1 14-02-2020; 20-02-2020 to 20-02-2021 0Not recruiting ChiCTR2000029816;http://www.chictr.org.cn/showproj.aspx?proj=49389 Clinical study for cord blood mesenchymal stem cells in the treatment of acute novel coronavirus pneumonia (COVID-19) UCB-MSCs; UCB-NK cells 60 Experimental groups: conventional treatment followed by i.v. infusion of 1) UCB-MSCs and 2) UCB-MSCs combined with UCB-NK cellsControl group: conventional treatment