This is a randomized controlled trial. double-blind, multi-center clinical study conducted at three different hospitals, on 21 patients who received intervention and 21 patients who received control treatment. The purpose of this study is to evaluate the efficacy and safety of intravenous administration of normoxic allogeneic umbilical cord-derived mesenchymal stem cell (UCMSC) in the treatment group, compared to the control group who are only given standard COVID-19 treatments and normal saline infusion
