4 03-03-2020; 31-01-2020 to 31-01-2021 NANot recruiting ChiCTR2000030484;http://www.chictr.org.cn/showproj.aspx?proj=50263 UC-MSCs and exosomes treating patients with lung injury following novel coronavirus pneumonia (COVID-19) UC-MSCs and exosomes 90 Experimental groups:Group 1: UC-MSCs i.v. 5×107 cells each time, once a week, twice per courseGroup 2: UC-MSCs i.v. 5×107 cells each time, once a week, twice per course, a total of 2 courses; exosomes i.v. administration, 180 mg each time, once a day, 7 days per course, 2 courses in totalControl group: same amount of placebo (stem cell solvent)