PF4 and Thrombocytopenia: Patients who never tested positive for SARS-CoV-2 infection underwent TTS/VITT because of the following mechanisms (21): IgG anti-PF4/heparin antibodies present; no proximate exposure to heparin, “severe” thrombocytopenia; temporal association (events observed 14 days after vaccination); FcɤRIIa-dependent platelet activation (resembles “spontaneous” or “autoimmune” HIT).