Photo: The Canadian Press

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."

"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."