Abstract Background. Vaccines are important for managing the COVID-19 pandemic caused by SARS-CoV-2. However, following widespread vaccination using a recombinant adenoviral vector encoding the spike protein antigen of SARS-CoV-2 (AZD1222, AstraZeneca), reports have emerged of some vaccine recipients developing unusual thrombotic events and thrombocytopenia. We investigated whether such patients could have a prothrombotic disorder caused by platelet-activating antibodies directed against platelet factor 4 (PF4), as is known to be caused by heparin and sometimes other environmental triggers.Methods. We summarized the clinical and laboratory features of 9 patients in Germany and Austria who developed thrombosis and thrombocytopenia events following AZD1222 vaccination. Serum from four patients was used to test for anti-PF4/heparin antibodies, both by immunoassay and by platelet activation assays performed in the presence of heparin, PF4, or both.Results. The 9 patients (8 female; median age, 36 [range, 22—49) presented with thrombosis beginning 4 to 16 days post-vaccination: 7 patients had cerebral venous thrombosis (CVT), 1 had pulmonary embolism, and 1 had splanchnic vein thrombosis and CVT; 4 patients died. None had received heparin prior to symptom onset. All four patients tested strongly positive for anti-PF4/heparin antibodies by immunoassay; all 4 patients tested strongly positive in the platelet activation assay in the presence of PF4 independently of heparin. Platelet activation was inhibited by high concentrations of heparin, Fc receptor-blocking monoclonal antibody, and intravenous immunoglobulin.Conclusions. The AZD1222 vaccine is associated with development of a prothrombotic disorder that clinically resembles heparin-induced thrombocytopenia but which shows a different serological profile.