Anti-platelet factor 4 immunoglobulin G levels in vaccine-induced immune thrombocytopenia and thrombosis: Persistent positivity through 7 months.

Background: Anti-platelet factor 4 (PF4) antibodies that activate platelets via FcγRIIA drive the pathophysiology of vaccine-induced immune thrombocytopenia and thrombosis (VITT). Evolution of these antibodies and their ability to activate platelets after initial treatment remains unknown. Objectives: To assess how clinical and platelet parameters, anti-PF4 antibody levels, and patient serum reactivity changes during follow-up after VITT presentation. Methods: We describe cases of seven discharged VITT patients that were followed from diagnosis up to 280 days (range 199-280) after vaccination. We measured anti-PF4 antibodies and PF4 levels in patient serum during follow-up and tested the ability of patient serum to activate healthy donor platelets and patient platelets over time. Results: Anti-PF4 immunoglobulin G antibody levels are very high at diagnosis (0.9-2.6 OD) and remain relatively high (>1.0 OD) in all patients, except one treated with rituximab, at 7 months post vaccination. All patients were on direct oral anticoagulants throughout follow-up and no patients had recurrent thrombosis. Patients' platelets during follow-up have normal FcγRIIA levels and responsiveness to platelet agonists. Patient diagnostic serum strongly activated control platelets, either alone or with PF4. Most follow-up serum alone was weaker at stimulating donor and patient platelets. However, follow-up serum beyond 150 days still strongly activated platelets with PF4 addition in three patients. Patient serum PF4 levels were lower than controls at diagnosis but returned within normal range by day 50. Conclusions: Explanations for reduced platelet activation during follow-up, despite similar total anti-PF4 antibody levels, remains unclear. Clinical implications of persistent anti-PF4 antibodies in VITT require further study.

Clinical outcomes and the impact of prior oral anticoagulant use in patients with coronavirus disease 2019 admitted to hospitals in the UK - a multicentre observational study.

Coagulation dysfunction and thrombosis are major complications in patients with coronavirus disease 2019 (COVID-19). Patients on oral anticoagulants (OAC) prior to diagnosis of COVID-19 may therefore have better outcomes. In this multicentre observational study of 5 883 patients (≥18 years) admitted to 26 UK hospitals between 1 April 2020 and 31 July 2020, overall mortality was 29·2%. Incidences of thrombosis, major bleeding (MB) and multiorgan failure (MOF) were 5·4%, 1·7% and 3·3% respectively. The presence of thrombosis, MB, or MOF was associated with a 1·8, 4·5 or 5·9-fold increased risk of dying, respectively. Of the 5 883 patients studied, 83·6% (n = 4 920) were not on OAC and 16·4% (n = 963) were taking OAC at the time of admission. There was no difference in mortality between patients on OAC vs no OAC prior to admission when compared in an adjusted multivariate analysis [hazard ratio (HR) 1·05, 95% confidence interval (CI) 0·93-1·19; P = 0·15] or in an adjusted propensity score analysis (HR 0·92 95% CI 0·58-1·450; P = 0·18). In multivariate and adjusted propensity score analyses, the only significant association of no anticoagulation prior to diagnosis of COVID-19 was admission to the Intensive-Care Unit (ICU) (HR 1·98, 95% CI 1·37-2·85). Thrombosis, MB, and MOF were associated with higher mortality. Our results indicate that patients may have benefit from prior OAC use, especially reduced admission to ICU, without any increase in bleeding.