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.

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.

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

The researchers thought the adenovirus might be linked to the rare clots occurring in some people.

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.".

It is thought the body starts to attack platelet factor four after confusing it for part of the foreign adenovirus to which it is stuck.

So antibodies are released into the blood, which clump together with platelet factor four and trigger the formation of dangerous blood clots.

The Cardiff team hope their findings can be used to improve adenovirus-based vaccines in the future to reduce the risk of these rare events!