Additional file 1: A detailed description of the methods and the full clinical trial protocol. Table S1. Quality-control characteristics of UC-MSCs used as treatment: identity and safety. Table S2. Quality-control characteristics of batch-1 UC-MSCs used as treatment: identity and safety. Table S3. Quality-control characteristics of batch-2 UC-MSCs used as treatment: identity and safety. Table S4. Evolution of respiratory characteristics from baseline (D0) to D28 Table S5. UC-MSCs posology per patient. Table S6. Subgroup analyses of the primary outcome. Table S7. Complete list of adverse events. Table S8. Summary of pre-specified infusion-associated adverse events for randomized subjects.