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Why Canada’s decision to delay 2nd doses of COVID-19 vaccines may not work for everyone

New research from two small pre-print studies suggests delaying second doses of COVID-19 vaccines by up to four months may not be the best approach for some older Canadians.

The research comes as some experts are also questioning whether Canada’s vaccination advisers, who recommended the delay, can keep up with rapidly evolving science during the pandemic.

Prior to the pandemic, the National Advisory Committee on Immunization (NACI), which has provided guidance to the federal government on vaccinations since 1964, met just three times a year to discuss issues related to vaccines for influenza, mumps, measles and other viruses.

But a year after the pandemic was declared, with new data emerging daily, NACI has been thrust into the spotlight and forced to evaluate new vaccines for a novel virus faster than ever before.

“NACI’s committees are basically made up of volunteers, many with heavy daily responsibilities during the pandemic,” said Dr. David Naylor, co-chair of Canada’s COVID-19 Immunity Task Force.

“There’s no precedent for NACI to operate at this pace, and everyone is adapting on the fly.”

NACI has met nine times since Canada approved its first COVID-19 vaccine on Dec. 10, but it has plans to ramp up in the coming months with another 13 meetings scheduled between now and the end of June.


Guido Armellin, 86, receives the COVID-19 vaccine during a clinic at a church in Toronto on March 17. A team was on-site administering the Pfizer-BioNTech vaccine to parishioners as part of a community outreach program to get seniors vaccinated at their place of worship. (Evan Mitsui/CBC)

The committee has previously overturned its initial guidance against immunocompromised individuals and pregnant women receiving COVID-19 vaccinations, as well as a controversial decision against the AstraZeneca-Oxford vaccine for those over 65.

Delay could leave cancer patients less protected, U.K. study suggests

Perhaps one of NACI’s most impactful recommendations on Canada’s vaccine rollout was the decision to delay second doses beyond manufacturing guidelines by up to four months, but emerging research signals it may not be the best approach for vulnerable Canadians.

A new pre-print study, which has not yet been peer reviewed, analyzed 151 older cancer patients and compared their immune response with 54 healthy adults after receiving the first and second doses of the Pfizer-BioNTech COVID-19 vaccine in the U.K.

The researchers concluded that delaying second doses to between eight and 12 weeks for most cancer patients left them “wholly or partially unprotected” and had implications on their health and the potential emergence of coronavirus variants. 

WATCH | Delaying some 2nd COVID-19 vaccine doses challenged by new data:

New early data suggests that Canada’s recommendation of delaying the second dose of COVID-19 vaccines to up to four months may not be effective in some older, more vulnerable patients, causing the vaccine advisory committee to re-examine its guidance. 2:36

“Our data advocates that bringing forward the second dose of the vaccine for patients who have cancer may benefit them,” said Leticia Monin-Aldama, lead author of the study and a researcher at the Francis Crick Institute in London.

“And that perhaps a sort of one-size-fits-all approach may not be ideal when delivering these vaccines to the population.”

NACI advocated for that universal approach to delay second doses by up to four months for all Canadians — the longest interval recommended by a country so far — based on limited real-world evidence and the reality of Canada’s vaccine supply.

The decision was also informed by findings from Dr. Danuta Skowronski, 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.

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

But Canada’s chief science adviser, Mona Nemer, has said the decision to delay second doses amounted to a “population level experiment” and advised against the delay in older Canadians on CTV’s Power Play this week, citing a lack of data to back up the decision.

Darryl Falzarano, a research scientist with the Vaccine and Infectious Disease Organization (VIDO) lab in Saskatoon, is also against the decision to increase the time between doses and said there is a growing body of research that suggests it’s not the safest approach for immunocompromised and older adults.

“The initial data look like delaying the dose of the mRNA vaccines would still provide reasonable protection to the population from severe or moderate disease, and so vaccinating more people was looked at as the greater good,” he said.

“Now, in certain populations — older people, people with comorbidities and cancers — likely delayed boosting for them is sub-optimal and possibly will lead to revised recommendations for those groups.”


Darryl Falzarano, a research scientist with the Vaccine and Infectious Disease Organization lab in Saskatoon, is opposed to increasing the time between doses and says there is a growing body of research that suggests it’s not the safest approach for immunocompromised and older adults. (Debra Marshall)

B.C. study analyzed long-term care residents

A second pre-print study released this week from researchers in British Columbia, which has also not been peer reviewed, cast further doubt on the dose delay for seniors and found that their immune response may not be as strong as in younger, healthier people.

The study analyzed antibody levels in a dozen long-term care residents in Vancouver a month after receiving their first dose of a COVID-19 vaccine, compared with 22 younger health-care workers — 18 of whom had not previously been infected by COVID-19 and four who had.

“The level of antibodies in older residents was fourfold lower, so significantly decreased,” said Dr. Marc Romney, a clinical associate professor at the University of British Columbia in Vancouver and one of the authors of the study. “The function of those antibodies in older people was also compromised.”

Romney said antibodies are just part of the picture, and he also plans to look at the immune system’s full response in future research. But he said the fact that antibodies in the elderly didn’t neutralize the virus as well as in the younger health-care workers suggests the dose delay may need to be revised for them.

“There is emerging evidence that demonstrates that there are some populations that will probably not fare as well and have the same degree of protection following single doses of a vaccine,” said Dr. Isaac Bogoch, an infectious diseases physician and member of Ontario’s COVID-19 Vaccine Distribution Task Force.

“These are groups you would want to shorten the time between dose one and two.”

WATCH | The science behind delaying the 2nd dose of COVID-19 vaccines:

Federal government scientists have put their support behind delayed second doses of COVID-19 vaccines — which several provinces were already doing — and ongoing research shows some of the benefits of the adapted strategy. 2:04

‘This isn’t a regular vaccine’

The speed with which NACI members are able to make these decisions has come under fire.

Falzarano said NACI is typically used to working under a “slow-moving” vaccine regulatory process where vaccines can take up to a decade to go from research to rollout.

“Their job is to review vaccines, but their experience is reviewing them under a much different scenario,” he said.

“They are normally looking at a full data set when they have to make decisions. They would normally make very conservative decisions, and now, they find themselves in a much different scenario than what they’re used to — and I think that’s highly challenging for them.”


Visitors to a mass vaccination clinic in Toronto on Tuesday fill in paperwork as they wait in line. The National Advisory Committee on Immunization initially recommended against giving the AstraZeneca-Oxford vaccine to seniors, but that guidance changed on March 16 after it reviewed data. (Frank Gunn/The Canadian Press)

NACI’s decision to recommend against the AstraZeneca-Oxford shot for seniors on March 1 came despite emerging evidence from around the world demonstrating its ability to prevent severe COVID-19 in older adults.

But that guidance changed on March 16 after more real-world data on the vaccine’s effectiveness was reviewed by NACI, and CBC News broke the story revealing documents on the federal government’s plans to allow those 65 and older to receive it.

Alyson Kelvin, an assistant professor at Dalhousie University and a virologist at the IWK Health Centre and the Canadian Centre for Vaccinology, all in Halifax, said NACI should include more experts in emerging viruses and vaccine development to help navigate the research in the pandemic.

“This isn’t a regular vaccine that’s gone through the typical workflow for vaccine approval and vaccine development because it’s an emerging virus,” said Kelvin, who is also evaluating Canadian vaccines at the VIDO lab in Saskatoon.

“You need somebody who understands that dynamic, instead of what we would normally depend on for our medicines or vaccines.”

Dr. Caroline Quach-Thanh, who chairs NACI, responded to criticism during a news conference on March 16, saying that as new evidence emerged on the efficacy of the AstraZeneca-Oxford vaccine in older adults, NACI was “busy with other files” that delayed its guidance.

“The committee is very busy, obviously, meeting weekly to discuss the emerging data on these important topics,” said Matthew Tunis, executive secretary to the committee.

“So there’s always inevitably going to be a bit of a lag between when a committee deliberates and when the advice is made public.”

Decisions take time, NACI chair says

Quach-Thanh responded to further questions about the delay in revising recommendations on CBC’s Power and Politics on Wednesday, noting that NACI isn’t equipped to review new evidence one day and make recommendations the next.

“It’s not possible, we can’t be that reactive,” she said. “I don’t think any advisory committee can be that reactive because it would mean that every time something changes, you move the needle one way or the next.


Dr. Caroline Quach-Thanh, who chairs the National Advisory Committee on Immunization, says NACI is currently re-examining its guidance based on new research, and new guidelines on the timing of second doses for seniors and the immunocompromised could come as early as next week. (Skype)

“Then it just means that you’re changing your recommendation every other day. So you need to gather that base of evidence before you change something.”

But even after NACI has finalized its recommendations, Quach-Thanh said, it takes an entire week to translate and upload them to the Public Health Agency of Canada’s website — precious time in a pandemic where new data emerges daily.

Quach-Thanh said the committee is currently re-examining its guidance based on new research, and new guidelines on the timing of second doses for seniors and the immunocompromised could come as early as next week. But Skowronski, with the BCCDC, said it’s too early to make that call definitively.

“This is a kind of a signal that we might want to follow, it’s of interest, but we cannot change or make policy on the basis of this sort of small study,” she said.

“It may come to pass that we will want to adjust depending upon how far we have come in achieving that goal of getting at least one dose into these individuals at highest risk.”

Skowronski defended the decision to delay second doses by up to four months in Canada and stressed that the benefits of vaccinating more vulnerable groups with an initial shot outweigh the risks of delaying a second.

“My preoccupation is in at least getting a first dose into those at high risk of severe complications, and we’ve not achieved that yet,” she said, adding that age was by far the biggest risk factor for severe outcomes from COVID-19.

“That’s job one. Let’s get that job one done, and then let’s debate the timing of the second dose.”

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White House says U.S. plans to send 1.5 million doses of AstraZeneca vaccine to Canada

The United States plans to send roughly 4 million doses of the AstraZeneca COVID-19 vaccine that it is not using to Canada and Mexico through loan deals with the two countries, the White House confirmed today.

White House press secretary Jen Psaki told reporters that a number of countries, including Canada and Mexico, have asked the U.S. for doses of the AstraZeneca vaccine, but those loan deals are still being worked out. 

Psaki confirmed today that the U.S. has “seven million releasable doses” of the AstraZeneca vaccine “available.”

“2.5 million of those, we are working to finalize plans to lend those to Mexico, and 1.5 million to Canada,” she said.

“It’s not fully finalized yet. It’s our aim and what we’re working toward, to Canada and Mexico. It’s a complex process and our team is working with the companies to move it forward.”


White House press secretary Jen Psaki takes a question from a reporter during a press briefing at the White House, Monday, March 1, 2021, in Washington. (Andrew Harnik/AP Photo)

“This virus has no borders,” a U.S. official told Reuters on condition of anonymity earlier in the day. “We only put the virus behind us if we’re helping our global partners.”

The “releasable” vaccines are ready to be used once they arrive, Reuters reported. Under the deal, the United States will share doses with Mexico and Canada now — with the understanding that they will send the United States doses in return. The official said that would take place later this year.

The Biden administration has come under pressure from allies worldwide to share vaccine doses — particularly the AstraZeneca vaccine, which is authorized for use in other countries but not yet cleared for use in the United States.

AstraZeneca has millions of doses made in a U.S. facility and has said that it would have 30 million shots ready at the beginning of April.

The deal does not affect President Joe Biden’s plan to have vaccine doses available for all adults in the United States by the end of May, an official told Reuters. The deal is likely to be announced publicly in the coming days.

Two officials said the vaccine would be delivered in “short order” once the deal was completed, but they declined to give a more specific timetable.

The Associated Press also quoted an unnamed official saying that a loan deal for 2.5 million doses to Mexico and 1.5 million to Canada is in the works.

U.K. clears AstraZeneca

News of the loan deal comes as the United Kingdom’s drug regulator reports that a “rigorous scientific review” has ruled out the Oxford-AstraZeneca vaccine as the cause of blood clots in veins. The regulator is doing a more detailed study looking at blood clots in the brain.

The U.K. Medicines and Healthcare Products Regulatory Agency said the benefits of the COVID-19 vaccine outweigh any risks.

Health Canada officials are attending a meeting of the European Medicines Agency, which is set to issue a report on blood clots and the AstraZeneca vaccine today.

Many European countries halted use of the AstraZeneca vaccine following reports of blood clots in about three dozen patients.

Dr. Howard Njoo, Canada’s deputy chief public health officer, said Canada is monitoring all the evidence closely. 

Health Canada has said the vaccine’s benefits are strong and it has not seen evidence to link the vaccine to blood clots.

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Canada on track to receive 36.5 million COVID-19 vaccine doses by July

Maj.-Gen. Dany Fortin said today that vaccine supplies have stabilized and the “ramp up” phase of the inoculation campaign is in full swing, with some 36.4 million shots now expected to arrive before the end of June.

Fortin, the military commander who leads vaccine logistics at the Public Health Agency of Canada, said that after weeks of uncertainty, the country now has firm commitments from major vaccine suppliers — and Canadians can be assured that there will be enough doses on hand to give everyone who wants a shot at least one dose by Canada Day.

Pfizer has dramatically increased the number of doses it will send to Canada after it successfully retooled its production plant in Belgium earlier this year. The company is expected to deliver 5.5 million shots to Canada by month’s end. More than a million doses of that product will arrive each week for the next month, Fortin said.

Combined with 2 million shots from Moderna and 500,000 AstraZeneca shots from the Serum Institute of India, Canada is projecting a total of 8 million doses will be delivered in the first three months of this year.

Another 25.1 million shots from Pfizer and Moderna combined will arrive between April and June — two million more than earlier projections — along with another 1.5 million Serum-made AstraZeneca doses. Canada is also expecting 1.9 million AstraZeneca shots from COVAX, the global vaccine-sharing initiative, over this three month period.

More AstraZeneca shots and doses from Johnson & Johnson’s pharmaceutical division, Janssen, will also be delivered in the second quarter but the exact numbers are not yet known.

Prime Minister Justin Trudeau said Tuesday that Janssen is experiencing production issues that have delayed deliveries of its one-dose product to Canada. That company has shipped roughly 4 million shots to the American market over the past two weeks.

All told, at least 36.5 million doses will be delivered to Canada by the end of June — possibly more.

Because the National Advisory Committee on Immunization amended its dosing guidance last week — the group suggested health care professionals can wait up to four months before delivering a second vaccine dose — there should be enough product in Canada to administer at least one dose to the roughly 30 million adults in Canada by the nation’s birthday.

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A behind-the-scenes look at why Canada delayed 2nd doses of COVID-19 vaccines

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.


Danuta Skowronski was poring over Pfizer-BioNTech vaccine data on a Friday night in mid-December when she had an “aha!” moment.

The epidemiology lead at the British Columbia Centre for Disease Control realized she could actually “correct” the data Pfizer had submitted to the U.S. Food and Drug Administration on the effectiveness of just one dose of its vaccine.

In clinical trials, Pfizer couldn’t accurately determine the efficacy of a single shot because participants had already received their second dose after three weeks, and there was no comparative one-dose study done.

Pfizer reported an efficacy of 52 per cent for one shot, compared to the more commonly cited 95 per cent after the second. 

But Skowronski, who has been working on vaccine effectiveness analyses for more than 15 years, realized the company had included in its analysis the two-week time period immediately after vaccination — before the body’s immune response typically kicks in.


Dr. Danuta Skowronski with the B.C. Centre for Disease Control is in favour of delaying second doses to four months after analyzing the data Pfizer submitted to the U.S. Food and Drug Administration. (Harman/CBC)

She told CBC News vaccines are never expected to protect “instantaneously,” and that there is always a “grace period” of a couple of weeks that factor into vaccine effectiveness.

“When we took that into account and reanalyzed it for them, what we found was that they were underestimating the efficacy of the first dose, and rather than the efficacy being 52 per cent it was actually 92 per cent, ” she said. “For us, that was a game changer.”

The finding has changed the face of Canada’s vaccine rollout. It led the National Advisory Committee on Immunization (NACI) to change its recommendation on the time people should wait between receiving doses of COVID-19 vaccines, extending it from three weeks to an unprecedented four months.

B.C. announced it would be delaying second doses earlier this week. Ontario, Quebec, Alberta, Manitoba and Newfoundland and Labrador quickly followed suit.

Canada is now an outlier in the global vaccination rollout. No other country in the world has delayed second doses up to four months, and there is no evidence yet on the long-term effect it could have on immunity to COVID-19. 

Some scientists say we are venturing into uncharted waters. Others are comfortable with the risk.

Why is Canada delaying second doses?

NACI says if second doses are stretched to four months across the country starting this month, close to 80 per cent of Canadians over 16 could get at least one shot of the Pfizer-BioNTech or Moderna vaccine by the end of June.

But Canada’s chief science adviser Mona Nemer says the decision to delay doses by four months amounted to a “population level experiment.”

“The comment from the Chief Science Advisor was most unfortunate,” said Skowronski. “It did not reflect the careful risk-benefit analysis that went into this decision, and frankly, that is a science and an art to be able to do that.” 

But aside from a vague reference to “real-world effectiveness” from Canada and other countries in NACI’s recommendations, little evidence has been communicated to Canadians to convince them that the massive change in vaccine rollout strategy is the right move.

NACI says its decision to delay second doses is based on emerging real-world data from Quebec, B.C., Israel, the U.K. and the U.S. that showed “good effectiveness” of between 70 and 80 per cent from a single dose of the vaccines “for up to two months in some studies.” 

But it also makes clear that these studies haven’t yet collected four months of data on the long-term effectiveness of a single dose, meaning NACI is betting on the “high levels of protection” shown so far.

“It’s shown us really good vaccine effectiveness two months after receipt of the first dose, and that the effectiveness isn’t decreasing over time,” Dr. Shelley Deeks, vice-chair of NACI and a lead author of the recommendations, said in an interview.

“After looking at it from all of these angles, and given that we are in a situation of limited supply, the committee came to a strong consensus that we recommend the interval to be extended to four months.” 

Deeks said NACI will continue monitoring vaccine effectiveness data as it comes out around the world to determine if it needs to further alter its recommendations — meaning another monumental change to Canada’s vaccine rollout strategy is possible.  

“If we need to reassess and revise the recommendations, we will,” she said. “But this will allow more Canadians to receive the first dose and have a vaccine in a more timely manner and will have an impact on serious disease.” 

‘Not based on evidence’

The move has effectively doubled Canada’s doses of COVID-19 vaccines overnight, but some scientists are critical of the move to experiment with delaying intervals.

“The decision is not based on evidence. It’s really based on an extrapolation of the evidence,” said Brad Wouters, executive vice-president of science and research at the University Health Network in Toronto. 

“We’ve only been giving this vaccine for two months, so we don’t have data out to four months — no one in the world has been waiting four months for a second dose.” 

WATCH | The science behind delaying the 2nd dose of COVID-19 vaccines

Federal government scientists have put their support behind delayed second doses of COVID-19 vaccines — which several provinces were already doing — and ongoing research shows some of the benefits of the adapted strategy. 2:04

Wouters says it’s unclear if the delay will impact the effectiveness of the second dose, and the decision comes with a lot of uncertainty in the months ahead. 

Skowronski says once good protection is established, it doesn’t suddenly disappear or “fall of a cliff.” Instead, protection against a disease wanes gradually after a vaccination, which buys researchers time to “re-evaluate the optimal timing of the second dose.” 

She said that longer intervals between a first and a second dose of a vaccine are generally preferred because shorter intervals can interfere with the immune boost response and longer intervals are often associated with ultimately higher antibody levels. 

Alyson Kelvin, an assistant professor at Dalhousie University and virologist at the Canadian Center for Vaccinology, says the clinical trials on COVID-19 vaccines ran with the shortest time frame possible so they could get data out quickly, but previous studies on other vaccines show longer intervals are generally better.

Skowronski says its unclear why Pfizer went with a three-week interval for their clinical trials, but it may have been due to pressure to get the vaccine out or because they didn’t expect to have such a good protection with the first dose.

“The only reason to go with a shorter interval is if you don’t get good protection with the first dose, and a second dose administered sooner could top it up a lot,” Skowronski said. 

“That’s a scenario that we are not dealing with here. We’re getting excellent protection after the first dose and we have a clear and present danger threat now with ongoing elevated pandemic disease risk on top of that scarcity of vaccine supply.”

Lack of clear communication for Canadians

While Skowronski is confident delaying the second dose is the right move for Canada, she and other experts feel the communication to Canadians from NACI on the decision could have been more clear.

She said it’s important to stress to Canadians that they still need a second dose eventually to have as much protection from COVID-19 as possible, and that they should take any vaccine offered to them to combat its spread.

WATCH | The evidence is there for the ‘concept of further delay’ of second doses: Dr. Naylor

Dr. David Naylor, Co-Chair of the COVID-19 Immunity Task Force, joined Power & Politics Wednesday to discuss the National Advisory Committee on Immunization’s new recommendation that second doses of COVID-19 vaccines can be administered up to four months after the first dose. 2:33

Dr. David Naylor, who co-chairs the federal government’s COVID-19 immunity task force, said the decision to delay doses is “defensible,” but agreed the decision could have been explained much more clearly to Canadians. 

“There didn’t seem to be an organized communications strategy overall,” he said. 

“The unhappy result is that a decision which might have been welcomed as a wider tide lifting many more boats and helping us end the epidemic more quickly has instead caused a real undercurrent of anxiety. I hope that subsequent communications will clear the air.” 

Wouters says he worries about how Canadians will interpret the move to delay doses, given the limited understanding the average person might have on the issue.

“There wasn’t a lot of information about why the decision was made, what the evidence was, what the process was,” he said. “There could certainly be a lot more transparency around the process and how that was done.”


Lisbeth Mendez comforts Luigini Parravano outside the Richmond Green Sports Centre, in Richmond Hill, Ont., on Monday. Parravano, along with her husband Mario Parravano, were among the first cohort of senior citizens aged 80+ as part of York Region’s mass COVID-19 vaccination program. (Evan Mitsui/CBC)

Dr. Allison McGeer, a medical microbiologist and infectious disease specialist at Toronto’s Mount Sinai hospital, says there is “overwhelming” evidence in favour of second doses.

“People who haven’t spent time in vaccination and thinking about vaccination are always really anxious about the stuff you don’t know in the future with vaccines,” she said. 

“I’m comfortable with those uncertainties, and with the fact that we can deal with them when we get there. But if you’re not comfortable with that, there is a tendency to really worry about the potential consequences of doing that.” 

Skowronski says Canadians should expect health experts to adapt to and absorb emerging evidence as it becomes available and incorporate that into recommendations.

“You don’t do business as usual in the midst of a crisis,” she said. “You don’t want experts that are holding steadfast to an earlier opinion or viewpoint while knowledge has amassed and moved on.

“You want your experts keeping pace with those developments and making decisions based on what is known as time evolves, especially during a dynamic crisis like an unfolding pandemic.” 


To read the entire Second Opinion newsletter every Saturday morning, subscribe by clicking here.

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What we know about spacing out COVID-19 vaccine doses

Federal and provincial health officials are poring over emerging data on the advantages and disadvantages of extending the time between shots of two-dose COVID-19 vaccines. Here are some of the factors they’re weighing and why it matters.

Why do provinces want to space out the doses beyond official recommendations?

More vaccines are arriving and the provinces aim to get them into the arms of willing Canadians as quickly as possible.

But demand exceeds supply, so researchers in British Columbia and Quebec are studying what happens when the interval between doses is extended. That way they can use the supply to vaccinate more people with a first shot sooner.

Late last year, Quebec decided to vaccinate more quickly and more widely by allowing a 90-day delay between doses.

But British Columbia went further on Monday, moving to a four-month interval for doses of the mRNA vaccines from Pfizer-BioNTech and Moderna.

Dr. Howard Njoo, Canada’s deputy chief public health officer, pointed to advances since Health Canada approved those vaccines.


A health-care worker prepares to administer a dose of the AstraZeneca-Oxford vaccine, in Santiago, Dominican Republic, in February. (Ricardo Rojas/Reuters)

“What’s happening is, I think, very encouraging,” Njoo said in a briefing on Tuesday.

“We have real-world data, the actual experience of what’s happening with the vaccination, for example in British Columbia and in Quebec, as they’re vaccinating seniors in long-term care facilities. We’re seeing quite a high level of protection.”

Njoo said experts are balancing vaccinating a large number of Canadians to achieve a good level of protection without compromising the effectiveness of the vaccines.

Dr. Sumon Chakrabarti, an infectious diseases physician in Mississauga, Ont., says the top priority is to protect older individuals and those who are at highest risk of severe consequences, hospitalizations and death.

Chakrabarti said the principle of getting as many people covered with one dose is a good one.

“We do know from other vaccines that increase in the interval between two shots doesn’t have any major consequence in decreasing efficacy and in some situations might actually make it better,” he said. “But keeping that in mind, we do have to be careful. I think that we don’t want to stray too far away.”

What’s the basis for the recommended dosing schedule?

Vaccine-makers tested their shots in clinical trials with certain times between doses.

Pfizer-BioNTech’s vaccine is meant to be given as two doses, 21 days apart, while Moderna recommends 28 days. For AstraZeneca-Oxford’s, the interval is eight to 12 weeks.

Health Canada approved the vaccines based on that clinical trial data. Both Pfizer and Moderna acknowledge that, in a pandemic, health authorities will make their own recommendations.

What’s the scientific basis for delaying?

Chakrabarti says there’s evidence, for example, to support delaying the second dose of the Hepatitis A vaccine by six to 36 months, and that’s true for other vaccines, too. But the COVID-19 vaccines haven’t existed long enough to know.

Efficacy for Pfizer-BioNTech’s vaccine was around 95 per cent after both doses and 52 per cent after the first, according to clinical data. For Moderna’s it was about 80 per cent after one dose and 94 per cent following the second.

WATCH | Stop confusing vaccine messaging, expert says:

Open communication about evolving decisions around COVID-19 vaccinations is very important to keep public trust, says Dr. Isaac Bogoch, a member of Ontario’s COVID-19 task force. 8:14

The benefits of a second dose include include longer-lasting protection says Tania Watts, a professor of immunology at the University of Toronto who is studying immune responses to COVID-19 vaccines in Canadians. 

She says everyone should eventually get a second dose. But “as we go to the broader population, yes, I think we will still get the benefit if you delay the second dose,” Watts said. 

Watts noted that when the mRNA vaccines were developed, the four-week interval for the “prime-boost effect” in the clinical trials was done for practical purposes.

“All things being perfect, we could stick to the protocol,” from the clinical trials, Watt said. “But, if you can save a lot more lives by not giving everyone the second dose at three weeks, but giving a lot more people the first, I think this is where the rationale comes, and I think it makes complete sense.”

What’s unknown?

The variants of concern that are more transmissible than the original coronavirus could throw a wrench into the works for some combinations of vaccines.

Watts said neutralizing antibodies that block the coronavirus from attaching and infecting cells dropped to almost nil in lab tests of those who received the Pfizer-BioNTech shot against the B1.352 variant that first appeared in South Africa.

“After two doses, which gives you stronger antibodies, you still had some partial protection,” she said.

Watts says Canada is at a critical juncture, watching to see if the variants will take off among partially vaccinated people.

Epidemiological or population-level studies are also needed to figure out how many antibodies are needed to prevent infection as well as the details of immune system memory.

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AstraZeneca doses set to arrive tomorrow — but questions remain about who gets them first

The first batch of Canada’s supply of the AstraZeneca-Oxford vaccine is set to arrive tomorrow — but public health officials still have some distribution issues to sort out before they can deliver those shots.

Health Canada approved the AstraZeneca product last Friday. The National Advisory Committee on Immunization (NACI), the independent panel that sets the guidelines for vaccine deployment, is not recommending that these shots be used in people aged 65 and over.

While Health Canada has determined the product is safe to use on all adults, NACI said there isn’t enough clinical trial data available to determine how effective this product is in preventing COVID-19 infection among people in this older cohort.

Health officials will be under pressure to quickly establish priorities for distribution of the AstraZeneca shots because 300,000 of the 500,000 doses set to arrive this week from the Serum Institute of India will expire in just a month’s time.

Dr. Theresa Tam, Canada’s chief public health officer, said NACI is prepared to update its guidance “as they see more and more real world data accumulating,” but for now the AstraZeneca product should be directed at younger Canadians.

“Don’t read their recommendations as sort of static. But this is what they’ve recommended at this point,” Tam said. “Just watch this space.”

It’s up to the provinces and territories to decide how to put these AstraZeneca shots to use. Some scheduling adjustments will be required because most jurisdictions are focused on vaccinating the elderly at this early stage of the immunization campaign.

Tam said some of the groups that were “potentially prioritized a little bit later on” will have a chance to get their shots earlier than planned because of the NACI guidance.

Most provinces have said that — after the elderly, front line health care workers and Indigenous adults are vaccinated — essential workers and people who face a greater risk of illness should be next in line for the second phase of shots.

Maj.-Gen. Dany Fortin, the military commander leading the federal government’s vaccine logistics, said the shots will be “expedited as quickly as possible” to prevent wastage.

WATCH: Procurement Minister Anita Anand says AstraZeneca shots will arrive Wednesday

Procurement Minister Anita Anand says the first shipment of AstraZeneca’s COVID-19 vaccine is scheduled to arrive in Canada on March 3. 1:06

Asked why Canada purchased vaccines that are set to expire during the first week of April, Procurement Minister Anita Anand said the federal government was responding to demands from the provinces to acquire more shots.

“They have repeatedly told the federal government that they want vaccines as soon as possible and that they’re ready to administer vaccines,” she said.

Beyond the question of who will get these shots, there’s a debate over just how long people should wait between the first and second doses.

NACI has recommended that provinces and territories follow the guidelines set by the manufacturers and approved by Health Canada regulators: 21 days between shots for the Pfizer product, 28 days for Moderna and between four and 12 weeks for the AstraZeneca doses.

Some provinces, notably Quebec, have ignored these guidelines from the beginning, preferring instead to administer as many first doses as possible to tamp down infection risk.

NACI ‘considering evidence’ on dosing intervals

Dr. Bonnie Henry, B.C.’s provincial health officer, announced Monday that the province would be extending the interval between doses for all three products to 16 weeks.

Tam said NACI is now “considering evidence” from the latest scientific studies about the intervals between shots and will provide an updated recommendation sometime this week.

Christine Elliott, Ontario’s health minister, said that while public health officials in her province have complied with NACI guidelines, they would shift gears to deploy first doses to more people if vaccine experts give them the green light to delay those second doses.

“We are anxiously awaiting NACI’s review of this to determine what they have to say and their recommendations,” Elliott said. “We want to make sure that the decisions that Ontario makes are based on science.”

Tam said data from B.C. and Quebec suggest there may be good reasons to wait longer.

“They’re vaccinating seniors in long-term care facilities and so on and we’re seeing quite a high level of protection. It also seems that the protection is obviously lasting even after the first dose,” she said.

In a recent analysis paper published in the New England Journal of Medicine, Dr. Danuta Skowronski of the British Columbia Centre for Disease Control and Dr. Gaston De Serres from the Institut national de sante publique du Quebec suggested that a single shot of the Pfizer vaccine might be almost as good as two.

The doctors found that, by waiting two weeks after vaccination to start measuring the rate of new infections, researchers recorded 92 per cent fewer COVID-19 cases among those who had received a single dose of the vaccine compared to those who got a placebo.

“With such a highly protective first dose, the benefits derived from a scarce supply of vaccine could be maximized by deferring second doses until all priority group members are offered at least one dose,” the doctors wrote in their paper.

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Moderna study suggests half doses offer strong immune response, but experts caution against changing approach

There’s now early evidence showing Moderna’s coronavirus vaccine may elicit a strong immune response even through half doses, prompting hope that further research could back up the results and eventually allow countries like Canada to stretch out vaccine supplies.

The company’s peer-reviewed findings, based on a Phase 2 trial, were published online by the journal Vaccine last week.

The study looked at the mRNA vaccine’s “immunogenicity” — its ability to provoke an immune response — through both anti-virus, spike-binding antibody levels and neutralizing antibodies, which help to block reinfection.

Researchers determined within a two-dose regimen that both the current amount of vaccine dose and half that amount being given each time were capable of “significant” immune responses.

Those findings are welcome news, though not yet worth changing dosing approaches, said Dr. Zain Chagla, an infectious disease specialist and associate professor at McMaster University in Hamilton.

“But it does bring up the urgent need to do a Phase 3-type clinical trial of full dose versus half dose and see what happens,” he said. 

“The implications, obviously, are you all of a sudden double your vaccine supply overnight if this seems to work out.”


During a recent study, researchers determined that half the amount of Moderna’s COVID-19 vaccine dose was capable of triggering a ‘significant’ immune response. (Jean-Francois Badias/The Associated Press)

Immune response ‘increased substantially’

The Moderna vaccine is one of two options currently approved and being used in Canada to combat the spread of COVID-19, with more than 40 million doses ordered by the federal government.

Based on full clinical trial results, the current approach requires two doses of the vaccine, spaced 28 days apart.

The company’s recently released findings looked at both a full dose of 100 micrograms and a half dose of 50 micrograms, given as two doses in a randomized, observer-blind, placebo-controlled trial.

At eight different U.S. sites, a total of 600 participants were divided into age cohorts and randomly assigned at a 1:1:1 ratio to receive either the two full doses, two half doses, or two placebo doses.

By 28 days after the first shot, anti-virus spike-binding antibody levels and neutralizing antibodies were higher among people who’d been given the full dose compared to the half dose.

But that difference was “less apparent” after participants received both rounds, Moderna’s research team found.  

Both binding antibodies and neutralizing antibodies “increased substantially” by the two-week mark after participants were fully vaccinated and remained elevated through day 57, the researchers wrote.


Boxes containing the Moderna COVID-19 vaccine are prepared to be shipped at a distribution centre in Olive Branch, Miss., in late December. (Paul Sancya/Reuters)

Questions over duration of protection, variants

Outside experts who spoke to CBC News all stressed the need for future research before changing Canada’s dosing approach, given the short two-month time period and small, homogeneous group studied by Moderna. 

Chagla also said there’s a clear need to understand longer-term immunity and how other elements of the immune system — such as T-cells, which target specific bodily invaders — might be affected as well.

“The point nobody can answer for you is how long you will have protection,” said Horacio Bach, an adjunct professor in the division of infectious diseases at the University of British Columbia’s faculty of medicine.

“Nobody knows. Nobody can tell you. There are no studies; that’s the reason it’s a global emergency.”

Dr. Noni MacDonald, a researcher focused on vaccine safety who is also a professor at Dalhousie University’s department of pediatrics in Halifax, stressed that while Moderna’s study did show similar immunogenicity with two different concentrations of the vaccine, it was also based on “old data.”

The research was conducted between late May and early July 2020 — long before the clear rise of multiple virus variants, which may be more transmissible or capable of evading the body’s immune response.

If the findings hold up against emerging variants, it could mean countries like Canada could one day “stretch what we have” when it comes to Moderna shipments, MacDonald said in an email exchange with CBC News.

But right now, that’s not yet a possibility. Already the company says its vaccine may be less effective against the B1351 variant, requiring it to develop an alternative version for booster shots.

Immunologist and microbiologist Nikhil Thomas says it’s important to ‘suppress the spread of these variants,’ as the coronavirus variant first identified in the U.K. is replicating faster and transmitting ‘at a higher frequency.’ 4:17

U.S. officials discussed half-dosing

Despite limited data on the benefits of using half-doses, particularly against emerging variants, there has been discussion south of the border over taking that approach, with the U.S. government also helping fund Moderna’s most recently-published research.

In January, Moncef Slaoui, then-chief adviser of the former U.S. president Donald Trump administration’s vaccine effort — one dubbed Operation Warp Speed — said officials were considering giving half-doses of the Moderna vaccine to American adults under the age of 55.

The same month, the U.S. Food & Drug Administration released a statement shooting that idea down, saying any changes to dosing or schedules of approved vaccines would be “premature and not rooted solidly in the available evidence.”

On this side of the border, Health Canada officials told CBC News the agency has not received an application from Moderna to amend its vaccine authorization, but would “thoroughly review” one if it was submitted.

Canada has other vaccines in line for approval — how they compare to the ones already being rolled out and how COVID-19 variants are a complicating factor. 2:03

Dosing strategies have long-term impact

While Bach suspects full clinical trials might yield a similar result to Moderna’s Phase 2 trial, he said it isn’t clear if the manufacturer would even allow countries to stretch their supplies.

He also agreed keeping the current guidelines in place is the ideal approach for the time being, rather than risking lives by adopting a dosing strategy that needs more evidence.

“We don’t know where we are going,” Bach said. “You can put people in danger.”

Still, there’s some potential in Moderna’s early results, according to Chagla. 

Knowing the adequate dosing strategy will matter down the line while developing those boosters for variants, he said. And the early evidence points to the potential for increasing vaccine supply to much of the developing world, where shots remain in short supply.

“The answer will have implicating effects for years, not just the vaccine roll-out over the next few months,” Chagla said.

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What you need to know about getting both doses of the COVID-19 vaccine

Getting vaccinated to protect yourself from the virus behind COVID-19 isn’t a one-and-done process in Canada right now.

The two approved options, from Pfizer-BioNTech and Moderna, both require a two-dose regimen, with each dose ideally spaced apart by a specific time interval that was used during clinical trials.

But that’s not how every region is handling the vaccine roll-out. While Ontario is striving to hold back enough doses so people get both shots in the recommended time-frames, B.C. is delaying second doses by up to a week or two past manufacturers’ guidelines, and Quebec is going even further, waiting up to three times longer.

The goal is to vaccinate as many people as quickly as possible, but there are still questions over just how effective these vaccines will be if policymakers stray too far from the guidelines.

What is the recommended dosing approach?

Official guidelines say the Pfizer-BioNTech vaccine is meant to be given as two doses, 21 days apart, while Moderna recommends spacing doses 28 days apart. 

Canada’s National Advisory Committee on Immunization, a federal body made up of scientists and vaccine experts, says every effort should be made to follow those dosing schedules.

But, since vaccine shipments are still trickling in, the panel offered some wiggle room — suggesting the second dose for either vaccine could be delayed up to six weeks at the most.

“The flexibility provided by a reasonable extension of the dose interval to 42 days where operationally necessary, combined with increasing predictability of vaccine supply, support our public health objective to protect high-risk groups as quickly as possible,” reads a statement released Thursday from Dr. Theresa Tam, chief public health officer of Canada, and the country’s provincial and territorial chief medical officers of health.


A nurse prepares a dose of the Pfizer-BioNTech COVID-19 vaccine at a clinic for care home workers at St. Michael’s Hospital, in Toronto, on Dec. 22. (Evan Mitsui/CBC)

Why are two doses, at a specific interval, even necessary?

The official guidelines for dosing intervals are based on each manufacturer’s clinical trial data, showing that the vaccines were most effective sometime after people got both rounds.

Vaccine efficacy for Pfizer-BioNTech’s option was around 95 per cent after both doses; for Moderna’s it was around 94 per cent following the second dose. (Certain high-risk groups weren’t part of those clinical trials, however, including anyone pregnant or immunocompromised.)

While research suggests there may be some level of protection from even just one shot, the consensus in Canada is that following guidelines based on the clinical trial data — as closely as possible — is the best bet to ensure people are protected.

“We also do not know how long lasting the protection will be,” said Dr. Howard Njoo, Canada’s deputy chief public health officer, during a briefing on Thursday. 

“We know it’s better with two doses, but with one single dose, we are not sure.”

WATCH | : Dr. Howard Njoo discusses spacing vaccine doses out 42 days:

Canada’s Deputy Chief Public health officer, Dr. Howard Njoo, addresses questions on whether the first and second dose of COVID-19 vaccine can be safely administered 42 days apart. 2:34

So can you get sick from COVID-19 if you only get one dose?

Potentially, yes. 

Clinical trials showed the level of protection from just one dose is lower for both vaccines, and it also takes time for your body to react — meaning you aren’t protected immediately after getting a shot.

The highest level of efficacy reported for Pfizer-BioNTech’s vaccine started a week after people got their second dose, and after at least two weeks following the second dose of the Moderna option.

In Quebec, health officials are currently examining how multiple residents of a long-term care home fell ill with COVID-19, despite being among the earliest in that province to receive a first round of the Pfizer-BioNTech vaccine.

That prompted questions over when the residents were infected, and Njoo noted the individuals may have already contracted the virus before getting vaccinated at all.

Long-term research will be helpful in confirming just how much protection people get from one dose of either of the approved two-dose vaccines, and how long it’s safe to delay the second shot.


The first care home in Quebec City to have been targeted for vaccination is seeing a steady increase in COVID-19 cases. (Evan Mitsui/CBC)

Why are different regions handling dosages differently?

Some provinces, such as Manitoba and Saskatchewan, are sticking with the dosing schedule set out by the vaccine manufacturers. 

But since federal recommendations suggest delaying a second dose up to 42 days at the most, other regions are considering a looser approach. 

Quebec — one of the country’s hardest-hit provinces for COVID-19 cases — is stretching the dosing time-frame to a maximum of 90 days.

The province’s Health Minister Christian Dubé stressed it’s about protecting as many people as possible with a first dose before April, when vaccine shipments from Ottawa are expected to ramp up. 

Njoo acknowledged some regions may be reacting to the realities on the ground. 

“The pandemic has intensified,” he said. “And there are more hospitalizations and deaths.”

Quebec Health Minister Christian Dubé says a second dose of a COVID-19 vaccine may be delayed from 42 to 90 days to allow more vulnerable people to receive their first shot. 1:05

Should you get doses of different vaccines?

It’s ideal for people to be given two doses of the same vaccine rather than mixing and matching, Njoo said.

That’s because without long-term research, federal officials are again stressing the need to stick as closely to the guidelines as possible, since clinical trials are the best-available data on how to ensure these vaccines are as effective as they can be at warding off COVID-19.


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Canada to receive one million COVID-19 vaccine doses a week starting in April: general

Maj.-Gen. Dany Fortin, the military commander leading Canada’s COVID-19 vaccine logistics, said today that manufacturers are expected to deliver up to one million doses a week starting in April.

In the spring, Fortin said, the country will shift from phase one of the vaccine rollout — immunizing particularly vulnerable people, such as long-term care home residents, some Indigenous adults and health care providers — to a wider rollout as deliveries become larger and more frequent.

The Canadian immunization campaign has gotten off to a slow start. A month into the inoculation efforts, barely one per cent of the population has received at least one shot of the Pfizer or Moderna products. Only 710,000 doses have been delivered to the provinces and territories.

The federal government is expecting up to six million doses — enough for three million people to be fully vaccinated using the Pfizer and Moderna two-dose products — by the end of March. But Fortin conceded Thursday the government is still negotiating a delivery schedule.

“We have a scarcity of vaccines in the first quarter,” Fortin said. April will mark the start of the what he’s calling the “ramp-up phase.”

The prospect of a million doses a week will be welcome news to provincial leaders who have been demanding more vaccine supply as COVID-19 cases spike.

While the vaccination campaign got off to a slow start, some provinces, notably Ontario and Quebec, have been fine-tuning their processes to administer doses faster.

“We have been sharing data with provinces and territories who, of course, understandably want more vaccines as they ramp up their vaccination programs. The challenge is we have limited quantities,” Fortin said. “The rub is right now … there’s perhaps a disappointment with the relatively small numbers that are being distributed,” Fortin said.

Other provinces are laggards. Tens of thousands of the doses the federal government has so far shipped are sitting in freezers. Manitoba and Nova Scotia have been particularly slow out of the gate, using less than half of the shots they have received.

According to CBC’s vaccine tracker, 419,209 doses have been administered so far.

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Government secures another 20M COVID-19 vaccine doses from Pfizer

Prime Minister Justin Trudeau said today the federal government has reached an agreement to purchase another 20 million doses of the Pfizer-BioNTech COVID-19 vaccine, bringing the total number of vaccine doses scheduled to arrive this year to 80 million.

That’s enough to vaccinate the entire Canadian population with the two doses required for the Pfizer-BioNTech and Moderna products.

Prime Minister Justin Trudeau said today the federal government has reached an agreement to purchase another 20 million doses of the Pfizer-BioNTech COVID-19 vaccine, bringing the total number of vaccine doses scheduled to arrive this year to 80 million.

That’s enough to vaccinate the entire Canadian population with the two doses required for the Pfizer-BioNTech and Moderna products.

“From our agreements with Pfizer and Moderna alone … we are on track to have every Canadian who wants a vaccine receive one by September,” Trudeau said at a press conference outside Rideau Cottage.

The news comes as Canada experiences a post-holiday surge in daily new cases, and as provinces warn that they are administering doses faster than they are being delivered.

The federal government published a delivery schedule this week that includes shipment dates and the number of doses expected to be delivered to the provinces over the next two months.

“This will make sure [the provinces and territories] have the vaccines and information they need to keep rolling out doses as quickly as possible,” Trudeau said. 

Trudeau’s announcement comes days after Procurement Minister Anita Anand said the federal government had chosen not to exercise its option to buy up to 16 million more doses of Moderna’s COVID-19 vaccine, although those doses would not have arrived until the end of 2021.

Canada gave the green light to Pfizer-BioNTech and Moderna’s messenger RNA vaccines last month. Independent regulators at Health Canada are still reviewing other vaccine candidates from AstraZeneca-Oxford and Janssen Inc., a subsidiary of Johnson & Johnson.

As of Monday night, more than 377,560 doses of COVID-19 vaccine had been administered in Canada, according to a third-party tally.

More to come

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