Patient Inclusion Criteria

Inclusion criteria of Cohort 1 #4 is adapted from the Flu guideline in China (2018) and community acquired pneumonia (CAP) guideline in China (2016) Cohort 1: Subjects must meet all of the following inclusion criteria at the time of randomization to be eligible for participation in this study: 1. Males and Females ≥18 years old 2. Diagnosed as influenza (IFV) infection within 3 days before randomization 3. Requires, at the time of randomization, supplemental oxygen ≥2 LPM due to hypoxemia 4. Subjects are severely ill 5. In the opinion of investigator, subjects will be hospitalized at least 1 week. 6. If female, subject must not be pregnant or nursing Cohort 2: Subjects must meet all of the following inclusion criteria at the time of randomization to be eligible for participation in this study: 1. Males and Females ≥18 years old 2. Hypoxemia 3. Subjects fulfill one of the following conditions: 1. IFV subjects who are eligible for all inclusion criteria of Cohort 1 except for acute hypoxemia at enrollment. 2. Subjects confirmed with non-IFV SAD viral infection. 4. Same in inclusion criteria #5 to #8 in Cohort 1.

Patient Exclusion Criteria

Cohort 1 and 2: Subjects who meet any of the following exclusion criteria are not to be enrolled in this study: 1. Subjects requiring mechanical, Bi-PAP or CPAP ventilation at randomization. 2. Life expectancy less than 30 days. 3. Subjects with conscious disturbance (slow response, drowsy, restlessness, anxiety, confusion, twitching or convulsion) 4. Subjects with unstable hemodynamics such as systolic blood pressure < 90 mmHg or septic shock 5. Subjects with BUN≧7.14 mmol/L 6. Subjects treated with inhaled anti-viral therapy and washout period ≦ 48 hours. 7. If Alanine Aminotransferase (ALT), Aspartate Aminotransferase (AST) or Alkaline Phosphatase (ALP) are ≥3x ULN and Total Bilirubin (TB) is ≥2x ULN. 8. Female subjects with positive pregnancy test result, breastfeeding or planning to breastfeed at any time through 30 days after the last dose of study drug. 9. Subjects taking any other investigational drug used to treat for another respiratory infection. 10. Psychiatric or cognitive illness or recreational drug/alcohol use that, in the opinion of the principal investigator, would affect subject safety and/or compliance.