Key Points Purification of staged human erythroblasts should facilitate a comprehensive cellular and molecular characterization of these cell populations. Quantification of human terminal erythropoiesis in vivo provides a powerful means for studying impaired erythropoiesis in human diseases.

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is associated with thrombotic complications in adults, but the incidence of COVID-19 related thrombosis in children and adolescents is unclear. Most children with acute COVID-19 have mild disease, but coagulopathy has been associated with multisystem inflammatory syndrome in children (MIS-C), a post-infectious complication. We conducted a multicenter retrospective cohort study to determine the incidence of thrombosis in children hospitalized with COVID-19 or MIS-C and to evaluate associated risk factors. We classified patients into one of three groups for analysis: COVID-19, MIS-C, or asymptomatic SARS-CoV-2. Among a total of 853 admissions (426 COVID-19, 138 MIS-C, and 289 asymptomatic SARS-CoV-2) in 814 patients, there were 20 patients with thrombotic events (TE) (including 1 stroke). Patients with MIS-C had the highest incidence (6.5%, 9/138) versus COVID-19 (2.1%, 9/426) or asymptomatic SARS-CoV-2 (0.7%, 2/289). In patients with COVID-19 or MIS-C, the majority of thrombotic events (89%) occurred in patients ≥12 years. Patients > 12 years with MIS-C had the highest rate of thrombosis at 19% (9/48). Notably, 71% of TE that were not present on admission occurred despite thromboprophylaxis. Multivariable analysis identified the following as significantly associated with thrombosis: age ≥12 years, cancer, presence of a central venous catheter, and MIS-C. In patients with COVID-19 or MIS-C, hospital mortality was 2.3% (13/564), but was 28% (5/18) in patients with thrombotic events. Our findings may help inform pediatric thromboprophylaxis strategies.

Background: Observational studies indicate that children hospitalized with COVID-19-related illness, like adults, are at increased risk for venous thromboembolism (VTE). A multicenter phase 2 clinical trial of anticoagulant thromboprophylaxis in