This supplementary appendix provides additional information about the study on thrombosis and thrombocytopenia after ChAdOx1 nCoV-19 vaccination. The methods section describes the HIMEA protocol used to test platelet function. Platelets from healthy blood donors were incubated with patient serum, saline buffer, and heparin at different concentrations. The HIMEA test measures impedance over time to assess platelet aggregation. A positive result requires specific patterns of aggregation in different conditions. Figure S1 shows brain CT scans and thrombus retrieval during venous thrombectomy in case 5. It illustrates cerebral venous sinus thrombosis and the use of mechanical endovascular thrombectomy to improve cerebral venous outflow. Figure S2 presents serial dilution of patient sera in an ELISA assay to detect IgG anti-PF4/polyanion antibodies. The study highlights the importance of identifying and managing thrombosis and thrombocytopenia following vaccination. It also discusses the use of endovascular treatment for cerebral venous sinus thrombosis, particularly in cases where medical therapy is ineffective. The findings emphasize the need for careful monitoring and appropriate interventions in patients with suspected heparin-induced thrombocytopenia (HIT). References are provided for further reading on endovascular treatment of cerebral venous sinus thrombosis.This supplementary appendix provides additional information about the study on thrombosis and thrombocytopenia after ChAdOx1 nCoV-19 vaccination. The methods section describes the HIMEA protocol used to test platelet function. Platelets from healthy blood donors were incubated with patient serum, saline buffer, and heparin at different concentrations. The HIMEA test measures impedance over time to assess platelet aggregation. A positive result requires specific patterns of aggregation in different conditions. Figure S1 shows brain CT scans and thrombus retrieval during venous thrombectomy in case 5. It illustrates cerebral venous sinus thrombosis and the use of mechanical endovascular thrombectomy to improve cerebral venous outflow. Figure S2 presents serial dilution of patient sera in an ELISA assay to detect IgG anti-PF4/polyanion antibodies. The study highlights the importance of identifying and managing thrombosis and thrombocytopenia following vaccination. It also discusses the use of endovascular treatment for cerebral venous sinus thrombosis, particularly in cases where medical therapy is ineffective. The findings emphasize the need for careful monitoring and appropriate interventions in patients with suspected heparin-induced thrombocytopenia (HIT). References are provided for further reading on endovascular treatment of cerebral venous sinus thrombosis.