There's growing consensus that offering the COVID-19 vaccine to as many people as possible may have more impact than holding back supplies to give recipients their second dose — but in Ontario, officials still plan to maintain the two-dose timeframe to ensure immunity.
The push for a change in approach comes after two vaccines have been approved to roll out in Canada.
Pfizer-BioNTech's version, approved first by Health Canada, is already being offered to tens of thousands of health-care workers.
A second option from Moderna, just approved on Wednesday, is now slated to roll out as well with nearly 170,000 doses expected to arrive in Canada by the end of the year.
Both require a two-dose approach, and original recommendations involved using half the available vaccine supplies while reserving a second dose in case supply chain issues disrupted the timeline, explained Toronto-based infectious disease specialist Dr. Allison McGeer, a member of Canada's COVID-19 immunity task force.
"That consensus is changing now," she added.
The two-dose regime involves injections given 21 days apart for the Pfizer-BioNTech vaccine, and 28 days apart for the Moderna version.
The latest findings suggest a single dose did provide solid protection against COVID-19 "in the short term," McGeer said.
U.S. FDA data shows 1st shot efficacy
Data released by the U.S. Food and Drug Administration in December showed that some level of protection from the Pfizer/BioNTech vaccine begins quickly after the first shot, with efficacy of a little more than 52 per cent — which spikes to roughly 95 per cent a week after the second dose.
A briefing note on the Moderna vaccine showed around 50 per cent efficacy after a first shot within the first two weeks, and beyond that timeframe, efficacy of more than 92 per cent even before someone received their second dose.
"In a setting in which we are seeing more cases every day, and we really want to do something as quickly as possible, it probably makes more sense to get everybody a dose now — knowing that there's going to be enough vaccine coming," said McGeer.
"And if that means some doses come a week late, that's probably fine."
Yet-to-be-published modelling from the University of Toronto, first reported by the Globe and Mail and obtained in a draft form by CBC News, shows taking a more flexible approach — by withholding fewer doses to vaccinate more people quickly — could avoid 34 to 42 per cent of symptomatic infections.
WATCH | Some provinces won't hold back COVID-19 vaccine doses for 2nd shot:
"If we could get more vaccines in the arms of long-term care residents and long-term care workers, this could potentially avert a lot of the potential infections in the coming weeks," said Ashleigh Tuite, an infectious disease epidemiologist and mathematical modeler at the university's Dalla Lana School of Public Health.
Ontario's COVID-19 Science Advisory Table is now preparing to make a formal recommendation to the province on whether to hold back the second doses, based on the research from Tuite and her team, the Globe reported.
Ontario to monitor, assess vaccine rollout
Ontario, however, isn't committing to changing its current approach, and officials say no formal recommendation has yet been made.
A spokesperson for the Ministry of Health did say in a statement that the province's COVID-19 Vaccine Distribution Task Force will continue to closely monitor and assess the vaccine rollout.
"While some individuals may have good COVID-19 immunity after only one dose, it's not guaranteed and a second dose is necessary," reads the statement provided to CBC News.
"We will continue to administer second doses to patients, ensuring they have optimal immunity from the vaccine, while continuing to vaccinate a growing number of new patients as additional doses of the vaccine are delivered."
Several provinces, including Saskatchewan, New Brunswick, and British Columbia, are already aiming to do the opposite by providing available vaccine doses widely.
"We aren't holding back doses because we want to protect as many people as possible, as quickly as possible," Dr. Bonnie Henry, B.C.'s chief medical officer of health, recently said.
But Matthew Miller, an infectious disease specialist from McMaster University, cautioned against drifting away from the strict two-dose timeframes before the vaccines' long-term efficacy is clear — particularly during the early phase of immunization campaigns targeting those at a high-risk of infection.
"I do think it's imperative to ensure we have the doses on hand to be able to guarantee that they're to experience the most efficacy possible," he said.
'We know it can save lives'
Advocates for more widespread vaccinations aren't calling for an end to the two-dose regimen, but McGeer stressed there's now less need to hold back doses based on emerging information on the stability of the supply chain.
The debate is playing out while a growing number of Canadians say they want to get a COVID-19 vaccine as soon as it's available to them.
Close to half of respondents said they would take the shot if it became available, according to a December online survey from the Angus Reid Institute, up from 40 per cent of respondents the month before.
Eventually, as more doses become available, all provinces will be scaling-up their immunization programs to the broader public, with a federal goal of vaccinating all Canadians who want the shot by the end of 2021.
Tuite said her stance on dispersing available doses more widely doesn't need to be a long-term strategy, since maintaining the tight two-dose timeframe will become easier as more shipments arrive in the months ahead.
"But right now, when a vaccine is scarce, and when we know it can save lives, I think it's important to think about how we can maximize that," she said.