Vaccines against SARS-CoV-2 are based on a range of novel vaccine platforms, with adenovirus-based approaches (like ChAdOx1 nCov-19) being one of them. Recently a rare and novel complication of SARS-CoV-2 targeted adenovirus vaccines has emerged: immune thrombocytopenia, either isolated, or accompanied by thrombosis (then termed VITT). This complication is characterized by low platelet counts, and in the case of VITT also by platelet-activating platelet factor 4 (PF4)-polyanion antibodies reminiscent of heparin-induced thrombocytopenia leading to a prothrombotic state often with clot formation at unusual anatomic sites. Here, we employ in vitro and in vivo models to characterize the possible mechanisms of these platelet-targeted autoimmune responses. We show that intravenous but not intramuscular injection of ChAdOx1 nCov-19 triggers platelet-adenovirus aggregate formation and platelet activation. After intravenous injection, these aggregates are phagocytosed by macrophages in the spleen and platelet remnants are found in the marginal zone and follicles. This is followed by a pronounced B-cell response with the emergence of circulating antibodies binding to platelets. Correspondingly, we detected -in addition to PF4 antibodies- anti-platelet antibodies targeting platelet glycoprotein receptors in 25% of patients with proven VITT (n=24). Our work contributes to the understanding of platelet associated complications after ChAdOx1 nCov-19 administration and highlights accidental intravenous injection as a potential mechanism of platelet targeted autoimmunity including PF/polyanion but also other platelet autoantibodies that determine the clinical presentation. Hence, safe intramuscular injection, with aspiration prior to injection, could be a potential preventive measure when administering adenovirus-based vaccines.