In rare cases – estimated to be around 4-6 of every one million people - some individuals who receive the AstraZeneca COVID-19 vaccine develop blood clots that can pose a risk to their health.
Now, scientists believe they have discovered the trigger as to why this happens.
The team – from Cardiff in Wales and the United States – have shown how a protein in the blood is attracted to a key component of the vaccine like a magnet. This attraction invokes an immune response that can see blood clots form.
However, all of this relies on a series of unlucky events, which is why the number of people who experience such clots is so low.
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."
The scientists' study is published in the journal Science Advances.
*Image by Angelo Esslinger from Pixabay