How risky are COVID vaccines?

You’ve probably gotten mixed messages about COVID vaccines. Misinformation is blanketing social media. Official recommendations are shifting constantly. Oxford-AstraZeneca’s vaccine in particular experienced several stop-and-gos with approval in Canada.

It’s enough for a lot of people to throw up their hands. “How am I supposed to know for sure that the vaccines are safe when experts don’t even seem to know?” they ask.

A recent survey found about 8% of Canadians will refuse a COVID vaccine and 28% are hesitant or skeptical. If Canada hopes to reach herd immunity and end the pandemic in this country, many among those 28% will need to be vaccinated.

It’s normal to be concerned about the safety of new vaccines. After all, no medication is risk free. Now that vaccine eligibility has been expanded to include most Canadian adults, let’s try to put those risks in perspective.

The approval process

There are three major vaccines in Canada: Pfizer-BioNTech’s, Moderna’s, and Oxford-AstraZeneca’s. The scientific evidence to date has shown that all three of these are very effective at protecting against serious forms of COVID (hospitalizations, death) and all of them are reasonably effective at preventing against minor illness (if you’d like to learn more about efficacy rates, check out this video from Vox).

Every vaccine went through three phases of clinical trials with tens of thousands of volunteers. When those results were published, health authorities such as Health Canada, reviewed the findings and made a decision about whether to approve it. In most countries, that happened at the beginning of 2021.

But, after approval, research on the vaccines has continued. Adverse side effects are constantly being documented and studied. The difference is that now, rather than tens of thousands of patients, there are millions.

What’s the deal with blood clots?

In a large enough patient population, there is a statistical certainty that some patients will suffer a bad health outcome or death within a period of time of vaccination.

As of May, more than 2.3 million doses of the Oxford-AstraZeneca vaccine had been administered to Canadians and of them, 28 people experienced serious blood clots within a month. That number was small enough that it was unclear whether the vaccine caused those clots. If you looked at another group of 2.3 million people who were not vaccinated, about 28 of them or more will experience a serious blood clot. But, responsibly, researchers began to investigate the possibility of a link.

Further study found that the blood clots were in fact caused by the vaccine, a phenomenon known as vaccine-induced thrombotic thrombocytopenia or VITT. While the precise number is still changing, the most recent Canadian estimate suggests your risk of suffering VITT after getting the Oxford AstraZeneca vaccine is about 1 in 60,000, which is rare. To put that figure in perspective, your lifetime risk of being struck by lightning is about four times higher, according to 2019 data from the National Weather Service.

The Pfizer-BioNTech and Moderna vaccines have a different mechanism from the Oxford-AstraZeneca vaccine and, after millions of doses administered globally, the others have shown no risk of VITT. Other serious side effects (such as severe allergic reactions) appear to be even rarer.

Why was Oxford-AstraZeneca approved then recalled?

So, if the risk is so small why was the vaccine recalled? And if a recall was the right thing to do, why wouldn’t our health authorities have waited until we could confirm there was nothing to worry about?

The answer has to do with another set of risks governments and regulators are trying to balance: the risks of COVID. Our risk tolerance for vaccines is extremely low, and rightfully so. Almost anyone’s risk of ending up in intensive care or dying from COVID is much higher. That risk is related to how old you are and the odds of you being exposed to the virus, which is a function of where you live, what you do for work, and, critically, how much virus is circulating in the community. Vaccines not only bring your risk of hospitalization or death down to almost zero, there is solid evidence that they also stop you from transmitting it to other people. So, more vaccines means less virus in the community.

At earlier stages in the pandemic, there was a lot of virus circulating in the community and less vaccine availability. So, it made sense to maximize the availability of vaccines by approving Oxford-AstraZeneca. Now that infection rates are declining and other vaccines are more widely available, the risk profile has changed, which is why regulators have decided to withdraw their approval.

Determining where to draw the line is difficult, and different health authorities will often disagree (which is why you’ve seen the Oxford-AstraZeneca vaccine approved elsewhere; why there are desperate calls to send excess supply to India, for example, where the toll of the virus now appears to be exceeding all other countries combined). Those thresholds may also change as more precise information about the risks is revealed, which is what happened in Canada.

But even after the final phase of clinical trials, there was enough evidence to confirm that overall the benefits of all the vaccines far outweighed the risk of harm. The risk of harm for Oxford-AstraZeneca’s vaccine remains infinitesimally low. In Canada, we’re now extremely fortunate to have other vaccines whose risks are even lower.

What’s my risk, personally?

You can no longer receive a first dose of the Oxford-AstraZeneca vaccine in Ontario. But perhaps you’re outside of Ontario, or are considering whether you should choose another vaccine for your second dose, or avoid your second dose altogether. What should you do?

David Juurlink, a Toronto doctor and toxicologist, referenced Cambridge University’s risk assessment tool outlining three scenarios for different age groups who are at low, medium, and high risk of exposure to the virus. The only scenario in which your risk of suffering VITT from the Oxford-AstraZeneca vaccine is higher than your risk of landing in an ICU from COVID is if you’re under 30 with low risk of exposure to the virus. As soon as your risk of exposure increases to medium, the tool suggests that 20-29 year olds become twice as likely to land in an ICU from a COVID infection than to suffer VITT.

For older age groups, the risks of infection are far higher regardless of your risk of exposure—in some cases hundreds of times higher. And remember, all of the vaccines including Oxford-AstraZeneca bring that risk down to almost zero.

“You’re 25 and living in a Unabomber-style cabin in rural Montana? By all means, take a pass (on the vaccine),” Juurlink tweeted. “You’re 61 and living in Toronto? Different story altogether. Especially if you spend a lot of time around others, either at work or at home.”

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