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.