Quebec's director of national health said he's still not sure when the province will begin administering COVID-19 booster shots — 43 days since officials started injecting people with the Pfizer-BioNTech vaccine.

Dr. Horacio Arruda said Tuesday that while he doesn't want Quebecers to wait more than seven weeks to receive a booster shot, he said he was still waiting to hear back from government scientists studying the efficacy of the vaccine among those who received their first of two injections.

Quebec has taken a different approach from other provinces, focusing on giving a first dose to as many people as possible before giving anyone a second, a strategy that Arruda maintains will save more lives and keep more people out of hospital at a time when vaccine supplies are limited.

"We did it because we don't have enough vaccine," he told reporters.

Vaccine maker Pfizer has said the second dose of its vaccine should be given within 21 days. Moderna, the maker of the other vaccine approved for use in Canada, has set the date for the second shot at 28 days. Ottawa's National Advisory Committee on Immunization, however, has said the second dose of both vaccines can wait up to 42 days.

Relatives of long-term care residents say they don't believe the government is making a science-based decision.

Quebec is "playing Russian roulette with our loved ones' lives," Joyce Shanks, member of the Maimonides Family Advocacy Committee, said Monday. Her group, which represents residents of the Maimonides Geriatric Centre in Montreal, is considering suing the government over its vaccine strategy.

Her father, a Maimonides resident, was one of the first people in Quebec to get a dose of vaccine, on Dec. 14. "We signed up for the two doses. We did not sign up to be part of a clinical trial," Shanks said.

Kathy Assayag, chair of the users' committee at the Jewish General Hospital in Montreal, said the government's vaccine strategy is "a mistake."

#Quebec is "playing Russian roulette with our loved ones' lives," Joyce Shanks, member of the Maimonides Family Advocacy Committee, said on Monday. #COVID19 #Vaccine

"It's a gamble and we cannot gamble with people's lives."

Experts, however, differ on whether Quebec's plan is a well-calculated risk or an ill-fated wager.

Dr. Caroline Quach, chair of the National Advisory Committee on Immunization, said the committee's guideline that patients can wait up to 42 days between injections is based on trials from vaccine makers. Trial participants, she said, were encouraged to return after 21 or 28 days from their first injection — depending on whether they received the Pfizer-BioNTech or Moderna vaccine — but she said they were allowed to return up to 42 days later.

"The problem after the 42 days is that we don't have any data," Quach said.

While it's possible a single dose remains effective after 42 days, she said, there's a decent risk its efficacy decreases, but no one knows how fast that could happen.

"We know that the second dose helps with the maturation of the antibodies," Quach said. "The two companies decided that a second dose was needed, so we have no data with only one dose and we have no data with extended intervals."

Dr. Andre Veillette, a research professor at the Universite de Montreal's department of medicine and a member of the federal government's COVID-19 vaccine task force, said the further Quebec moves away from what's been proven in clinical trials, the higher the chance the vaccine won't have the same results.

"I think it's a gamble," he said. "I think there's not enough information."

Veillette said he's particularly worried about older people who generally don't respond as well to vaccines as younger people do. Even in a situation with constrained supply, he said the government should stick to the vaccine schedule that's been proven in clinical trials.

"Take the drops as they come and give them the way they're supposed to be given," Veillette said.

While the results of the vaccination campaign in Israel have raised concerns about the amount of protection given by the first dose of vaccine, Dr. Donald Sheppard, chair of the department of microbiology and immunology at McGill University's faculty of medicine, said the data matches what was demonstrated in the clinical trials.

The protection offered by both vaccines 14 days after the first shot exceeds 90 per cent, he said. Studies of other vaccines indicate the timing of the second dose isn't critical, Sheppard added. But, he said, there's no specific data on the consequences of delaying the COVID-19 vaccines.

Like other provinces, Quebec has been forced to manage with fewer COVID-19 vaccines than anticipated, following Pfizer's recent decision to suspend deliveries to upgrade its European production facility. As a result, Quebec doesn't expect to receive any vaccine shipments this week. The Health Department said Tuesday it had received 238,100 doses of vaccine and had administered 224,879.

Given the shortage of vaccine and the high number of COVID-19 cases in Quebec, Sheppard said he thinks the government made the right calculation — though if there were fewer cases or more doses, he added, there wouldn't have been the need to take the risk.

"But that's not where we are right now, in January, in Quebec," he said.

This report by The Canadian Press was first published Jan. 27, 2021.

This story was produced with the financial assistance of the Facebook and Canadian Press News Fellowship.