This study is a multicenter, randomized, open, parallel-controlled study. Qualified subjects will randomly be assigned to the experimental arm or the control arm according to the ratio of 1:1, with age (> 60 years or ≤ 60 years), smoking status (yes/no) and forced expiratory volume in one second/prediction (FEV1 %pred > 60% or ≤ 60%) as the random stratification factors.

Subjects in the experimental arm and the control arm will receive hydrogen-oxygen mixed gas inhalation (Hydrogen-Oxygen Generator with Nebulizer, AMS-H-03, output: 3 L/min (hydrogen concentration: 66.7%, oxygen concentration: 33.3%)) and oxygen inhalation (OLO-1 Medical Molecular Sieve Oxygen Generator, output: 3 L/min (oxygen concentration: 33.3%), Shanghai Ouliang Medical Devices Co., Ltd.), respectively; the treatment duration will not be less than 8 hours per day, for 12 weeks.

Subjects in the experimental arm and the control arm will also receive other medications (excluding antiviral drugs) by the investigator as clinically indicated. Six visits are required for each subject in this study, including Visit 1 (D-7~-1), Visit 2 (D1), Visit 3 (D14±3d), Visit 4 (D28±3d), Visit 5 (D56±7d), Visit 6 (D84±7d).

Inclusion Criteria:

• 1） Male or female, aged between 18 and 75 (including boundary values) at screening.

  2) Severe or critically ill patients who have been diagnosed with a novel coronavirus during hospitalization (COVID-19).

  3) After treatment, the patients have met the discharge criteria of "COVID-19 Diagnosis and Treatment Guideline", and the time from hospital discharge is at least 1 month at the time of enrollment. The clinical symptoms of the subjects did not worsen significantly as compared with that at the time of discharge, and the COVID-19 nucleic acid test results are negative for at least 2 consecutive times (one of which could be the nucleic acid test before discharge).

  4) Forced vital capacity/per predicted (FVC% pred) ≥ 50%. 5) 50% ≤ FEV1 %pred ≤80%。 6) Subject (or legally authorized representative) provides written informed consent prior to initiation of any study procedures. Understands and agrees to comply with planned study procedures.

  7) Agrees not to participate in other drug/device studies until the study is completed.

Exclusion Criteria:

• 1) With one of the following respiratory diseases:

  1. Subjects with asthma history, or cannot rule out asthma based on the diagnosis of investigator;

  2. Subjects with chronic obstructive pulmonary disease (COPD);

  3. Subjects with following respiratory diseases such as active tuberculosis, lung cancer, sarcoidosis, pulmonary hypertension, pneumothorax, uncontrolled pleural effusion through intervention, pulmonary embolism, etc.;

  4. Lung volume reduction: subjects have had lung volume reduction surgery, pulmonary lobectomy, or bronchoscopic lung volume reduction surgery.

     2) Subjects with pulmonary heart disease. 3) Patients who are scheduled for elective surgery during the study period, such as thoracic and abdominal major surgery.

     4) Subjects, judged by investigators, with previous or current diseases, which may affect the participation in this study or the outcome of this study: such as cancer, diseases of heart, liver, kidney, hematopoietic system and other vital organs or systems, etc.

     5) Patients who have undergone surgery within 1 month prior to screening and have not fully recovered.

     6) Occurrence of congestive heart failure, uncontrolled or unstable angina or myocardial infarction, cerebrovascular accident, or history of pulmonary embolism within 6 months prior to screening.

     7) Patients with active tuberculosis infection within 12 months prior to screening.

     8) Pregnancy or lactating women, or women of childbearing potential not agree to either abstinence or use at least one primary form of contraception from the time of screening till the study is completed.

     9) Subjects with mental disorders or other conditions that are unable to cooperate effectively with the conduct of the clinical trial.

     10) Subjects intolerance to inhalation therapy. 11) Others whom the investigator or sub-investigator judged inappropriate for participation in the study.

This study is conducted in a double-blind design with a primary unblinding, and a terminal unblinding is performed after data review, database lock, and finalization of the statistical analysis plan.

Both the investigational device (AMS-H-03 Hydrogen-Oxygen Generator with Nebulizer) and the control device (OLO-1 Medical Molecular Sieve Oxygen Generator) were placed in the shell with the same appearance, with the same operating interface and only ventilation function. During the treatment, neither the investigator nor the subject would be aware of the treatment allocation The study devices will be masked by independent non-blinded personnel of the statistical company according to the corresponding operating procedures, and no specific personnel participating in the clinical operation shall be present during the process.