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Jeanne Hennebury, 93, receives the Pfizer-BioNTech COVID-19 mRNA vaccine from Vena Anderson at a pharmacy prototype clinic in Halifax on March 9, 2021.

Andrew Vaughan/The Canadian Press

Elderly people mount a weaker immune response than do younger adults after receiving a first dose of the Pfizer-BioNTech COVID-19 vaccine, according to a new Canadian study being released as the country’s immunization experts re-evaluate their backing of an interval as long as four months between doses for everyone.

It’s unclear whether the subpar immune response means the elderly are at higher risk of catching the coronavirus and falling seriously ill after one dose than younger people – a possibility that is complicating the already fraught deliberations over how to best deploy Canada’s limited supply of shots.

Earlier this month, Canada’s National Advisory Committee on Immunization (NACI) recommended a “first doses fast” strategy, paving the way for provinces to stretch the interval between shots to a maximum of four months, longer than the three or four weeks recommended by vaccine makers.

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Since then, several new studies have emerged from overseas that suggest a single dose of the new messenger RNA vaccines may not offer as much protection as hoped to the elderly, cancer patients and transplant recipients.

Now, NACI is considering tweaking its recommended interval for some of those groups, said NACI chair Caroline Quach-Thanh, a pediatric infectious diseases specialist and medical microbiologist at Montreal’s Sainte-Justine hospital.

“The data that have emerged are in the elderly and immunocompromised,” she said on Thursday. “The problem is, that doesn’t mean that because those two groups have data that the other at-risk groups are not important. That’s where we’re struggling.”

Dr. Quach-Thanh said she expects NACI to update its guidance as early as next week.

The new British Columbia study looked at the levels and performance of antibodies in 12 Metro Vancouver long-term care residents one month after they received their first dose, then compared them with 22 younger health-care workers, four of whom had recovered from COVID-19 before receiving a first shot.

“We were able to show that, essentially, the magnitude of [the LTC residents’] antibody response after the first dose was blunted compared to younger people and also the function of those antibodies was impaired,” said Marc Romney, medical leader for microbiology and virology at St. Paul’s Hospital in Vancouver and one of the authors of the new paper.

The study, released Thursday, has not yet been peer-reviewed.

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Both B.C. Provincial Health Officer Bonnie Henry and Danuta Skowronski of the BC Centre for Disease Control cautioned against drawing conclusions from the study and others like it, saying it’s more useful to look at how the delayed-dose strategy is panning out in the real world in places like Britain, where the approach appears to be contributing to a steep drop in cases, hospital admissions and deaths.

Relying on a single study to argue in favour of scrapping the delayed-dose strategy for the elderly is, “a step too far,” said Dr. Skowronski, the BCCDC’s epidemiology lead of influenza and emerging respiratory pathogens.

“That’s too sweeping a conclusion given the small sample size – 12 long-term care facility recipients,” she said. “The unifying, clarifying background to each of these studies is we don’t have immunological correlates of protection.”

That means scientists don’t know for sure whether high antibody levels measured in a lab equal strong protection against SARS-CoV-2, especially in the case of vaccines that rely on new messenger RNA technology made by Pfizer-BioNTech and Moderna.

Dr. Skowronski pointed out that B.C. has seen deaths among nursing-home residents plunge, as has Quebec, an early adopter of the delayed-dose strategy.

Currently, Quebec has 35 active cases of COVID-19 in nursing homes, down from a second-wave high of 903 recorded on Dec. 20.

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Ontario, which has already offered both doses to all of its nursing-home residents on schedule, is also reporting a huge drop, with just eight active cases in nursing home residents as of Thursday, down from more than 1,600 in mid-January.

Both Dr. Quach-Thanh of NACI and Dr. Skowronski of the BCCDC said that as Canada’s vaccine supply ramps up – the country is set to receive more than two million doses this week alone – provinces should feel free to consider shortening the interval.

But Dr. Skowronski said B.C. isn’t there yet because it still hasn’t offered a first shot to everyone over 70, the group likeliest to die of COVID-19.

“We still have not achieved job one, which is to give at least a single dose of protection to those highest-risk individuals,” she said. “When we’ve done that, then let’s talk about timing of second dose, which I remain open to adjusting.”

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