10 24-02-2020; 17-02-2020 to 17-04-2020 0Not recruiting ChiCTR2000030173;http://www.chictr.org.cn/showprojen.aspx?proj=49229 Key techniques of umbilical cord mesenchymal stem cells for the treatment of novel coronavirus pneumonia (COVID-19) and clinical application demonstration UC- MSCs 60 Experimental group: UC-MSCsControl group: Conventional treatment