PF4 and autoimmune thrombocytopenic purpura: It has also been found that immune thrombocytopenia was induced after vaccination with the COVID-19 vaccine Vaxzevria, and that the resulting antibodies directed against platelet factor-4 (PF4)/heparin complex induced platelet activation by binding to the FCGR2A receptor on the surface of platelets, triggering thromboembolism and even death [48].