Scientists believe they have discovered the reason for some people experiencing extremely rare blood clot complications after getting the AstraZeneca COVID-19 vaccine.

A new study from a team of British and US researchers suggests that an interaction between the vaccine and a particular protein in blood could trigger thrombosis in some people.

The team’s findings suggest that the way the viral vector – which is used to transport COVID-19’s genetic material into cells – binds to a protein called platelet factor four, which could trigger an immune response that can result in blood clots.

Professor Alan Parker, an expert from Cardiff University’s School of Medicine and an author of the study, said thrombosis “only happens in extremely rare cases because a chain of complex events needs to take place to trigger this ultra-rare side effect”.

After the serious side effect prompted some countries to restrict its use and promote alternative vaccines, an investigation was started into the causes of the clots and any potential preventions.

The search involved AstraZeneca’s own scientists, who joined after the team published early findings.

The latest study has been published in the journal Science Advances.

Despite the restrictions on its usage, the AstraZeneca vaccine is thought to have saved more than a million lives around the world, while preventing 50 million cases of COVID-19.

Image: Getty Images

This article first appeared on Over60.