PF4 and deep vein thrombosis: Because of the widespread application of AdV-based vectors for SARS-CoV-2 immunization, it has been noted that in very rare cases, immunization would result in the severe adverse event of vaccine-induced thrombosis with thrombocytopenia syndrome (56, 57), which has been attributed to the induction of autoimmune antibodies directed against platelet factor 4 (PF4), leading to the activation of platelets and granulocytes (58–60).