Trigger of rare blood clots with AstraZeneca jab found by scientists

Scientists believe they have found “the trigger” that leads to extremely rare blood clots after the Oxford-AstraZeneca Covid vaccine.

The team – in Cardiff and the US – have shown in exquisite detail how a protein in the blood is attracted to a key component of the vaccine.

They think this kicks off a chain reaction, involving the immune system, that can culminate in dangerous clots.

The vaccine is thought to have saved about a million lives from Covid.

However, concerns about rare blood clots shaped how the vaccine has been used around the world including an alternative being offered to the under-40s in the UK.

It also started a scientific detective hunt to figure out what was going on and if it could be prevented. The Cardiff team were given emergency government funding to find the answers.

AstraZeneca’s own scientists also joined the research project after earlier results from the team were published.

A spokeswoman for AstraZeneca stressed that clots were more likely to occur because of a Covid infection than the vaccine, and that the complete explanation for why they occur had not yet been established.

“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,” she added.

There were two initial clues for the researchers investigating the rare blood clots:

  • The greater risk of clots was seen only with some of the vaccine technologies
  • People with clots had unusual antibodies that were attacking a protein in their blood called platelet factor four

The vaccines used in the UK all try to deliver a snippet of the Covid-virus’s genetic code into the body to train the immune system.

Some package that code up inside spheres of fat, while the AstraZeneca one used an adenovirus (specifically a common cold virus from chimpanzees) as its microscopic postman.

The researchers thought the adenovirus might be linked to the rare clots occurring in some people. So they used a technique called cryo-electron microscopy to take images of the adenovirus in molecular-level detail.

Their study, published in the journal Science Advances, reveals the outer surface of the adenovirus attracts the platelet factor four protein to it like a magnet.

Prof Alan Parker, one of the researchers at Cardiff University, told BBC News: “The adenovirus has an extremely negative surface, and platelet factor four is extremely positive and the two things fit together quite well.”

He added: “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.”

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