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Lailamie Viloria, 48, a personal support worker at Fudger House Long Term Care home in Toronto receives a COVID-19 vaccine at St. Michael's Hospital in Toronto, Dec. 22, 2020.Melissa Tait/The Globe and Mail

Administering the vast majority of Canada’s COVID-19 vaccine doses right away would avert significantly more coronavirus infections than reserving half of the country’s allotment as second doses for the first recipients, according to new modelling from researchers at the University of Toronto.

Yet some provinces – including Ontario – are planning to keep half of their initial shipments in the freezer in case the vaccine supply chain breaks down, meaning fewer vulnerable people will be inoculated in the early weeks of the vaccine campaign.

U of T’s projections came to light as Health Canada authorized Moderna, Inc.’s COVID-19 shot on Wednesday, making Canada just the second country to give a green light to two vaccines against the pandemic virus.

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The approval of Moderna’s shot adds urgency to the question of whether provincial governments should hold back half of their shots. That’s because the Moderna vaccine, which is easier to store and transport than the Pfizer-BioNTech shot, is set to be distributed widely in nursing homes, where the vaccines will be in a life-and-death race against a surging coronavirus.

“The thing that will best protect people against short-term illness and outbreaks and save lives is getting the first dose into as many people as you can,” said Allison McGeer, an infectious-disease physician at Sinai Health System and a member of the federal government’s COVID-19 immunity task force.

Dr. McGeer said that fact only became clear in the past week or two, as Pfizer-BioNTech and Moderna released data from their late-stage clinical trials showing the first dose conferred relatively strong protection against COVID-19 disease, at least in the short run.

Both vaccines are given as two injections either 21 or 28 days apart. In Moderna’s case, the vaccine was 50.8-per-cent efficacious in the two weeks after the first dose, and 92.1-per-cent efficacious after 14 days from the first dose, but before the second jab.

The second dose, Dr. McGeer and other experts agree, is crucial to ensuring immunity lasts as long as possible. They say everyone should get the second dose on schedule, but if supply issues delay that injection by a week or two, it shouldn’t hamper how well the vaccines work.

Ashleigh Tuite, an epidemiologist at U of T’s Dalla Lana School of Public Health who worked on the new modelling, said she and her colleagues projected that frontloading vaccine doses would avert between 34 and 42 per cent more symptomatic coronavirus infections, compared with a strategy of keeping half the shipments in reserve.

“It makes much more sense to just get as many people their first doses as soon as possible,” Dr. Tuite said.

On Tuesday, Dr. Tuite presented her projections to a meeting of Ontario’s COVID-19 Science Advisory Table, which is preparing to make a formal recommendation to the government on whether to hold back the second doses.

Peter Juni, scientific director of the science table and a professor of medicine and epidemiology at U of T, said his personal view is that it is “absolutely justifiable” from a scientific perspective to inject the available doses as soon as possible, and count on future shipments to supply the follow-up dose.

The decision ultimately comes down to risk management, and to how much the provincial government trusts the vaccine supply, he added.

Nova Scotia, Saskatchewan and British Columbia have said they won’t hold back doses, while other provinces have decided to reserve some vials or are still working out their plans.

“I have a real challenge leaving vaccine in a fridge when there are so many people at risk right now,” British Columbia’s Provincial Health Officer Bonnie Henry said earlier this month.

In Ontario’s case, a spokeswoman for Health Minister Christine Elliott declined to answer questions about whether the government would change its publicly stated approach and start administering all of the vaccines that it receives from the federal government right away.

“While some individuals may have good COVID-19 immunity after only one dose, it’s not guaranteed and a second dose is necessary,” Alexandra Hilkene said in a statement.

She referred The Globe And Mail to previous statements from retired general Rick Hillier, chair of Ontario’s vaccine task force, who said in early December that the province would hold back 50 per cent of the first shipments of the vaccines in order to ensure that the second shots are available for individuals at the correct time interval.

Ontario is expecting to receive 53,000 doses of the Moderna vaccine the week of Dec. 28. The first shots are destined for long-term care homes in hard-hit parts of the Greater Toronto Area, Ms. Hilkene said.

If all of those doses were injected right away, more than half of the residents of Ontario’s approximately 79,000 long-term beds would have some vaccine-induced protection against COVID-19 by the middle of January.

Major-General Dany Fortin, the federal government’s point person on distributing vaccines, said Wednesday that provinces shouldn’t be concerned about the reliability of the vaccine shipments in the coming weeks.

“We have no reason to [have] doubt on the distribution of the product. It’s been progressing very well to date,” he told reporters at a press conference in Ottawa.

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