Platelet count, fibrinogen, D-dimer, and treatment over time. The X-axis indicates the days following Ad26.COV2.S vaccination. The left Y-axis indicates platelet count and fibrinogen concentration; the right Y-axis indicates D-dimer concentration. Normal range for platelet count (140-450 ×109/L), fibrinogen (202-430 mg/dL), d-dimer (<500 ng/mL). *>14 000 is the maximum d-dimer value reported by our laboratory. Hospitalization, the gray-shaded column, indicates the time from presentation to our emergency department until discharge from the hospital. IVIG (1 g/kg) was administered to increase platelet count to allow for safe initiation of anticoagulation. Cryoprecipitate (first dose 10 units; second dose 5 units) was given to improve fibrinogen levels to allow for safe initiation of anticoagulation. Prednisone 1 mg/kg per day with taper. Therapeutic argatroban was dosed for target activated partial thromboplastin time of 60 to 80 seconds. Apixaban 10 mg twice daily ×7 days followed by 5 mg twice daily.