Health

Is AstraZeneca vaccine safe for people who've had blood clots? Your COVID-19 questions answered

AstraZeneca has been in the headlines recently over concerns that some people were developing blood clots after receiving the vaccine. Here are some answers to questions you may have.

What about those who are immunocompromised? And what if you refuse to take it?

A medical worker prepares a syringe from a vial of the AstraZeneca coronavirus vaccine at the vaccination centre in Ebersberg near Munich on Monday. Some countries recently suspended use of the vaccine over blood clot concerns, but many have since resumed. (Matthias Schrader/The Associated Press)

The information in this article was current at the time of publishing, but guidelines and advice can change quickly. Check with your local public health unit for the most current guidance, and find the latest COVID-19 news on our website.

We're still answering your COVID-19 questions. We've received more than 71,000 since the start of the pandemic. But vaccine questions are the main theme right now. We publish a selection of answers online and also put some questions to the experts during The National and on CBC News Network. Send your questions and stories to COVID@cbc.ca, and we'll address as many as we can.

The AstraZeneca-Oxford COVID-19 vaccine has been in the headlines recently over concerns that some people were developing blood clots after receiving the shot. Some European countries suspended use of the vaccine, but many have since resumed.

You may be wondering what all of this means for you when it comes to taking the vaccine. Here's what the experts are saying.

What are the latest findings on AstraZeneca's safety?

A recent review from the European Union's drug watchdog found the vaccine is not linked to an increase in the overall risk of blood clots. The European Medicines Agency (EMA) also concluded that the benefits of protecting against COVID-19 — which itself results in clotting problems — outweigh the risks.

At the same time, the EMA said it could not definitively rule out a link between the vaccine and specific, rare types of blood clots associated with thrombocytopenia, or low levels of blood platelets.

WATCH | AstraZeneca vaccine is safe, says EU drug regulator:

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The AstraZeneca COVID-19 vaccine is safe and does not appear to cause blood clots, says the European Medicines Agency, after a review by an expert committee. But the EMA couldn't rule out a link to blood clots entirely. 1:22

Specifically, it noted 18 cases of an extremely rare type of blood clot called cerebral venous sinus thrombosis (CVST), a condition that is much more common in women than men. Most of the cases occurred within 14 days of receiving the AstraZeneca shot, and the majority were in women under the age of 55.

In its investigative document, the EMA said it would expect to see just 1.35 cases of CVST in the time period it looked at — but instead its researchers saw 12.

"A causal link with the vaccine is not proven, but is possible and deserves further analysis," the agency said in its findings.

Around the same time the EMA released its report, researchers in Germany and Norway announced they had found a mechanism that could cause the AstraZeneca vaccine to create the blood clots in very rare circumstances, in addition to identifying a possible treatment for it.

Dr. Theresa Tam, Canada's chief public health officer, acknowledged the possible link in a statement on Sunday.

"It is possible that the vaccine may be associated with very rare but serious cases of blood clots associated with thrombocytopenia," she said.

Overall, Health Canada has maintained that the benefits of the AstraZeneca COVID-19 vaccine continue to outweigh the risks. 

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Canada's Chief Public Health Officer Dr. Theresa Tam says the benefits of the AstraZeneca COVID-19 vaccine outweigh the rare risks. 1:53

Meanwhile, AstraZeneca announced on Monday that scientists found no increased risk of clots among the more than 20,000 people who received at least one dose of the shot in a late-stage study in the United States. The vaccine has not yet been given the green light in the U.S.

The company said the study also showed the vaccine provided strong protection against disease and complete protection against hospitalization and death across all age groups.

But just hours after the encouraging results were reported, the U.S. National Institute of Allergy and Infectious Diseases (NIAID) issued a statement expressing concern "that AstraZeneca may have included outdated information from that trial, which may have provided an incomplete view of the efficacy data."

AstraZeneca defended the data in a statement, saying it  included cases up to Feb. 17, as the study rules specified.


The company said it would "immediately engage" with the data safety monitoring board and release updated vaccine efficacy data within 48 hours.

Is AstraZeneca safe for people with a history of blood clots?

Amid the developments, many readers are still wondering if having a history of blood clots means they're at a higher risk of developing them after getting an AstraZeneca jab.

Some of the experts we spoke to said no, the vaccine is safe, even for people with a history of blood clots.

"Anyone who has a history of blood clots might have an increased risk of blood clots at any point anyway," Dr. Lynora Saxinger, an infectious disease specialist at the University of Alberta in Edmonton, said in a recent CBC News interview.

"But I don't believe that there would be a rationale to make a recommendation against using [the vaccine]," she said.

A medical worker receives a dose of the AstraZeneca vaccine in Taipei, Taiwan, on Monday. (Ann Wang/Reuters)

Dr. Cora Constantinescu, an infectious diseases specialist from the Vaccine Hesitancy Clinic at Alberta Children's Hospital in Calgary, said if you are still worried about blood clots, you should be more concerned about catching the coronavirus than the vaccine.

"If you looked at five million people hospitalized with COVID-19, you would expect 100,000 to 500,000 of them to have clots," Constantinescu said in a recent interview with CBC News Network.

"Keep in mind the risk of the disease itself is so much higher, and the more you wait to [get the vaccine], the less protected you are."

On the other hand, Dr. David Fisman, an epidemiologist at the University of Toronto's Dalla Lana School of Public Health, said Germany's data offered a "compelling picture" that the rare blood clots were potentially linked to the vaccine in rare cases.

"I find myself in disagreement with Health Canada's guidance on the use of AstraZeneca," Fisman said.

"I do think that the use of this vaccine should be suspended in Canada until we have more data. At a minimum, I do not think it should be used in women aged 20 to 50 until we know more."

Have blood clots been associated with the other vaccines?

If the overall rate of reported blood clots after AstraZeneca is no more than the rate in the general population, a number of readers, including Carolyn W., wanted to know why we're not hearing similar issues with the Pfizer and Moderna vaccines.

"That's a great question," Constantinescu said.

In terms of a blood clot, she said, the numbers appeared similar "across the board" for all of the vaccines, including the mRNA vaccines.

Constantinescu tried to put the figures into perspective.

"If you took five million people, you would expect 5,000 to 15,000 cases of blood clots versus the 37 that were noted," she said. "So if anything, even in the vaccinated population, there seemed to be a lower baseline rate than you would [have] in the general population."

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There haven't been any substantial reports of clotting during the clinical trials for the vaccines, Saxinger noted.

Most of the vaccine data has been "basically equal in the vaccine versus placebo recipient group," she said. "And there certainly hasn't been a strong population-based signal specifically for [deep vein thrombosis or pulmonary embolism] so far."

What if you're immunocompromised?

If you've got an autoimmune disease or are taking an immunosuppressant drug, it's probably a good idea to talk to your doctor.

That's because each condition is so different, Dr. Isaac Bogoch, an infectious disease physician in Toronto and a member of Ontario's COVID-19 Vaccine Distribution Task Force, advised in an earlier article.

In general, the experts we spoke to said AstraZeneca is probably safe for immunocompromised people, but we're still learning more.

The National Advisory Committee on Immunization (NACI), which makes recommendations on the use of newly approved vaccines in Canada, notes there is currently no data on COVID-19 vaccination in individuals who are immunosuppressed, as they were not included in the clinical trials.

"The concern is not so much about a safety issue," Saxinger said, noting that AstraZeneca is considered as safe as any other non-live vaccine. You can read more about different types of vaccines here.

British Prime Minister Boris Johnson reacts after receiving his first dose of the AstraZeneca vaccine in London on March 19. (Frank Augstein/Reuters)

The real issue, Saxinger said, is "whether or not your particular immune change will allow you to get a fully protective response."

"That's where we're still learning more."

But even if immunocompromised people have slightly less protection than others, they should go ahead and get the vaccine, she said, because even a partially protective response against a severe COVID-19 infection is worthwhile.

Constantinescu added that it's likely immunocompromised people may need booster doses in the future.

If you're offered AstraZeneca, can you refuse?

Sure. No one is going to force you to take any one vaccine if you don't want it. COVID-19 vaccines aren't mandatory.

But that doesn't mean you will immediately be offered another vaccine. That would depend on the province you live in, Constantinescu said.

For example, Quebec's director of public health has said that people who refuse to take the AstraZeneca-Oxford vaccine will be bumped to the back of the line and have to make another appointment.

And waiting could be risky, especially as some provinces are seeing case counts rise, Constantinescu said.

"You are putting yourself at risk waiting for a different vaccine when we know that this vaccine works really well at preventing hospitalization, severity and death," she said.

Have a question? 

Send your questions to COVID@cbc.ca or leave them in the comments.

With files from Adam Miller

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