How investigators are checking the safety of AstraZeneca-Oxford's vaccine

Canadians wondering about the safety of the AstraZeneca-Oxford COVID-19 vaccine — after some countries paused distribution amid reports of blood clots — should keep in mind there's no indication the vaccine caused those clots, say experts, and that there are regulatory safeguards in place.

Authorities in Denmark, Iceland, Norway and other European countries suspended their use of the shot following reports of blood clots in some people who were vaccinated. European regulators are investigating.

Many other countries, including Germany, France, Poland, Nigeria, the United Kingdom and Canada, continue to use the vaccine, citing a lack of any evidence of a link.

Danielle Kaplan, 60, is on the waiting list for the vaccine in Toronto. She wants to receive it but is also looking for some reassurance.

"It just makes me feel a little insecure and I would like this clarified," Kaplan said.

Here's some nuance and context on the clotting investigation:

What's the first priority?

Step one is to look for any possibility of a cause-and-effect relationship between receiving the vaccine and an adverse event or medical complication.

Bad outcomes can sometimes happen after someone receives a vaccine. But that doesn't mean the vaccine caused the problem and that's why regulators worldwide rule out, or in, any possibilities after checking with a fine-toothed comb.

Dr. Supriya Sharma, Health Canada's chief medical adviser, says there's no scientific basis for a link between blood clots and the AstraZeneca shot.

"There's not a good biological explanation about why a vaccine of this type, injected into a muscle, would cause that kind of adverse event," Sharma said in an interview with The Canadian Press.

The World Health Organization (WHO) called the investigation a "precautionary measure."

"It's very important to understand that, yes, we should continue to be using the AstraZeneca vaccine," WHO spokesperson Margaret Harris told a briefing.

What do regulators do to find out?

It's standard, after a vaccine hits the market, to review records for any hint of what scientists call a "biological mechanism" behind a potential link. 

Most turn out to be nothing, doctors say.

The clot reports from Europe all go to centralized agencies focused on looking at the safety of medicines and vaccines, says Dr. Lisa Barrett, an infectious diseases physician at Dalhousie University in Halifax.

"Right now, they're assessing whether the frequency of these particular events, these blood clots of various types and kinds, are likely or not likely related to vaccines," Barrett said on CBC Radio's The Current on Friday. "It is looking much less likely that this is related to vaccines."

Blood clots can happen for a variety of reasons, Barrett said.

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Despite some European countries temporarily halting use of the AstraZeneca-Oxford vaccine after 30 cases of blood clots, experts maintain it is still safe to use in Canada.

Then what?

Step two is to see whether more patients experienced the event than what's expected naturally in the population.

Dr. Anand Kumar, an infectious diseases specialist working at a Winnipeg intensive care unit, says you have to view this in the context of the number of vaccinations and the usual frequency of the adverse event.

The European Medicines Agency said, as of Wednesday, 30 cases of blood clots had been reported, including three deaths. That's among five million people vaccinated with the AstraZeneca shot within the European Union.

"I have a high degree of confidence that they'll look into this and within a couple of weeks they'll say, 'Something we need to keep an eye on but nothing to be worried about,'" Kumar said.

Hypothetically, if a problem is found then authorities would also check if the blood clots happened more often among those with a history of them.

What else could explain it?

Vaccinologists point to contamination of a certain batch of the vaccine as a potential explanation.

But there's no evidence of that in Europe and Canada's doses of the vaccine come from a different source.

Should Canadians be concerned?

Barrett said Canadians should be mindful of the events in Europe and should be reassured that people with expertise are looking carefully at the data.

The safety bar for vaccines needs to be high, she says, because they're given to people who are well, unlike drugs taken by people who are sick.

"I hope that what people take from this is that we're being transparent about any possibilities of issues and resolving them," she said.