Definition
Fever was defined as an axillary temperature of 37.3°C or higher. Hypoxemia was defined as PaO2/FiO2 < 300 mmHg [18]. Acute respiratory distress syndrome (ARDS) was diagnosed according to ARDS Berlin definition, i.e. severe hypoxemia (PaO2/FiO2≤ 300 mmHg with PEEP ≥ 5 cmH2O), associated with bilateral opacities on chest X-ray, which could not be fully explained by cardiac failure or fluid overload [29]. Obesity was considered if patients with a body mass index of more than 28 kg/m2. Pneumonia severity was assessed by CURB-65 [30] and the Pneumonia Severity Index (PSI) [31]. Severe pneumonia was according to modified ATS criteria [23]. Viral and bacterial/fungal superinfection was defined as the detection of both viruses and bacteria/fungi [13, 18]. Mixed viral infection was defined as concurrent detection of both virus detection of ≥ 2 viruses. Leukocytosis: leukocyte count > 10,000/mm3. Neutrophilopenia: neutrophilic granulocyte count < 1500/mm3. Lymphocytopenia: lymphocyte count < 1000/mm3. Anemia: hemoglobin <120 g/L for men and <110 g/L for women. Thrombocytopenia: platelet count < 150,000/mm3.