Patient platelet count and functional activation in VIT. Patient platelet count (panel A) and investigation of platelet activation using a modified SRA with exogenous addition of spike protein (panel B) or vaccine (panel C). The platelet count fully recovered by day 7 of treatment with dexamethasone (Dex) and IVIg. Serum from the patient (green squares) caused dose-dependent platelet activation and serotonin release with spike protein (93%; 100 µg/mL) and with vaccine (53%; 50 µL/mL). This effect was not observed with plasma from patients who recovered from severe (n = 5; red circles) or mild infection (n = 3; blue circles) with COVID-19. The activation was inhibited with FcγRIIa blockade using the monoclonal antibody IV.3 (5 µg/mL) or IVIg (400 µg/mL).