A Clinical Trial of Human Umbilical Cord Mesenchymal Stem Cell Injection for the Treatment of Severe Acute Respiratory Distress Syndrome

Primary Objective: To evaluate the safety and tolerability of human umbilical cord mesenchymal stem cell injection in the treatment of moderate/severe acute respiratory distress syndrome.Secondary Objectives: To explore the efficacy and appropriate dosage of human umbilical cord mesenchymal stem cell injection in the treatment of moderate/severe acute respiratory distress syndrome.Exploratory Objective: To explore the immunogenicity and pharmacokinetic/pharmacodynamic (PK/PD) characteristics of a single dose of human umbilical cord mesenchymal stem cell injection in patients with moderate/severe acute respiratory distress syndrome.