Why rare blood clots could be a side effect of AstraZeneca COVID-19 vaccine

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The European Medicines Agency has ruled a link is “possible” between the AstraZeneca COVID-19 vaccine and rare but serious blood clots. Now the EU drug regulator says the disorder should be listed as a side effect of the shot.

The agency still recommends AstraZeneca for adults of all ages, but Britain has paused it for people under age 30.

Canada continues to pause the product for under age 55 but is reviewing the data.

Of the 20 million doses given in the U.K. so far, 79 people developed clots. Nineteen of those people died. There is no clear link between the deaths and the vaccine.

Meanwhile, medical experts are learning more about why and how the “unusual” clots could happen.

“These clots tend to occur in unusual areas, like the brain or the gut, and they also tend to be quite aggressive,” Dr. Menaka Pai told Global News.

The Hamilton hematologist and thrombosis medicine physician says the AstraZeneca vaccine appears to trigger antibodies directed against platelets, the sticky blood cells that form clots.

“So these antibodies attack the person’s platelets… They switch them on and they cause blood clotting,” said Pai.

Then the antibodies destroy the platelets, leading to the low platelet count seen in some patients after the vaccine.

“There was a very low rate of clotting cells (platelets) in some of the patients with these rare clotting complications, which would be a very unusual feature for some of your garden-variety clotting complications,” Dr. Thalia Field told Global News.

“This seems to be an extremely rare autoimmune, kind of body-attacking-itself, type of reaction,” said Field.

The Vancouver stroke neurologist says typical risk factors for blood clots, including blood vessel damage, hormone replacement therapy and family history, don’t seem to play a role in this reaction.

“It’s not the sort of thing that (doctors) are consistently seeing in people with a history of clots or necessarily other risk factors,” said Field.

“That’s the million-dollar question: Who is more at risk? Is somebody more susceptible?” said Pai.

Both doctors point out the risk of serious clots with COVID-19 is still much higher than the possible risk with AstraZeneca.

While even typical blood clots affect one or two people in every 1,000, about one in 200 people with COVID-19 will develop clots.

The risk goes up as the person gets sicker – if they’re hospitalized, it’s one in 20. And the rate is one in five if they need intensive care.

COVID-19 can cause inflammation in every organ of the body, and hospitalization means lying in bed for long periods of time, leading to blood pooling. Both can cause clots.

“So I would definitely recommend getting vaccinated. We know COVID is also associated with stroke, it’s associated with lung problems, it’s associated with chronic fatigue and other problems with thinking,” said Field.

“If (AstraZeneca) were the vaccine available to me, I would get it. I would feel comfortable with my family members getting it as well,” added Field.

Aviva Rappaport of Vancouver had the same rare clotting disorder possibly linked to the vaccine: cerebral venous thrombosis. It happened out of the blue two years ago, before the pandemic.

“I know that it’s a rare type of stroke so hearing that a vaccine was linked to (cerebral venous thrombosis) was a little bit surprising,” said Rappaport.

The 29 year old says it took about a year of rehabilitation to feel normal again. She struggled to speak and understand language.

Despite their daughter’s ordeal, her parents chose to follow the doctors’ advice.

“They both got the AstraZeneca vaccine here in B.C., even though it has been linked to this type of stroke and they saw me go through everything,” said Rappaport.

“I have faith and trust in the medical community… and my parents and sister do too.”

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