Future of AstraZeneca COVID-19 vaccine in question in Canada over blood clots, supply issues


The future use of the AstraZeneca-Oxford COVID-19 vaccine in Canada is now in question due to concerns over the increased risk of rare but severe blood clots connected to the shot, an unpredictable future supply and a significant amount of alternative vaccines.

Ontario and Alberta will no longer give the AstraZeneca-Oxford vaccine as a first dose and future supply would instead be reserved for optional second shots.(Evan Mitsui/CBC)

The future use of the AstraZeneca-Oxford COVID-19 vaccine in Canada is now in question due to concerns over the increased risk of rare but severe blood clots connected to the shot, an unpredictable future supply and a significant amount of other vaccines.

Alberta was the first province to confirm it would stop administering first doses of the AstraZeneca vaccine, citing a scarcity of supply. The province will instead prioritize mRNA vaccines like Pfizer-BioNTech and Moderna while reserving existing AstraZeneca for second doses.

Ontario's Chief Medical Officer Dr. David Williams announced in an impromptu news conference late Tuesday afternoon the vaccine would also no longer be offered to Ontarians as a first dose and future supply would instead be reserved for optional second shots.

But unlike Alberta, Ontario's decision was made largely due to the rising rate of the rare but serious blood-clotting condition connected to the shot known as vaccine-induced immune thrombotic thrombocytopenia (VITT).

Those of you who received your first dose of AstraZeneca did the right thing to protect yourselves, your loved ones and communities, and I continue to encourage everyone to sign up for a vaccine as soon as it’s your turn.


Williams said that in the past several days there have been a growing number of reports of VITT in Ontario. Out of more than 850,000 AstraZeneca doses given, there are now eight cases in the province as of Saturday at a rate of about one in 60,000 shots administered.

Other provinces have yet to follow suit, but there are growing signs the vaccine will not be prioritized in provincial and territorial rollouts across the country.

Quebec's Ministry of Health and Social Services said in a statement to CBC News that while the AstraZeneca vaccine is still technically available to people over 45, the remaining doses have almost all expired and mRNA vaccines will be offered if a second dose is unavailable.

Manitoba's Chief Provincial Public Health Officer Dr. Brent Roussin said Sunday that the possibility of stopping the use of the AstraZeneca vaccine was being discussed "at many levels."

British Columbia Provincial Health Officer Dr. Bonnie Henry said Tuesday that while second doses of the AstraZeneca vaccine will be available to those who already had a shot in the province, mRNA vaccines will also be an option.

New international research is expected in the coming days and weeks on the safety and efficacy of mixing and matching COVID-19 vaccines and Canadians who have received an initial dose of AstraZeneca will likely have the option of combining it with an mRNA shot.

"I cannot emphasize enough how important AstraZeneca has been, in particularly March and April when we had limited amounts of the mRNA vaccine and we had high case rates and it's a very good vaccine and very protective," she told .

"But as we have case rates coming down thankfully and hopefully staying down, and we have a lot more other vaccines available, we'll be looking at using the Pfizer and Moderna that's coming in."

VITT data 'evolving' in real time

The abrupt change in vaccine rollout strategy comes after growing concerns from health experts over the risk of VITT, which they say calls its continued use in Canada into question.

Dr. Andrew Morris, an infectious diseases specialist at the University of Toronto, says the use of the AstraZeneca vaccine in Canada is no longer justifiable, particularly in younger people.

"For the people who are in their 30s and 40s, it just doesn't make sense," he said. "They're at really low risk of dying from COVID and they're assuming a risk of dying from this."

Health Canada approved the AstraZeneca vaccine for all Canadians over 18 in February, but provinces and territories have largely restricted its use in those over 40 due to the increased risk of VITT in younger age groups compared with the risk of death from COVID-19.

Though extremely rare, VITT is much more severe than a typical blood clot because it can cause cerebral venous sinus thrombosis (CVST), where veins that drain blood from the brain are obstructed and can potentially cause fatal bleeding.

Pharmacist Kyro Maseh administers a dose of the AstraZeneca COVID-19 vaccine to Matthew Stone, 46, at a pharmacy in Toronto on Apr. 20, 2021. Ontario announced Tuesday the vaccine will no longer be offered to Ontarians as a first dose.(Evan Mitsui/CBC)

Dr. Menaka Pai, a clinical hematologist at McMaster University and a member of Ontario's COVID-19 Science Advisory Table, says that the risk of VITT is changing almost daily and that different guidelines are to be expected.

"The reality is this is what evolving science looks like," she said. "We're asking people to make decisions in real time with the info that we have and then the info evolves a week later and we can be regretful about the decision from a week ago but hindsight is 20/20."

Pai says each region of the country needs to look at its specific epidemiological situation in the pandemic and determine whether the risk of VITT outweighs the risk of severe outcomes from COVID-19.

"What works in a hotspot where you don't have enough mRNA vaccines to quickly cover everyone [may not work] in parts of the Maritimes where you actually have a lot of places with low prevalence," she said.

"If you don't have the privilege of choice because your pandemic is out of control, I think that changes the math a lot and it changes the decision-making."

VITT risk 'increasing' as cases identified

Canada has reported 12 confirmed cases of VITT as of Monday out of more than 2.3 million AstraZeneca doses administered, according to the Public Health Agency of Canada. Quebec, Alberta and New Brunswick have each reported one death; New Brunswick has only 40 COVID-19 deaths overall.

"In a province with low COVID-19 risk like New Brunswick, the risk of death from VITT outweighs the risk of death from COVID-19 at any age," said Dr. David Fisman, an epidemiologist at the University of Toronto's Dalla Lana School of Public Health.

The Ontario Science Table now estimates the frequency of VITT in Canadians who have received the AstraZeneca vaccine at 1 in 55,000, but that number is a moving target with new data being released from countries around the world daily.

"Risk estimates have been increasing as people have started looking for the complication," said Fisman. "It doesn't make sense to use a vaccine where short term harms outweigh short term benefits, when people can stay safe until they get mRNA vaccine."

The National Advisory Committee on Immunization (NACI) came under fire last week for its messaging around its recommendations to provinces and territories that mRNA vaccines such as Pfizer-BioNTech and Moderna, which don't have a risk of VITT, were "preferred."

PHAC said in a statement to CBC News that as the supply of mRNA vaccines like Pfizer and Moderna increase this month, it is "expected" that provinces and territories will "continue to review and adjust their vaccination programs."

Morris says he believes more cases and deaths from VITT will occur in Canada in the future if we continue to use the AstraZeneca vaccine as a significant part of our vaccine rollout.

"You have to be pretty certain that the people who you're giving it to have a really, really, really high risk of death from COVID," he said. "So that the risk of death that you're exposing them to is exceeded by the risk of death that you're preventing."

AstraZeneca use in other countries

Denmark has completely halted the use of the AstraZeneca vaccine due to the risk of VITT, in addition to the Johnson & Johnson vaccine, which uses a similar adenoviral vector method. The move that is expected to set the country back weeks in its vaccine rollout.

The United Kingdom has used the AstraZeneca vaccine as a significant part of its vaccination campaign to successfully control its pandemic and is now able to relax public health restrictions due in large part to the use of the shot.

But VITT was not yet identified when that rollout was in full swing at the beginning of the year and the U.K. restricted the vaccine to those over 40 last week and retroactively reported 242 cases of VITT and 49 deaths out of 28.5 million doses given up to April 28.

A waitress serves drinks as customers sit at tables set up outside pubs in Soho, in London, on the day some of England's third coronavirus lockdown restrictions were eased by the British government on April 12.(Alberto Pezzali/The Associated Press)

The Ontario Science Table says that VITT can present anywhere from four to 28 days after vaccination and Canadians should seek medical attention if they have any of the symptoms listed on its website.

Pai says Canadians who are concerned about having taken an initial dose of the AstraZeneca vaccine despite not having VITT symptoms should rest assured that they made a smart choice at a critical point in the pandemic.

"You guys made an amazing, amazing decision based on the information in front of you," she said. "And now doctors, like me are trying to educate you if there is a very rare risk."


Adam Miller is a senior health writer with CBC News. He's covered health, politics and breaking news extensively in Canada, in addition to several years reporting on news and current affairs throughout Asia.

    With files from Ben Shingler and Phil Ling

    Credit belongs to : www.cbc.ca

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