Scientists identify reaction that may cause rare blood clots after AstraZeneca Covid vaccine

Scientists claim to have discovered a “potential mechanism” that triggers rare blood clots in some people who receive the Oxford-AstraZeneca Covid vaccine.

Links between the vaccine and rare, sometimes fatal, blood clots have prompted some countries to restrict its use to older adults or favor alternative shots.

To be sure, medical experts have repeatedly said the benefits of the vaccine outweigh the risks associated with it, with one study concluding that Covid-19 poses a much bigger threat of blood clots than vaccination.

Scientists from Cardiff University in the U.K. and Arizona State University in the U.S. worked with AstraZeneca to investigate vaccine-induced blood clots, publishing their findings on Wednesday in the journal Science Advances.

The study suggested that the shot’s viral vector — the vaccine ingredient used to transport the coronavirus’ genetic material into a recipient’s cells — could be the issue.

In the Oxford-AstraZeneca vaccine, the viral vector is an adenovirus found in chimpanzees. The adenovirus is engineered to match Covid-19’s spike protein — a key part of its structure used to invade human cells.

Johnson & Johnson‘s Covid-19 vaccine also uses an adenovirus to carry spike proteins from the coronavirus into human cells to trigger an immune response and has similarly been linked to rare blood clots.

The team behind the study said they believed the chimpanzee virus used in the Oxford-AstraZeneca vaccine had a specific interaction that could prompt the body’s defenses to act against itself.

According to the study, the viral vector in the Oxford-AstraZeneca vaccine, in rare cases, enters the bloodstream, where it can bind to a protein called platelet factor 4 (PF4).

Once the viral vector has bound to the protein, the immune system views it as foreign, the study’s authors said. “Misplaced immunity” may trigger a release of antibodies against PF4, which then binds to and activates cells that help blood clot, causing those cells to cluster together and generate blood clots.  

The study’s authors emphasized that the phenomenon occurs in “a very small number of people.”

Alan Parker, a professor at Cardiff University’s School of Medicine, said if a procedure responsible for the rare vaccine-induced blood clots was established, it may help to prevent and treat the disorder. 

“Vaccine-induced immune thrombotic thrombocytopenia [blood clots] only happens in extremely rare cases because a chain of complex events needs to take place to trigger this ultra-rare side effect,” he said in a press release Wednesday.

“We hope our findings can be used to better understand the rare side effects of these new vaccines, and potentially to design new and improved vaccines to turn the tide on this global pandemic.”