12 23-02-2020; 01-02-2020 to 31-08-2020 NARecruiting ChiCTR2000030116;http://www.chictr.org.cn/showproj.aspx?proj=49901 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) UC-MSCs 16 Experimental group: different stem cell dosesControl group: none specified