of long-term care residents and vocal academics opposed the plan, saying it was risky.

The 80-per-cent real-world efficacy of the first dose in Quebec is short of the 92- to 95-per-cent efficacy the researchers calculated from clinical trials conducted by vaccine manufacturers Pfizer-BioNTech and Moderna. The Quebec researchers said the age and health status of nursing home residents and the close contact with COVID-19 among health care workers help explain the gap. The efficacy may still rise over time, they added.

“Clinical trials are done in the best conditions,” Dr. De Serres said. “They also generally exclude people with serious conditions. Residents in long-term care are not part of clinical trials. But this is still in the same magnitude of effect.”

The researchers said they see no indicators that the efficacy of the vaccine starts to fade quickly. “We have good effectiveness in the vaccine without any evidence of reduced effectiveness over time,” said Dr. Nicholas Brousseau, a physician-researcher at the INSPQ and the chair of Quebec’s immunization committee. “We aren’t specifying a length of time to wait, but we favour a longer rather than shorter interval to maximize public health benefits.”

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How a single dose might work against variants remains an open question, although Dr. Brousseau said leaving older vulnerable people unvaccinated is likely the greater danger. He added, “there is no scientific data to support the theory” that partial immunity from a single dose could help create new variants.

Dr. De Serres and Dr. Danuta Skowronski, the epidemiology lead for influenza and emerging respiratory pathogens at the B.C. CDC, wrote a letter to the New England Journal of Medicine published Thursday urging a delay in second doses to reach more vulnerable people and prevent thousands of deaths.

“With such a highly protective first dose, the benefits derived from a scarce supply of vaccine could be maximized by deferring second doses until all priority group members are offered at least one dose,” the letter said.

Dr. Howard Njoo, Canada’s deputy chief public health officer, said the Quebec results “are encouraging, showing there’s a good increase in terms of our level of protection, even after one dose.” But, he added, the balance between offering maximum protection of two doses and reaching the most people with one dose is still up for discussion. “The questions raised by the authors are legitimate ones.”

Outbreaks and cases in Quebec nursing homes dropped by 95 per cent in the past month, but the researchers say part of that drop is attributed to lower community spread. Less than 25 per cent of residents in Quebec retirement homes have received a first dose but those homes have had an 80-per-cent drop in cases.

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