PF4 and Thrombocytopenia: Herein, we report a case of a young woman who developed a combination of severe thrombocytopenia and extensive venous thrombosis of the inferior vena cava and lower limb, concurrent with detectable anti-PF4 antibody and a lupus anticoagulant, occurring 2 weeks after a booster dose of the Pfizer-BioNTech COVID-19 mRNA vaccine tozinameran, BNT162b2.