Why experts say 2nd vaccine doses should be rolled out now

This is an excerpt from Second Opinion, a weekly roundup of health and medical science news emailed to subscribers every Saturday morning. If you haven’t subscribed yet, you can do that by clicking here.


Canada’s controversial decision to delay second doses of COVID-19 vaccines far beyond other countries and manufacturing guidelines was a calculated risk — and some experts say that without question it worked.

More Canadians were given partial protection from one dose, as opposed to what would’ve been half as many fully protected with two. That helped to drive our surging third wave down in much of the country at a critical time.

But some experts say it’s time to shift to prioritizing second doses for Canadians because of an increase in vaccine supply, a drop in COVID-19 levels and the growing threat of more contagious coronavirus variants that could jeopardize the progress made.

“There is no question that for the whole of Canada, from the perspective of lives saved, that giving single doses to people and asking them to defer their second dose was the best idea,” said Dr. Allison McGeer, a medical microbiologist and infectious diseases specialist at Toronto’s Mount Sinai Hospital.

“Of course I want my two doses of vaccine. But if the price of that is somebody else maybe dying because of it — that’s just not OK. I think we can all see that.”

The National Advisory Committee on Immunization (NACI) made the unparalleled recommendation to delay second doses by up to four months for all Canadians in March, based on limited real-world evidence and the reality of Canada’s vaccine supply.

It said if second doses were stretched by up to four months across the country, close to 80 per cent of Canadians over 16 could get at least one dose by the end of June.

That decision was not without its critics, who pointed to the fact that the vaccine manufacturers recommended sticking to the approved clinical trial interval of three weeks for Pfizer-BioNTech and Moderna and four weeks for the Oxford-AstraZeneca shot.

At the time, Canada’s chief science adviser, Mona Nemer, said the decision to delay doses amounted to a “population level experiment.”

The decision was also informed by findings from Dr. Danuta Skowronski, the epidemiology lead at the British Columbia Centre for Disease Control (BCCDC), who determined that one dose of the vaccine was actually more effective than clinical trials had initially shown.

“It was a calculated benefit risk,” she told CBC News. “It was public health doing its job. This is what we expect of public health authorities: to take the emerging evidence during an emerging crisis and consider what is best for the population they serve.”

Skowronski said there was never reason to doubt that a delay in second doses of COVID-19 vaccines would allow for more time to generate heightened antibody responses and improve the boost response of the second shot — much like for other vaccines.

And emerging research appears to be proving her right.

Dr. Danuta Skowronski said there was never reason to doubt that a delay in second doses of COVID-19 vaccines would allow for more time to generate heightened antibody responses and improve the boost response of the second shot. (Harman/CBC)

Research suggests delaying 2nd doses was ‘right move’

A new Canadian preprint study, which has not yet been peer reviewed, found the Pfizer-BioNTech and Moderna vaccines’ effectiveness grew from 48 per cent 14 to 21 days after the first dose to 71 per cent after 35 to 41 days.

The observational study looked at close to 325,000 people in Ontario from mid-December to mid-April and concluded that while vaccine effectiveness was lower for those aged 70 and over initially — it grew to comparable levels found in younger adults after 28 days.

“I think that’s an important finding — that we just need to be patient after the first dose, that the first dose effectiveness is actually pretty good,” said Dr. Jeff Kwong, an epidemiologist and senior scientist at the Toronto-based research organization ICES.

“That provides good reassurance that the strategy of getting as many people a first dose as possible was the right move.” 

A recent study in The Lancet medical journal looked at more than 23,000 vaccinated healthcare workers in the United Kingdom from December to February and found the Pfizer-BioNTech vaccine was at least 70 per cent effective at preventing COVID-19 three weeks after the first dose.

Another Lancet study looked at more than 1.3 million people in Scotland during the same time period and found the Pfizer shot was more than 90 per cent effective at preventing hospitalization due to COVID-19 four to five weeks after the initial dose. 

That study also analyzed the AstraZeneca-Oxford vaccine in the same population and found it was 88 per cent effective at curbing hospital admissions from COVID-19.

Skowronski said it’s not surprising research is showing a single dose provides “substantial protection,” which she determined was more than 90 per cent with mRNA vaccines such as Pfizer and Moderna in a letter published in the New England Journal of Medicine.

Time to revise the strategy

But now she believes it’s time to revise the strategy. 

“The second dose is needed and one of the reasons is to increase the cross-coverage against other circulating variants,” she said. 

“Now that we have the vaccine supply and now that we have first-dose coverage across especially those high risk priority groups, I think that providing the second dose now is a good idea.”

A man wearing a mask walks in downtown Vancouver on April 6. A new Canadian study concluded that while vaccine effectiveness was lower for those aged 70 and over initially — it grew to comparable levels found in younger adults after 28 days. (Ben Nelms/CBC)

Variants may be spreading behind the scenes in Canada

Provinces initially followed NACI’s guidance, but have recently shortened the time between shots to as low as two months as supply has increased.

NACI released new recommendations Friday that said second doses should now be offered “as soon as possible,” with priority given to Canadians “at highest risk of severe illness and death from COVID-19” either before or in tandem with other eligible populations.

While the NACI guidance didn’t mention the threat of variants specifically, experts say the variant first identified in India known as B1617 could potentially pose a significant threat to Canada’s future vaccine effectiveness.

Prof. Ashleigh Tuite, an infectious diseases epidemiologist and assistant professor at the University of Toronto’s Dalla Lana School of Public Health, said B1617 may be spreading behind the scenes in Canada — much like the variant first identified in the U.K. first did.

WATCH | Provinces shorten timelines for 2nd COVID-19 vaccine dose:

Several provinces have announced plans to shorten the timeline for second doses of COVID-19 vaccines as supply increases. 1:59

“We knew that it was here, we knew that we were importing cases and we were in a situation where we had declining cases overall, but we were starting to see signs that B117 was starting to take hold,” she said. “What we’re starting to see the signs of right now is something similar [with B1617].”

But unlike for B117, Tuite said Canada isn’t able to screen for B1617 as easily through genetic sequencing, so we don’t have a “full picture” of the number of cases here aside from a category of variant-positive cases classified as “unknown.”

Maria Sundaram, an infectious diseases epidemiologist and post-doctoral fellow at the University of Toronto’s Centre for Vaccine Preventable Diseases who co-authored the study on vaccine effectiveness in Ontario, said Canada is fortunate B1617 hasn’t played a “major role” in our epidemic yet, but it’s something we need to “keep our eye on in the future.” 

WATCH | Variant found in India linked to U.K. COVID-19 resurgence:

A resurgence of COVID-19 in the U.K. is linked to the B1617 strain first found in India and it has sparked concerns it may delay plans to emerge from lockdown. 1:56

B1617 is currently the dominant strain driving a third wave in the U.K., leading to a surge in not only COVID-19 cases but hospitalizations as well, causing the nation to accelerate its vaccination program in order to avoid jeopardizing the reopening of the British economy.

A recent cluster of COVID-19 in Newfoundland & Labrador that has grown to at least 60 cases was confirmed as being caused by B1617, while outbreaks of the variant have also been detected in B.C., Quebec, Alberta and Ontario.

Dr. Michael Warner, medical director of critical care at Michael Garron Hospital in Toronto, said if Canada doesn’t follow suit we could see more breakthrough infections in the partially vaccinated and risk a similar resurgence in COVID-19.

“We need to get the people at the highest risk of being exposed to COVID and the highest risk of being severely ill from COVID their maximum protection,” he said. “And I think we have enough supply that we can do that without cannibalizing first shot opportunities for everyone else.” 

McGeer said the fact that it took until March for B1617 to become dominant in the U.K. means that Canada still has precious time to protect our most vulnerable with vaccines — if we act fast. 

“We have a couple of months probably before [B1617] is a significant issue.… That means we have a couple of months to get second doses into everybody and we should be comfortably able to do that,” she said. 

“But a part of me is still worried about what this virus can do.”


This is an excerpt from Second Opinion, a weekly roundup of health and medical science news emailed to subscribers every Saturday morning. If you haven’t subscribed yet, you can do that by clicking here.

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