This is a Phase IIb study consisting of two cohorts to evaluate efficacy, safety and pharmacokinetics of DAS181 in IFV infection. An approximate total of 280 subjects will be enrolled into this study.

Cohort 1: It is designed as a two stage, multi-center, randomized, double-blind, two parallel doses, placebo-controlled study that will investigate the efficacy of DAS181 for the treatment of serious IFV in hospitalized patients who suffered from acute hypoxemia requiring supplemental oxygen therapy.

Cohort 2: It is designed as an open-label study that will investigate the efficacy of DAS181 for the treatment of patients with sever IFV but not eligible for Cohort 1 and non-IFV SAD viral infection, e.g. parainfluenza virus, human metapneumovirus, enterovirus and all strains of β-coronaviruses (including but not limited to SARS-CoV-2), with hypoxemia requiring supplemental oxygen therapy.

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.

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.

Stage 2, Ansun will know the dosage.