The "trigger" behind a rare Covidvaccine complication has been discovered, according to new research.
Scientists say they have figured out why some people have experienced blood clots after receiving the Oxford/AstraZeneca jag.
Concerns about the rare reaction has shaped how the vaccine has been used around the world.
In Scotland and across the UK, those under the age of 40 were offered an alternative to the AstraZeneca jag in a bid to avoid the risk of blood clots.
However, scientists now believe they have identified why the worrying complication sometimes happens.
They think the vaccine kicks off a chain reaction, involving the immune system, that can culminate in clots. This is due to protein in the blood being attracted to key components of the jag.
The experts - lead by a team from Arizona State University and Cardiff University - investigated the reasons behind reactions and side effects.
The findings could help prevent dangerous reactions in the future as researchers have now began looking at how the complications 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.
Back in May, the UK's medicines safety regulator said there had been 242 clotting cases and 49 deaths, with 28.5 million doses of the vaccine administered.
However, scientists and authorities have also argued that the benefits of taking the vaccine outweigh the risk.
A spokeswoman for AstraZeneca stressed that clots were more likely to occur because of a Covid infection than the vaccine.
A complete understanding of why the infection could cause clots has not yet been reached.
"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.
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