OXFORD, U.K. — The trigger for rare blood clots occurring in patients who receive the Oxford-AstraZeneca COVID-19 vaccine may be a type of protein in the blood that is attracted to one element of the vaccine, according to a new study in the Science Advances journal, cited by the BBC.


The University of Oxford’s vaccine uses a genetically modified common cold virus from chimpanzees to carry blueprints for the coronavirus’s spike protein. This virus vector then helps program an immune response against a real coronavirus.


The new study found that if the vaccine enters the bloodstream, it can attract a type of protein called platelet factor 4.


From there, in extremely rare cases, the body’s immune system can confuse platelet factor 4 for the virus vector, and release antibodies to attack it.


When this happens, the antibodies and platelet factor 4 can cluster together, resulting in blood clots.


The BBC points out that vaccine-induced clots like these have been linked to just 73 deaths out of nearly 50 million doses of AstraZeneca given in the U.K., while AstraZeneca said the vaccine is thought to have saved more than a million lives around the world and prevented 50 million cases of COVID.


The clots are more likely to occur because of a COVID infection than the vaccine, according to a spokesperson for the company.