1 04-02-2020; 05-02-2020 to 30-04-2021 0Not recruiting ChiCTR2000029569;http://www.chictr.org.cn/showproj.aspx?proj=49062 Safety and efficacy of umbilical cord blood mononuclear cells conditioned medium in the treatment of severe and critically novel coronavirus pneumonia (COVID-19): a randomized controlled trial UC-MSCs CM 30 Experimental group: conventional treatment combined with UC-MSCs CMControl group: conventional treatment