Scientists have identified a possible reason the AstraZeneca Covid-19 vaccine may cause blood clots, after use of the shot was limited globally to prevent the rare side effect.

The preclinical research, conducted with Astra, found the interaction between the vaccine and a protein known as platelet factor 4 could be behind the cases of thrombosis with thrombocytopenia syndrome, according to a study published on Wednesday in the journal Science Advances by scientists from US and UK universities, as well as Astra. The research is not definitive, the company said.

The Astra vaccine, co-developed with the University of Oxford, has been hampered globally by a possible link between the shot and rare cases of blood clots, with the UK limiting its use to those over 40 and the US yet to authorise the vaccine. In May, German scientists published a hypothesis that the side effect was linked to the adenovirus vector the inoculation uses...