Health Canada said that extremely rare blood clots “may be linked” to use of the AstraZeneca vaccine.
In a statement released Wednesday (April 14), the agency said that its findings are in line with other regulatory agencies. The potentially AstraZeneca-linked blood clots are specifically called central venous sinus thrombosis (CVST) and are associated with low levels of blood platelets. The vaccine showed an effectiveness of about 62 per cent in preventing symptomatic COVID-19 two weeks following the second dose.
“As a result, the Department has updated warnings in the product information to inform Canadians of the possible side effects and to provide information about the signs and symptoms and when to seek prompt medical attention following vaccination,” the agency said.
However, “no specific risk factors” have been identified so Health Canada is not restricting the use of the vaccine for any population. Dr. Supriya Sharma, chief medical adviser at Health Canada, said that people who receive the AstraZeneca vaccine should look for the following symptoms four to 20 days post-shot: really serious persistent headaches, persistent serious abdominal pain, pain in your limbs that’s quite severe and persistent or any unusual bruising other than at the injection site.
Wednesday’s update comes the day after the Public Health Agency of Canada was alerted of the first case of blood clots in a Quebec woman who had received the AstraZeneca vaccine. She has been treated and is recovering at home.
Sharma said that her agency still believes that “the benefits of AstraZeneca vaccine outweigh the risks in all age groups” and that individuals should follow government recommendations on COVID-19 vaccines.
“Get whichever vaccine is available to you. The longer you wait to get vaccinated the longer you’re not protected,” Sharma said.
Sharma said there have been 484,000 doses of the AstraZeneca vaccine administered in Canada – although that figure is about 10 days out of date – but that Canada is looking at international data, including from the U.K. to determine the risks of the vaccine.
Health Canada has determined the rate of potentially AstraZeneca related blood clots to be four in one million based on a U.K. data set because it is “the most complete,” Sharma said, based on the number of doses doled out there and their post-vaccination program.
Overall, Sharma said the rate can vary from one in 100,000 and one in 250,000 – the latter of which is equivalent to four in one million.
Sharma said that comparisons between the AstraZeneca blood clots and those caused by other medications, such as birth control, are are to compare because they are different forms of clots.
However, Sharma said that the risks of blood clots from the following are:
- In women ages 15 to 45: 1 in 3,300
- Birth control pill: 1 in 1,600
- Pregnant women: 1 in 300
- Post-partum: 1 in 100
- Hospitalized with COVID-19: 1 in 5
“The risks of regular clots with COVID is much, much higher than with other things we talk about in terms of benefits and risks and it’s much much much higher than having this very very severe clot,” Sharma said.
The CVST clots are “extremely, extremely rare,” she said.
“One of the hypotheses in what’s causing this is really an activation of the immune system that then leads to platelets being activated, causing clots and then because those platelets are consumed in the process, the overall count goes down,” Sharma said, noting that this is similar to what can be seen as a result of Heparin, a blood thinner medication.
“We still have Heparin on the market. We still use it in prevention, we still use it in syringes to prevent clots… it’s helpful sometimes putting in context with other types of clots, but really, the thing we’re looking at is this very, very rare chance of having this severe clot that you can diagnose and treat and potentially getting sick with COVID, getting seriously ill and potentially dying.”
The National Advisory Committee on Immunizations recommendations to not use the AstraZeneca vaccine for people under the age of 55 remains in effect.
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