Diagnostic criteria
The diagnostic criteria were based on the Chinese Novel Coronavirus Infection Treatment Protocol (Trial Version 10). Positive nucleic acid or antigen was diagnosed as Coronavirus infection. Patients were divided into four categories according to their severity as follows:
Mild COVID-19: Main manifestation was upper respiratory tract infection, such as dry throat, sore throat, cough, and fever.
Medium COVID-19: Persistent high fever for >3 days and/or cough and shortness of breath; respiratory rate (RR) <30 breaths/min and oxygen saturation > 93% when air is inhaled at rest. Characteristics of COVID-19 infection based on imaging of pneumonic manifestations
Heavy COVID-19: Any of the following criteria were met and could not be explained by anything other than COVID-19 infection (i) shortness of breath with RR≥30 beats/min; (ii) oxygen saturation at rest ≤93% on air inhalation; (iii) arterial blood oxygen partial pressure (PaO2)/inhaled oxygen concentration (FiO2)≤300 mmHg; (iv) Progressive clinical symptoms with significant progression of >50% of the lesion within 24–48 h on lung imaging.
Critical COVID-19: One of the following conditions was met: (i) respiratory failure that required mechanical ventilation, (ii) shock, and (iii) ICU monitoring and treatment combined with other organ failure.