17 24-02-2020; 24-02-2020 to 01-02-2021 NARecruiting NCT04293692;https://clinicaltrials.gov/show/NCT04293692 Therapy for pneumonia patients infected by 2019 novel coronavirus UC-MSCs 48 Experimental group: conventional treatment plus 4 times of 0.5×106 UC-MSCs·kg−1 body weight suspended in 100 mL saline containing 1% human albumin, i.v. at days 1, 3, 5 and 7Control group: conventional treatment plus 4 times of placebo (100 mL saline containing 1% human albumin), i.v. at days 1, 3, 5 and 7