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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.

Tracking Canada’s COVID-19 vaccine rollout plans: A continuing guide

Coronavirus tracker: How many COVID-19 cases are there in Canada and worldwide? The latest maps and charts

Moderna, Pfizer, AstraZeneca or Johnson & Johnson: Which COVID-19 vaccine will I get in Canada?

Canada pre-purchased millions of doses of seven different vaccine types, and Health Canada has approved four so far for the various provincial and territorial rollouts. All the drugs are fully effective in preventing serious illness and death, though some may do more than others to stop any symptomatic illness at all (which is where the efficacy rates cited below come in).


  • Also known as: Comirnaty
  • Approved on: Dec. 9, 2020
  • Efficacy rate: 95 per cent with both doses in patients 16 and older, and 100 per cent in 12- to 15-year-olds
  • Traits: Must be stored at -70 C, requiring specialized ultracold freezers. It is a new type of mRNA-based vaccine that gives the body a sample of the virus’s DNA to teach immune systems how to fight it. Health Canada has authorized it for use in people as young as 12.


  • Also known as: SpikeVax
  • Approved on: Dec. 23, 2020
  • Efficacy rate: 94 per cent with both doses in patients 18 and older, and 100 per cent in 12- to 17-year-olds
  • Traits: Like Pfizer’s vaccine, this one is mRNA-based, but it can be stored at -20 C. It’s approved for use in Canada for ages 12 and up.


  • Also known as: Vaxzevria
  • Approved on: Feb. 26, 2021
  • Efficacy rate: 62 per cent two weeks after the second dose
  • Traits: This comes in two versions approved for Canadian use, the kind made in Europe and the same drug made by a different process in India (where it is called Covishield). The National Advisory Committee on Immunization’s latest guidance is that its okay for people 30 and older to get it if they can’t or don’t want to wait for an mRNA vaccine, but to guard against the risk of a rare blood-clotting disorder, all provinces have stopped giving first doses of AstraZeneca.


  • Also known as: Janssen
  • Approved on: March 5, 2021
  • Efficacy rate: 66 per cent two weeks after the single dose
  • Traits: Unlike the other vaccines, this one comes in a single injection. NACI says it should be offered to Canadians 30 and older, but Health Canada paused distribution of the drug for now as it investigates inspection concerns at a Maryland facility where the active ingredient was made.

How many vaccine doses do I get?

All vaccines except Johnson & Johnson’s require two doses, though even for double-dose drugs, research suggests the first shots may give fairly strong protection. This has led health agencies to focus on getting first shots to as many people as possible, then delaying boosters by up to four months. To see how many doses your province or territory has administered so far, check our vaccine tracker for the latest numbers.

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.

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.

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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