ABSTRACT
Background: Vaccine-induced Thrombotic Thrombocytopenia (VITT) is a rare but potentially life-threatening complication of the ChAdOx1 COVID-vaccine, which involves binding of IgG antibodies against platelet factor 4 (PF4) to the platelet Fc-gamma receptor. This causes platelet activation with thrombosis and thrombocytopenia. Because of the similarity with heparin-induced thrombocytopenia (HIT), heparin is avoided in the acute treatment of VITT. There is limited information about the long-term persistence of anti-PF4- antibodies and their clinical relevance. Aim(s): To describe long-term clinical and serological outcomes after VITT. Method(s): A case series of patients from Leuven University Hospitals with confirmed VITT and at least 6 months of follow-up. All patients provided informed consent. Anti-PF4 antibodies were measured via chemiluminescence (HemosIL AcuStar HIT-IgG( PF4-H), Werfen) and enzyme-linked immunosorbent assay (ELISA) with immobilised polyvinylsulfonate/PF4 complexes (PF4-IgG Immucor, GTI Diagnostics);with an optical density (OD) cut-off of 0.4. Aggregation of platelets after exposure to patient plasma with 0, 1 or 200 IU/ml of heparin was measured by whole-blood impedance aggregometry (HIMEA) (Roche multiplate analyser). Result(s): Three middle-aged women presented with thrombosis with thrombocytopenia and a positive anti-PF4 ELISA 9 to 16 days after first ChAdOx1 vaccination. Their clinical presentation, lab results and treatment are summarised in Table 1. All patients recovered rapidly after non-heparin anticoagulation with (case 2-3) or without (case 1) intravenous immunoglobulins. All patients received subsequent COVID-vaccination with an mRNA-based vaccine without thrombocytopenia or symptoms. Anti-PF4- antibodies remained elevated in two patients after 3 months and in one out of three after more than 6 months, but HIMEA results for all follow-up tests became negative (Figure 1). Conclusion(s): We report good short-and long-term outcomes of three cases of VITT, including successful subsequent vaccination with an mRNA vaccine. Anti-PF4- antibodies can persist for at least several months. In contrast with the initial presentation, these persistent anti-PF4- antibodies did not trigger platelet activation in our patients. (Table Presented).