13 22-02-2020; 01-03-2020 to 31-12-2021 0Not recruiting ChiCTR2000030088;http://www.chictr.org.cn/showproj.aspx?proj=49902 Umbilical cord Wharton's jelly derived mesenchymal stem cells in the treatment of severe novel coronavirus pneumonia (COVID-19) UC-Wharton's jelly MSCs 40 Experimental group: i.v. injection of Wharton's jelly MSCs (1×106 MSCs·kg−1), cell suspension volume 40 mLControl group: i.v. 40 mL saline