2. The criteria for the severity of COVID-19
Severe COVID-19 patients were diagnosed when conforming to one of the
following criteria: 1) Respiratory distress with respiratory frequency ≥
30/min; 2) Pulse Oximeter Oxygen Saturation ≤ 93% at rest; 3)
Oxygenation index (artery partial pressure of oxygen/inspired oxygen
fraction, PaO2/FiO2) ≤ 300 mmHg.