Two studies establishing a causal link between the AstraZeneca COVID-19 vaccine and severe thrombotic complications have recently been published. One article from a German-Austrian-Canadian collaboration and one from a group from Norway have been published online in the New England Journal of Medicine.

Both articles have concluded that the unusual severe thrombotic events along with thrombocytopenia are caused by antibodies directed against platelet factor 4 (PF4), which result in aggregation of platelets as well as thrombosis, and leads to decreased platelet count elsewhere. This eventually results in bleeding.  Patients therefore develop both severe thrombosis and severe bleeding. The syndrome is similar to the heparin-induced thrombocytopenia (HIT)… (Medscape)