This clinical trial is set out to evaluate the safety and efficacy of allogeneic human dental pulp mesenchymal stem cells in the treatment of severe pneumonia caused by COVID-19; to explore the effects of human dental pulp mesenchymal stem cells in the treatment of severe pneumonia of COVID-19 in terms of reducing mortality and improving clinical prognosis; and to discover a new therapeutic strategy for COVID-19 using allogeneic human dental pulp mesenchymal stem cells.

This clinical trial is set out to evaluate the followings:

1. the safety and efficacy of allogeneic human dental pulp mesenchymal stem cells in the treatment of severe pneumonia caused by COVID-19;
2. to explore the effects of human dental pulp mesenchymal stem cells in the treatment of severe pneumonia of COVID-19 in terms of reducing mortality and improving clinical prognosis; and
3. to discover a new therapeutic strategy for COVID-19 using allogeneic human dental pulp mesenchymal stem cells.

Inclusion Criteria:

1. Aged 18-65 years;
2. Voluntarily participate in this clinical trial and sign off "informed consent form";
3. Diagnosed with severe pneumonia of COVID: respiratory distress, RR >30 times / min; resting oxygen saturation of 93% or less; arterial partial pressure of oxygen / oxygen concentration 300mmHg; SARS-CoV-2 nucleic acid test was positive.
4. Chest imaging confirm COVID-19 featured lesions in lung.

Exclusion Criteria:

1. Receive any clinical trial drug treatment for COVID-2019 within 30 days before the screening assessment;
2. Severe liver disease (e.g., Child Pugh score >=C or AST> 5 times of the upper limit);
3. Patients with known severe renal insufficiency (estimated glomerular filtration rate <=30mL / min/1.73 m2) or patients receiving continuous renal replacement therapy, hemodialysis, peritoneal dialysis;
4. Co-infection of HIV, hepatitis B, tuberculosis, influenza virus, adenovirus or other respiratory infection viruses;
5. Female patients who have no sexual protection in the last 30 days prior to the screening assessment;
6. Pregnant or lactating women or women using estrogen contraception;
7. Patients who are planning to become pregnant during the study period or within 6 months after the end of the study period;
8. Other conditions that the researchers consider not suitable for participating in this clinical trial.