AbstractBackgroundAnti-Platelet Factor 4 (PF4) IgG antibodies that activate platelets via FcγRIIa have been shown to be an important part of the pathophysiology of vaccine-induced immune thrombocytopenia and thrombosis (VITT). There is now extensive literature on its presentation and initial management. There is no literature however on what happens to these patients following discharge.MethodsWe collected clinical data and samples from seven patients presenting with VITT and followed them up for 82-145 days. We also collected clinical samples from them at last follow-up. Testing for anti-PF4/heparin antibodies was done using an anti-PF4/heparin enzymatic immunoassay. Flow Cytometry was used to look at FcγRIIa levels on patient platelets. Light Transmission Aggregometry with patient serum and healthy donor / patient platelets was used to analyse platelet responsiveness, in the presence and absence of PF4.FindingsAll patients were discharged on direct oral anticoagulants. Two patients remain completely symptom free, three have ongoing headaches, two have residual neurological deficits. Two patients developed mild thrombocytopenia and worsening headache (but without cerebral venous sinus thrombosis) and were retreated, one of these with rituximab. All patients, except the one treated with rituximab, had similar anti-PF4 antibody titres at 80-120 days to their levels at diagnosis. Platelets from patients at follow-up had normal levels of FcγRIIa and had normal responses to thrombin and collagen-related-peptide. Patient serum from diagnosis strongly activated healthy donor platelets in the presence of PF4. Serum from follow-up was much weaker at stimulating platelets, even in the presence of PF4.InterpretationThis study shows that despite similar PF4 antibody titres at diagnosis and during follow-up, there are further differences in patient serum, that are not apparent from currently used testing, that result in lower levels of platelet activation during the follow-up period. Further understanding of these factors are important in order to assess duration of anticoagulation for these patients.FundingThis work was supported by an Accelerator Grant (AA/18/2/34218) from the British Heart Foundation (BHF) and by a National Institute for Health Research (NIHR) grant.Key pointsPF4 antibody titres do not reduce up to 4-months post ChAdOx1 nCoV-19 in patients with VITTDespite similar PF4 antibody titres, diagnostic serum is more potent at activating platelets in the presence of PF4 than follow-up serum.