inclusion criterion, at least one of the following conditions: 1) respiratory rate >= 30 breaths/min, 2) peripheral capillary oxygen saturation (SpO2) <= 93% while breathing room air, 3) PaO2/FiO2 <=300 mmHg. Patients with critical respiratory syndrome, needing mechanical ventilation at onset, were not included. Only confirmed cases of COVID-19, defined by a
positive result on a reverse-transcriptase–polymerase-chain-reaction (RT-PCR) assay of a specimen collected on a nasopharyngeal swab, were considered.