As with all COVID-19-related science, data is still evolving. At this point, it’s not entirely clear whether the risk of blood clots differs between the first and second doses of the AstraZeneca COVID-19 vaccine.
Experts believe it’s similarly rare, but it’s not impossible.
“The risk remains, but it doesn’t seem to be higher when people take the second dose,” Dr. Marc Berthiaume, director of the Bureau of Medical Science at Health Canada, said Wednesday.
“The data we have is limited when it comes to the adverse events associated with second use of the AstraZeneca vaccine.”
To better understand, experts are looking to the U.K., which has had the shot in circulation since January — much longer than in Canada.
More than 22 million first doses of the AstraZeneca vaccine have been administered in Britain. Of those, about 209 cases of rare clotting have been reported.
Canada’s National Advisory Committee on Immunization says the estimated risk is about one in 100,000 people, although more research is needed — and that number is subject to change.
That’s about a one-in-a-million risk, according to Dr. Supriya Sharma, the chief medical adviser with Health Canada.
“But again, that’s the information we have from the U.K.”
In Canada, over 700,000 doses of the AstraZeneca vaccine have been administered. There have been seven reported cases of the rare clotting — called vaccine-induced immune thrombotic thrombocytopenia (VITT) — and two deaths.
Looking at the U.K.’s experience, it appears cases of VITT are “very, very rare” with the first dose, and “even more rare” with the second, said Dr. Ishac Nazy, associate professor of medicine at McMaster and director of the McMaster Platelet Immunology Laboratory.
His lab is the only one in Canada equipped to test for VITT and VIPIT (vaccine-induced prothrombotic immune thrombocytopenia) and has been testing dozens of samples from across the country that are potentially linked to the AstraZeneca vaccine.
He’s confident in the two-dose regimen for AstraZeneca and stresses that Canadians should still get the first vaccine offered to them.
“If you got the first dose successfully, then your anxiety should actually be less,” he said.
“Why (clotting) happens to certain people and not others… it’s still elusive, but if it’s something intrinsic, if you don’t get it on the first one you shouldn’t get it on the second one. If you are getting it on the second one, we don’t know why you didn’t get it on the first one. It’s hard to speculate.”
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The answer to the “why” is still being analyzed around the world.
“You can appreciate, given the rarity of the event, how much harder it is to try to figure out why it’s happening,” he said.
Each case has to be reviewed and vetted to determine whether there’s a relationship with the vaccine or not, said Dr. Sonia Anand, a professor of medicine and epidemiology at McMaster University.
“But we’re very early into the incidence of clotting with a second dose of a viral-vector vaccine,” she said.
“The positive side is there’s an increasing awareness of this in Canada. We know how to diagnose it and we know how to treat it.”
The blood clot risks with viral-vector vaccines — while minimal — are fuelling part of the conversation around potentially mixing and matching doses.
While there’s no definitive data on the safety and efficacy of the strategy, health experts are generally confident in the idea. And despite no official guidelines in the U.K. or Canada, there are reasonable exemptions for people who did experience clotting after their first dose.
In the U.K., the government recommends those who experienced a very rare blood clot with low platelets after having the first dose of the AstraZeneca vaccine get a different second dose. The same goes for similar conditions, like patients with a history of HITT or HIT Type 2.
Health Canada takes the same stance.
“Unless you’ve had this very rare clotting event or you had an allergic reaction to it, then you should go ahead and get your second dose of AstraZeneca,” Sharma said Wednesday.
Nazy agrees this is the right move, though “it doesn’t mean people should shop around,” he said. Canada is slated to receive 6.4 million AstraZeneca COVID-19 vaccines by the end of June, a government official told Global News on Tuesday, meaning the hundreds of thousands of Canadians who got their first shot will soon be up for their second.
“If you were willing to take the first dose of AstraZeneca, then you should have no problem taking the second dose. The clotting event is real, but it’s very, very rare and the benefits outweigh the risks,” he said.
Those risks include ones directly associated with contracting COVID-19. British researchers in April found there is a much higher risk of brain blood clots from the infection itself than there is from vaccines against the disease.
“The benefit of the second dose is that it ensures long-term immunity… We need to take this into account,” Berthiaume said in French at the Health Canada briefing.
“This second dose will get people a longer-lasting protection and there’s a lower risk of adverse events and it will help people fight COVID-19.”
Nazy acknowledges that the fast-moving science has created some confusing messaging but believes the right decisions are being made right now.
“Decisions are being based on experiences and data that is available — not necessarily just COVID and its vaccines, but based on previous information from other diseases,” he said.
“It might sound like a guessing game but it really isn’t.”
— with files from Reuters