Dr. Andreas Greinacher led the team that discovered the link between the AstraZeneca vaccine and a rare blood clotting disorder, but he wants people to know the risk of developing the clots is low. Greinacher found that in some patients, the AstraZeneca vaccine was triggering antibodies that set off a platelet response. That means the the antibodies made the platelets, small blood cells that help form clots to stop bleeding, behave as though they were treating a wound and trigger clotting. At a press conference on Wednesday, Greinacher said that the causal link is still unknown, and there could be co-factors in the small number of patients who developed vaccine-induced prothrombotic immune thrombocytopenia (VIPIT). VIPIT is the name given to the type of clotting that seems to occur as a rare reaction to the AstraZeneca vaccine. “It could very well be that in these individual patients are two or three co-factors present at the same time, which together at the time point of vaccination trigger these antibodies,” he said.