UC-MSCs in the Treatment of Severe and Critical COVID-19 Patients With Refractory Hypoxia

Inclusion Criteria: * The age ranges from 18 to85 (inclusive), regardless of gender. * Patients who still have refractory hypoxia after sufficient standard treatment for severe and critical COVID-19 pneumonia following the suggestion of the Chinese diagnostic criteria for COVID-19 pneumonia in the Diagnosis and Treatment Plan for COVID-19 Pneumonia (Tentative tenth Edition). * 20kg/m2≤body mass index(BMI)≤30 kg/m2. * Volunteer to participate in this clinical study and sign the written informed consent Exclusion Criteria: * Those who are using immunosuppressive drugs or long-term immunosuppressive drugs after organ transplantation. * T lymphocyte abnormality (allogeneic may be considered, depending on clinical opinion), HIV positive. * Highly allergic or have a history of severe allergy, especially Interleukin-2 allergy. * Pregnant and lactating women. * Patients with severe autoimmune disease history; Allergies to all biological reagents in this treatment, such as Interleukin-2. * Patients with serious complications, including chronic cardiac insufficiency (NYHA cardiac function grading assessment of cardiac function grade III - IV), chronic renal insufficiency (CKD stage 3 and above); Chronic liver insufficiency (Child-Pugh score\>7); Patients with malignant tumors. * Patients with pulmonary embolism or acute coronary syndrome. * Patients with severe organ dysfunction caused by other diseases. * There are other situations that the researcher thinks are not suitable to participate in this clinical study