Safety and Efficacy Study of Allogeneic Human Dental Pulp Mesenchymal Stem Cells to Treat Severe COVID-19 Patients

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.