Confirmatory enzyme-linked immunosorbent assay tests showed an increase in antibodies against platelet factor 4 (PF4), which presumably activated a large number of platelets and caused a rare form of immune thrombotic thrombocytopenia, later named vaccine-induced prothrombotic immune thrombocytopenia (VIPIT). This evidence concerns the gene PF4 and autoimmune thrombocytopenic purpura.