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Nurse practitioner Leane Mathias administers the Pfizer-BioNTech COVID-19 vaccine to Caitlin Aleck at the Tsleil-Waututh First Nation's COVID-19 immunization clinic, in North Vancouver, on March 25, 2021.DARRYL DYCK/The Globe and Mail

Only a small percentage of the millions of Canadians who’ve received their first vaccine dose have gone on to develop COVID-19, data show, and an even smaller share of that total have fallen seriously ill or died – proof of the benefits of doling out first doses fast, the Public Health Agency of Canada says.

Nearly 2,300 Canadians have contracted the virus more than two weeks after receiving their first dose of a coronavirus vaccine, or 1.3 per cent of cases since the vaccine rollout began, according to PHAC. At least 53 of those people have died because of COVID-19.

The new data serve as a reminder that the first dose doesn’t provide foolproof protection against severe outcomes of COVID-19, which helps explain why public-health officials have urged Canadians to keep their guards up until they receive a second injection.

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The PHAC figures, which were provided in response to questions from The Globe and Mail, were released amid an ongoing debate about Canada’s decision to extend the interval between vaccine doses to as long as four months, longer than the three or four weeks recommended by vaccine makers.

Proponents of the longer interval argue that, in the face of vaccine scarcity, modelling projects that a “first-doses-fast” strategy will prevent more coronavirus infections, hospital admissions and deaths than offering the full protection of two doses to a smaller number of people right away.

But opponents say delaying second doses strays too far from the clinical trial evidence, and counts as ethically dubious because, in some cases, recipients consented to the first dose on the understanding they would receive the second on schedule.

“Having 30 million doses available right at the start of the immunization campaign would have been ideal. But we had to go through a period of shortage of vaccines,” said Gaston De Serres, a medical epidemiologist with Quebec’s national public-health institute, known by the French abbreviation INSPQ.

“There’s no doubt that you would have far more deaths with a limited number of doses if you go for two doses after three weeks than if you give one dose to many, many more people.”

As of April 26, PHAC was aware of 2,274 confirmed COVID-19 infections reported at a minimum of 14 days after receiving only the first dose. (Two weeks is considered the minimum amount of time it takes for the body’s immune response to mature enough to confer some protection against SARS-CoV-2, the virus that causes COVID-19.)

Of those cases, 203 were admitted to hospital and 53 died.

That 2,274 number works out to 1.3 per cent of all COVID-19 cases reported to PHAC since vaccines began being administered, “indicating that the vast majority of cases are occurring among unvaccinated individuals, and supporting the benefits of the extended-dose strategy,” Anna Maddison, a spokeswoman for PHAC, said by e-mail.

Of the 53 deaths, 91 per cent were in people 80 or older, the age group least likely to mount a strong immune response after immunization.

The Globe requested information on infections after vaccination from every province except the Atlantic provinces, which have recorded relatively few infections throughout the pandemic. Only British Columbia and Manitoba responded with numbers; public-health officials in other provinces were not yet ready to publish data.

The BC Centre for Disease Control said that as of April 7, it was aware of 618 COVID-19 cases reported more than two weeks after patients had received at least a first dose. That’s the equivalent of 0.14 per cent of vaccinated individuals. Eighteen of those people died.

Of those 618 cases, 42 were among people who were a week or more past their second dose. Two of the 18 deaths were in people who had received both doses. Most of the partly or fully vaccinated people who died of COVID-19 were over the age of 80, BCCDC spokeswoman Heather Amos said by e-mail.

Manitoba’s data show similar trends. As of April 26, the province was aware of 195 infections among people who had received their first dose more than 14 days earlier, or about 0.06 per cent of first-dose recipients. Seven died, all of them seniors.

“When you look at the total number of infections versus just that first dose of vaccine … it ends up being extremely, extremely low,” Jason Kindrachuk, a University of Manitoba virologist, said of Manitoba’s figures.

However, Dr. Kindrachuk and other experts cautioned that looking at what are known as breakthrough infections in isolation doesn’t shed much light on how well vaccines actually work in the real world.

“You can’t take much from that information. It’s only part of the analysis required to assess the benefits of vaccination,” said Danuta Skowronski, the epidemiology lead for influenza and emerging respiratory pathogens at the BCCDC.

To properly estimate vaccine effectiveness in the field, epidemiologists compare vaccinated populations with unvaccinated populations, then control for confounding factors such as the age and co-morbidities of vaccine recipients, and the state of their local epidemic at the time they received their shots.

Dr. Skowronski said the BCCDC is working feverishly to prepare proper estimates of COVID-19 vaccine effectiveness in the province, with hopes of sharing the information in the coming weeks.

Canada’s National Advisory Committee on Immunization (NACI), a group of experts that advises PHAC, has looked to evidence primarily from abroad to conclude that the first dose provides enough protection to justify delaying second doses in the context of constrained supply.

The expert group also said doses should be given closer together if supply allows, which appears likely now that a huge influx of shots are scheduled to arrive in Canada in May and June.

Still, NACI’s reassurance is cold comfort to at-risk groups who have been asking provincial governments to give them their second doses on time, including the elderly, cancer patients, people who take immune-suppressing medications and frontline health workers. (Some provinces have allowed exceptions, including Ontario, which decided to give second doses on schedule to certain cancer patients and transplant recipients.)

Michele Miron, an emergency medicine physician at Belleville General Hospital in eastern Ontario, found out her appointment was cancelled two days before she was scheduled to receive her second dose. She has been calling on the provincial government to speed up second doses for health-care workers at a time when the hospital system is under immense strain and can’t afford COVID-19-related staff absences.

“I think it’s critically important that we protect the integrity of the [health care] system,” Dr. Miron said. “Protect us so we can continue to protect and treat and care for our communities.”

With reports from Marieke Walsh

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