Scientists believe they now understand why AstraZeneca’s COVID-19 vaccine causes rare blood clots, raising the prospect of tweaking the shot to negate the risk.

A new study in Science Advances suggests the chimpanzee adenovirus vector used in the vaccine can act like a magnet for platelet factor 4 (PF4) proteins circulating in the blood.

The UK and US researchers say the resulting vector-PF4 complex can trigger an autoimmune reaction that ultimately leads to the rare thrombosis with thrombocytopenia syndrome (TTS) seen among recipients of Vaxzevria and the Johnson & Johnson COVID-19 vaccines.

The proposed mechanism, which builds upon previous research published in April, is the most comprehensive explanation for the rare clotting events to date.