Natural history of anti-PF4 antibodies in patients with vaccine-induced immune thrombocytopenia and thrombosis

Vaccine-induced Immune Thrombocytopenia and Thrombosis (VITT) is a new clinical syndrome discovered during the first half of 2021, when very rare cases of thrombocytopenia combined with unusual thrombosis have been reported one to two weeks after vaccination with Adenovirus-based vaccines against SARS-CoV-2. As in autoimmune heparin-induced thrombocytopenia (aHIT), VITT patients develop a high serum titer of IgG antibodies directed against platelet-factor-4 (PF4) even in the absence of heparin exposure. Initial data on the natural history of antiPF4 antibodies in large VITT-patients cohorts have been recently published suggesting a longer antiPF4 titer durability compared to what has been previously described in aHIT patients. Here we present the results of an analysis of a cohort of 10 VITT-patients, followed with serial determinations of quantitative serum antiPF4 antibodies titer and functional HIPA testing for a median follow-up duration of 6 months.