Umbilical Cord(UC)-Derived Mesenchymal Stem Cells(MSCs) Treatment for the 2019-novel Coronavirus(nCOV) Pneumonia

Inclusion Criteria: * 18-80 years old ,no gender restriction * According to Diagnosis and Clinical Management of Pneumonia caused by 2019-nCoV infection(Trial Version 6), patients are diagnosed with severe or critical 2019-nCoV pneumonia * Women of childbearing age should have a negative blood pregnancy test before the start of dosing and agree to take effective contraceptive measures during the trial until the last follow-up (28 days) * Previous detection of Nucleotide or antibody of 2019-nCoV pneumonia was positive * Voluntarily participate in this clinical study and sign a written informed consent. If the patient cannot obtain informed consent, he can authorize his legal representative. Exclusion Criteria: * Liver SOFA score of more than 3 points; * HIV positive * Highly allergic constitution or history of severe allergies; * Pregnant and lactating women; * Patients with malignant tumors; * Patients with previous history of pulmonary embolism; * Participating in clinical trials of other drugs within 3 months before enrollment. * be thought by researchers to be inappropriate to participate in this clinical study.