Pathologic Antibodies to Platelet Factor 4 after ChAdOx1 nCoV-19 Vaccination

This case series describes 23 patients who developed thrombosis and thrombocytopenia 6–24 days after receiving the first dose of the ChAdOx1 nCoV-19 (AstraZeneca) vaccine. Most patients had cerebral venous thrombosis; others presented with pulmonary embolism, deep vein thrombosis, or arterial thrombosis. Lab findings revealed elevated D-dimer and low to normal fibrinogen levels. All but one tested positive for anti–platelet factor 4 (PF4) antibodies despite no exposure to heparin, indicating a heparin-independent platelet-activating immune response. Functional HIT assays confirmed platelet activation in multiple cases. The study recommends avoiding platelet transfusions, initiating non-heparin anticoagulation, and considering intravenous immune globulin. The authors conclude that a rare, PF4-dependent thrombotic syndrome can follow ChAdOx1 vaccination, and early recognition is essential for appropriate treatment.

This case series describes 23 patients who developed thrombosis and thrombocytopenia 6–24 days after receiving the first dose of the ChAdOx1 nCoV-19 (AstraZeneca) vaccine. Most patients had cerebral venous thrombosis; others presented with pulmonary embolism, deep vein thrombosis, or arterial thrombosis. Lab findings revealed elevated D-dimer and low to normal fibrinogen levels. All but one tested positive for anti–platelet factor 4 (PF4) antibodies despite no exposure to heparin, indicating a heparin-independent platelet-activating immune response. Functional HIT assays confirmed platelet activation in multiple cases. The study recommends avoiding platelet transfusions, initiating non-heparin anticoagulation, and considering intravenous immune globulin. The authors conclude that a rare, PF4-dependent thrombotic syndrome can follow ChAdOx1 vaccination, and early recognition is essential for appropriate treatment.