This was a randomized, multi-center, open-label clinical trial. Eligible patients were recruited from seven hospitals in China. On the basis of standard-of-care, patients in treatment group inhaled H2-O2 (66% hydrogen; 33% oxygen) at 3 L/min via nasal cannula by using the Hydrogen/Oxygen Generator (model AMS-H-03, Shanghai Asclepius Meditech Co., Ltd., China) until discharge. Patients in control group received standard-of-care (consisting of oxygen therapy) alone until discharge.

This was a randomized, multi-center, open-label clinical trial. Eligible patients were recruited from seven hospitals in China. On the basis of standard-of-care, patients in treatment group inhaled H2-O2 (66% hydrogen; 33% oxygen) at 3 L/min via nasal cannula by using the Hydrogen/Oxygen Generator (model AMS-H-03, Shanghai Asclepius Meditech Co., Ltd., China) until discharge. Patients in control group received standard-of-care (consisting of oxygen therapy) alone until discharge. Clinical assessments included the five-category ordinal scale, four-category ordinary scale of dyspnea, coughing, chest distress and chest pain (0: None; 1: Mild; 2: Moderate; 3: Severe; 4: Very severe) and adverse events, performed on admission, at enrollment, at day 2 and 3 after randomization, and the day before discharge. The primary endpoint was the proportion of patients with improved disease severity (by at least one scale). Secondary endpoints comprised the change from baseline in oxygen saturation and symptom scales.

Inclusion Criteria:

• Patients with lab-confirmed Covid-19 aged 18 to 75 years.
• Had dyspnea both on hospital admission and at enrollment.
• The patients volunteered to participate in this study and signed the informed consent

Exclusion Criteria:

• Have other systemic diseases other than COVID-19 -- a disease that, according to the investigator's judgment, might increase the risk of developing adverse outcomes or affect the outcome measures after participation in the study.

Women who are pregnant or breastfeeding or plan to be pregnant during the study.

Subjects with one of the following respiratory diseases:

• Subjects in critical or unstable conditions. Critical (meeting any of the following symptoms): 1. Respiratory failure occurs and mechanical ventilation is needed; 2. Shock occurs; 3. Other organ failure, ICU monitoring and treatment is needed.
• Having immunosuppressive diseases (HIV), severe neurological disease affecting upper respiratory tract control, or other risk factors that the investigator believes can cause a significant risk of pneumonia in subjects.
• Subjects with mental disorder and cognitive impairment.
• Subjects who do not follow the study steps.
• Patients with mental retardation, poor motivation, drug abuse (including drugs and alcohol) or other disease history restricting the effectiveness of informed consent in this study.
• Use of antioxidants, including large doses of vitamin C and vitamin E.
• Subjects who are not suitable for participation in this study in the judgment of investigator.