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Scientists find clues to what triggers rare blood clots in AstraZeneca coronavirus vaccine

The Washington Post logo The Washington Post 12/2/2021 Ellen Francis
Syringes on a production line at a factory in South Korea on Feb. 22, 2021. © SeongJoon Cho/Bloomberg Syringes on a production line at a factory in South Korea on Feb. 22, 2021.

LONDON — Scientists say they have identified clues to what triggered blood clots in rare cases of people who took AstraZeneca’s coronavirus vaccine.

The very rare but sometimes deadly clots led to pauses in the vaccine’s rollout early this year in Europe, and had public health experts worried it could fuel hesitancy, even as they stressed that the benefits of the vaccine far outweighed any risks.

The vaccine, developed in collaboration with the University of Oxford and widely used in European countries and others such as Canada and India, has not been approved in the United States. The United States did pause use of the Johnson & Johnson vaccine, after a handful of blood clot cases were reported among the millions of people who received those shots, but lifted it after a safety review.

The blood clot cases reported after some injections of the Oxford-AstraZeneca vaccine have divided regulators over how and whether to administer it, with France, Britain and Canada limiting use of the shot in younger people, while Denmark and Norway stopped administering it altogether. Scientists have said an antibody response may have triggered the clotting in some cases, with one researcher in Germany likening the search to “looking for the needle in the haystack.”

What you need to know about AstraZeneca’s coronavirus vaccine

In a new study published Wednesday in the Science Advances journal, researchers, including some at Cardiff University and Arizona State University, say they have found that a protein in the blood, “platelet factor 4,” can bind to a key element of the AstraZeneca vaccine, potentially triggering a reaction, with antibodies attacking the protein. However, questions remain over whether the binding is a direct cause of the blood clots.

A statement from AstraZeneca, whose scientists joined the research, said the “preclinical research further advances our knowledge about the potential mechanisms underlying the extremely rare condition … which is treatable for the majority of people.”

The pharmaceutical company based in England said the full reasoning behind why the clotting condition happens still needs to be determined, and noted that coronavirus infections were more likely to cause clotting than the vaccine. “Although the research is not definitive, it offers interesting insights, and AstraZeneca is exploring ways to leverage these findings as part of our efforts to remove this extremely rare side effect.”

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The condition has been linked to 73 deaths out of nearly 50 million doses administered in the United Kingdom, according to a British government agency, which notes that blood clots have also been reported with coronavirus infections. “This compares with the clear demonstrable benefits from the covid-19 vaccination” program, which has helped prevent more than 80,000 deaths, the UK Health Security Agency said.

One of the authors of the latest study, Alan Parker, acknowledged that more research was needed.

“We’ve been able to prove the link between the key smoking guns of adenoviruses and platelet factor four. What we have is the trigger, but there’s a lot of steps that have to happen next,” Parker, a professor at Cardiff University, told the BBC.


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