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A couple walks to a COVID-19 vaccination clinic in Montreal on March 23, 2021.Paul Chiasson/The Canadian Press

Isaac Bogoch and David Naylor are doctors with the Department of Medicine at the University of Toronto.

As the COVID-19 epidemic grinds on, mass vaccination is clearly our only ticket to a new quasi-normalcy. Public-health experts on the National Advisory Committee on Immunization recently indicated that a “first-doses-fast” vaccine strategy is the best way to slow a worrisome third wave already building in several provinces. The math is compelling. All the approved vaccines confer substantial protection after one dose, and covering more people rapidly saves lives by interrupting infection and transmission.

Most provinces have concurred and are stretching intervals between doses to as long as 16 weeks. In response, some scientists and physicians have objected strongly, insisting that provinces should follow the protocols of randomized trials that gave two doses of the powerful new mRNA vaccines three to four weeks apart. This polarized debate has not helped vaccine confidence at a precarious time. Perhaps a very Canadian compromise can be found by considering the U.S. and British vaccine programs and the need for both public health and clinical perspectives.

The United States and Britain both have generous vaccine supplies but have adopted divergent delivery strategies. The U.S. is relying on the Pfizer and Moderna mRNA vaccines and aiming to deliver two doses spaced no more than four weeks apart. So far, it has given at least one dose to 25 per cent of its population; 14 per cent have received two doses. Britain, in contrast, went for the first-doses-fast approach, spacing jabs 12 weeks apart. The numbers for coverage thus show 43 per cent with at least one dose; 3.6 per cent have received both. Excellent trial evidence supported that strategy for the Oxford/Astra-Zeneca vaccine, heavily used in Britain. But British officials also delayed second doses of the Pfizer mRNA vaccine with good results. The outcome in both countries has been a significant decline in the size and toll of substantial epidemic waves.

For better or worse, the British example is more relevant to Canada given our low vaccine coverage to date. Critics may claim that “half-immunized” persons will be petri dishes for new viral mutations. In fact, the three major variants of concern emerged in vaccine-free times and places. Instead of speculating about mutation risks, it seems smarter to acknowledge the stunning efficiency of the British strategy at lowering the toll of COVID-19 in a huge wave dominated by the B.1.1.7 variant – which is growing rapidly in prevalence here in Canada.

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.

So where’s the compromise? For starters, it makes no sense that those who consented to a first shot with a promise of a booster in three to four weeks should suddenly find weeks changed to months. As well, the elderly warrant special consideration because of their much higher risk of hospitalization and death from COVID-19. Yes, B.C. and Quebec have achieved dramatic declines in new cases and deaths in long-term care facilities despite waiting many weeks to deliver second doses of mRNA vaccines to seniors in those settings. However, there’s no data on 16-week waits – nor will such evidence be forthcoming from the U.K., where the maximum interval is 12 weeks.

We do have laboratory evidence suggesting a risk of attenuated mRNA vaccine effectiveness among groups other than the elderly, e.g. persons who’ve undergone solid organ transplants or being treated for cancer. There’s no corroborating clinical evidence as yet, nor do these studies explore all facets of the vaccine immune response. But these examples illustrate the importance of more research to sort out when a long delay may be associated with substandard protection after a first vaccine dose, waning of immunity or serious risk of a poor response to a second booster.

What, then, is the bottom line? A first-doses-fast approach is critical to achieving wide protection that will slow the pace and peak of the third waves that seem to be building in multiple provinces. But that doesn’t require a one-size-fits-all solution. There’s room for a clinical perspective that shortens the vaccine dose interval for the highest-risk individuals and subgroups, while still adhering to a public-health perspective of greatest good for the greatest number.

Making exceptions that shorten dosing intervals for some must be done with simple criteria and due regard for the value of wider, faster coverage. And as always, SARS-CoV-2 may have other plans. That’s why Canadians involved in the vaccine rollout must keep learning from other countries, from the natural experiments inherent in provincial pluralism, from global reports and from robust research into domestic vaccine effectiveness and safety. That’s also how, by continuously adapting and learning, Canada can find its own unique way out of this pandemic.

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