We enrolled 34 COVID-19 patients admitted to the West District of Union Hospital of Tongji Medical College from February 5th to February 13rd, 2020. COVID-19 was diagnosed upon admission based on the New Coronavirus Pneumonia Prevention and Control Program (4th edition).5 Severe COVID-19 was defined as having either one of the flowing criteria: (1) Respiratory distress with respiratory rate more than 30 times/min; (2) Oxygen saturation ≤93% in resting state; (3) PaO2/FiO2≤300 mmHg (1 mmHg = 0.133 kPa);