Images are unavailable offline.

Amy Oh, a registered nurse, administers a Pfizer BioNTech COVID-19 vaccine at a UHN pilot site in downtown Toronto on Dec. 15, 2020.

Fred Lum/The Globe and Mail

Ontario has changed its COVID-19 vaccination plan to give a first dose to as many people as possible and no longer hold second doses in reserve as the province has lagged behind Canada’s already-slow immunization pace.

The province announced the change Monday, 14 days after the launch of Canada’s vaccination campaign. Ontario has administered 13,200 of its allotted 96,000 doses and faced criticism for shutting down clinics over the holidays.

“We are not holding or reserving doses, and are vaccinating as many people as possible, counting on confirmed shipments of the vaccine that will arrive over the coming weeks for second doses,” said Ontario Ministry of Health spokeswoman Alexandra Hilkene in an e-mail statement.

Story continues below advertisement

‘We are not prepared’: The flaws inside Public Health that hurt Canada’s readiness for COVID-19

Mandatory masks, shuttered theatres and confusing rules: The 1918 Spanish flu pandemic and its similarities with today

Alberta reports first case of new COVID-19 variant in person who returned from U.K.

The change came amid conflicting practices between the provinces and evolving guidelines from the vaccine’s manufacturer.

Quebec’s Health Ministry said in a statement Monday it is still following the guidelines of vaccine manufacturer Pfizer-BioNTech by holding back half of the doses it has received for second doses. Those are expected to start at the end of the week, following the manufacturer guidelines that the second vaccination be administered 21 days after the first. Quebec has so far administered 19,643 of its 56,000 shots.

Other provinces including Nova Scotia, Saskatchewan and British Columbia are not holding back doses and giving a first shot to as many people as possible. Others are still making plans.

Pfizer recommended holding back second doses for the first shipment of the vaccine, Christina Antoniou, director of corporate affairs for Pfizer Canada, said Monday.

“As we move into the first quarter of 2021 and are bringing in larger volumes of vaccine doses, there is greater predictability of expected volumes so we are a little more flexible on this guideline,” Ms. Antoniou said. “However, we still consider it to be a safe approach for the points of use to continue storing a portion of the doses received, to ensure no delay in the second dose deployment.”

Last week, researchers from the University of Toronto published modelling that showed giving a first dose to as many people as possible would give more people partial immunity and decrease serious illness and death, compared with holding back second doses.

Canada has vaccinated slightly fewer than 4,000 people a day since the first of 249,000 doses of the Pfizer-BioNTech vaccine began arriving in the country Dec. 13. The provinces combined have administered about 55,540 vaccines. Moderna Inc. is expected to ship 168,000 doses of its vaccine by the end of December. No other shipments have been publicly confirmed.

Story continues below advertisement

A representative of Health Canada did not answer a request for information on new vaccine shipments Monday, saying in an e-mail that the offices were closed.

Canada is lagging behind the United States, Britain and Israel in vaccination rates but is ahead of Denmark and Germany. Much of the rest of the world has yet to start.

Maxwell Smith, bioethicist and assistant professor at Western University and member of the province’s vaccine distribution task force, said that when Ontario received its first shipment of vaccines in December, it was unclear whether there would be enough supply of the vaccine to rely on steady shipments when the province launched its vaccination program on Dec. 14. When the province received 90,000 vaccines last week, it reassured officials that the province will reach its goal of receiving 2.4 million doses by March, he added.

The first dose of the Pfizer vaccine gives 50 per cent of people immunity while the second dose provides 95-per-cent immunity, vaccine trials showed.

“On the one hand, it might be the case that if you used all available doses as soon as you get them, then it’s true that you could vaccinate more people,” Dr. Smith said. “But if we don’t have a guarantee that we’ll get a second dose in time for those who already got a first dose, we don’t have the evidence to suggest that the second dose would be effective at such a high interval.”

While Quebec and other provinces continued to vaccinate through the Christmas holidays most vaccine clinics in Ontario were closed, prompting criticism from public health experts that vaccinations should not be interrupted at a time when infections are at record highs.

Story continues below advertisement

The COVID-19 vaccination clinic at Toronto’s University Health Network was closed during the holidays, but the clinic is on pace to finish administering all vaccines reserved for first doses by the end of this week, according to Susy Hota, medical director for infection prevention and control at the UHN.

“We’re at a bad point in this pandemic, so the greater impact would be to get broader coverage and try to vaccinate as many people as possible with the first dose, recognizing that worst-case scenario is that people can’t get the second dose,” Dr. Hota said.

Immunization experts say it is important to refine immunization regimes but supply is the most urgent problem.

“The biggest issue is and has always been, when will they get the number of vaccines that will allow them to ramp up administration?” said David Levine, the former director of the Montreal health region who was in charge of vaccinating the city’s population during the H1N1 outbreak 11 years ago. “The key moment will be when millions of doses start arriving, not thousands. We vaccinated a million people in nine weeks in Montreal in 2009. The key factor was that the vaccine was there.”

Caroline Quach, an infectious-diseases specialist and microbiologist at the University of Montreal, said Quebec’s choice to vaccinate residents and staff at nursing homes has slowed rollout of the limited vaccines available. Vaccinating health care staff in the next phase will go more quickly but still will not move as fast as mass clinics that will come with larger vaccine shipments.

“A big problem is nobody knows when the next shipment will come,” she said. “It’s supposed to be in the first quarter of 2021, so let’s hope it’s in January and not March.”

Story continues below advertisement

Sign up for the Coronavirus Update newsletter to read the day’s essential coronavirus news, features and explainers written by Globe reporters and editors.