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Ontario has a stockpile of 45,000 doses that are set to expire by the end of May. Provincial Health Minister Christine Elliott has pledged that none will go to waste.

Nathan Denette/The Canadian Press

The question: I live in Ontario and I’m eligible to get a second dose of the AstraZeneca vaccine. Should I get it now? Or should I wait for another type of vaccine?

The answer: Ontario, as with many other provinces, stopped providing AstraZeneca as a first dose after the vaccine was linked to rare, but potentially fatal, blood clots.

However, the province has decided to keep using AstraZeneca for second doses, which are necessary to boost and extend the immunity conferred by the vaccine.

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Last week, Ontario was in a race to get shots into arms as quickly as possible because 45,000 of its doses were set to expire on May 31.

Then on Saturday – faced with the likelihood that some of those doses would go to waste – Ontario received permission from Health Canada to extend the expiry date to July 1.

The Health Canada approval followed a review of vaccine “stability data.”

Ontario will continue to receive new shipments of the AstraZeneca vaccine, which will be allocated as second doses for the 860,000 people in the province who have already received the first shot.

The province’s outgoing Chief Medical Officer, Dr. David Williams, has said that the recipients of AstraZeneca will also be given the option of getting another type of vaccine for their second dose – such as the vaccines made by Pfizer and Moderna, which use mRNA technology and are not associated with a blood-clot risk.

So, basically, it’s up to the AstraZeneca recipients to decide what they think is best for themselves.

It’s not necessarily an easy decision, given the fact that there are questions about the vaccines that can’t be answered with any certainty.

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Rob Kozak, a scientist and clinical microbiologist at Sunnybrook Health Sciences Centre in Toronto, believes there is merit to getting fully inoculated sooner rather than later.

“The data show that two doses of the AstraZeneca vaccine provide better protection than a single dose.” That extra protection may be critically important considering the emergence of highly contagious variants of the coronavirus that causes COVID-19.

But other medical experts don’t think AstraZeneca should be used for second doses because the shots will still put people at risk of the rare blood-clotting condition known as vaccine-induced immune thrombotic thrombocytopenia or VITT.

“We honestly don’t know the actual level of risk. The data is still evolving,” says Michelle Sholzberg, head of the division of hematology-oncology at St. Michael’s Hospital in Toronto.

Based on the latest case reports from Canada and other countries, the risk of developing VITT after a first dose ranges from one in 26,000 to one in 127,000.

“The data from the U.K. suggests that the risk is far lower with the second dose than with the first – that it might be in the range of one in 600,000,” Dr. Sholzberg says.

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Even so, VITTs are not like ordinary blood clots, she stresses. “They can be very severe and can result in death.”

With so much uncertainty, Dr. Sholzberg believes it’s unrealistic to expect the public to weigh the risks and benefits of getting the second AstraZeneca vaccine. “It’s hard to make an informed decision when you don’t have all the information,” she adds.

Dr. Andrew Morris, an infectious-diseases physician at Sinai Health and the University Health Network in Toronto, goes one step further. “This is one of those situations where the decision should not be left up to the public.”

Regulatory bodies, he adds, should intervene by removing AstraZeneca from the Canadian market.

Over the coming weeks and months, Canada is expected to receive a steady supply of Pfizer. “It’s clear now that we don’t need AstraZeneca,” Dr. Morris says.

What’s more, mixing different types of vaccines might actually produce better immune protection than relying on the same shot for both doses.

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Researchers in various countries are currently studying the safety and effectiveness of using COVID-19 vaccines interchangeably although the definitive results from these trials won’t likely be available until later this summer.

So, what should you do in the meantime?

Dr. Kozak says we can’t lose sight of the fact that COVID-19 remains a serious public-health threat.

“The way I see it, the sooner you get the second dose the better. I can’t say that’s the right decision for everyone, but it’s what I’ve been telling my friends and family to do,” he says.

Paul Taylor is a former patient navigation adviser at Sunnybrook Health Sciences Centre and former health editor of The Globe and Mail.

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