Persistently High Platelet Factor 4 Levels in an Adolescent with Recurrent Late Thrombotic Complications after SARS-CoV-2 mRNA Vaccination.

Thrombosis after severe acute respiratory syndrome coronavirus 2 vaccination is a serious complication in patients with a thrombophilic predisposition. Herein, we present a 17-year-old female who had underlying antiphospholipid syndrome (APS) and developed deep vein thrombosis (DVT) 6 months after her second BNT162b2 vaccine dose. Although she had no family history of thrombosis, she had previously developed DVT at 6 years of age, with thrombus formation in the right common iliac vein and the inferior vena cava, along with concomitant left pulmonary infarction. The patient had received anticoagulant therapy for 6 years after DVT onset, with subsequent treatment cessation for 5 years without recurrence. She received the BNT162b2 vaccine at 17 years of age, 1 week before a routine outpatient visit. Platelet factor 4 elevation was detected 14 days after the first vaccination, persisting for 5 months without thrombotic symptoms. Six months after the second vaccine dose, the DVT recurred and was treated with a direct oral anticoagulant. The vaccine was hypothesized to exacerbate the patient's APS by activating coagulation. Platelet factor 4 levels may indicate coagulation status. When patients predisposed to thrombosis are vaccinated, coagulation status and platelet activation markers should be monitored to prevent DVT development.