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Impaired neutrophil extracellular trap (NET) formation compromises the host defense after engraftment following hematopoietic stem cell transplantation (HSCT) despite adequate neutrophil counts. The aims of the present study were to determine reference ranges for the activity of key enzymes of NET formation-neutrophil elastase (NE) and myeloperoxidase (MPO)-in a healthy population and to unravel the recovery dynamics of NET formation over time following HSCT, along with NE and MPO enzymatic activities. Reference ranges of NE and MPO activity were derived from 50 healthy volunteers. During 2017 to 2018, 11 consecutive pediatric patients undergoing allogeneic or autologous HSCT were recruited at a single referral center for pediatric hemato-oncology. Patients were followed for up to 1 year following engraftment. The mean reference value was 7.5 § .4 mU for NE activity and 2.17 § .4 U for MPO activity in the healthy population, and enzymatic activity of MPO was significantly higher in males. At 3 weeks following neutrophil engraftment, all study participants demonstrated extremely low enzymatic NE activity, whereas MPO activity was above the lower normal reference range at all time points. Reduced NE activity corresponded to the inability to form NETs. Neutrophil function improved over time, but partial impairment persisted for 7 months following transplantation. The ability of neutrophils to form NETs was significantly impaired for 3 weeks after engraftment in the setting of HSCT, exposing patients to bacterial infections. NE activity might serve as a surrogate marker for the capacity of neutrophils to form NETs.

Real world studies have demonstrated impressive effectiveness of the BNT162b2 COVID-19 vaccine in preventing symptomatic and asymptomatic SARS-CoV-2 infection. We describe an outbreak of SARS-CoV-2 infections in a hospital with high vaccine uptake. We found low secondary attack rate(7%), suggesting low infectivity of vaccinated persons with vaccine breakthrough SARS-CoV-2 infections.

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