4 07-02-2020; 06-02-2020 to 30-09-2020 2Recruiting NCT04269525;https://clinicaltrials.gov/show/NCT04269525 Umbilical cord (UC)-derived mesenchymal stem cells (MSCs) treatment for the 2019-novel coronavirus (nCOV) pneumonia UC-MSCs 10 Experimental group: UC-MSCs 3.3×107 cells per 50 mL per bag, 3 bags each time; UC-MSCs will be infused i.v. on the 1st, 3rd, 5th and 7th days after enrolment, once each dayControl group: none specified