Safety and effectiveness of human umbilical cord mesenchymal stem cells in the treatment of acute respiratory distress syndrome of severe novel coronavirus pneumonia (COVID-19)

INTERVENTION: Two groups:Different stem cell doses; CONDITION: Novel Coronavirus Pneumonia (COVID‐19) PRIMARY OUTCOME: Time to leave ventilator on day 28 after receiving MSCs infusion; INCLUSION CRITERIA: Patients are eligible for admission if they meet all of the following criteria within 5‐7 days of their first ICU admission. 1) Aged 18‐75 years; 2) The patient meets the ARDS diagnostic standard (Berlin standard) and needs intubation ventilator‐assisted or ECMO‐assisted treatment; 3) The primary disease caused by 2019‐nCoV virus infection; 4) Imaging data suggest bilateral lung lesions; 5) Voluntarily participate in clinical research; fully understand, know, and sign informed consent.