This study is designed to analyze the impact of Wharton's Jelly Mesenchymal Stem Cells on moderate to severe ARDS due to COVID19 in adult patients who require mechanical ventilation. It is a Phase IIa double-blind randomized controlled trial.~30 patients are planned.~Patients will be randomized and will receive, with the conventional treatment recommended to treat ARDS:~Either Wharton's Jelly MSCs in a solution of albumin 4% (40% of final volume), NaCl 0,9% (50% of the final volume) et ACD formule A (10% of the final volume) . Treatment will be administered intravenously during 10 minutes following that scheme:~Day 0 (or 1): 1. 10^6 MSC/kg (maximum 80.10^6 MSC)~Day 3 (or 4): 0.5 . 10^6 MSC/kg (maximum 40. 10^6 MSC)~Day 5 (or 6): 0.5 . 10^6 MSC/kg (maximum 40. 10^6 MSC) An interval of 2 days will be respected between 2 infusions.~Either a placebo, which contains the same solution of albumin, NaCl 0.9% and ACD without cells. The volume will be of 75 ml, infused in 10 minutes.~The main objective is to investigate efficacy of WJ-MSCs, compared to a placebo, on respiratory function evolution during the first 14 days of study treatment in patients with SARS-CoV-2 related moderate to severe ARDS.~Secondary objectives are to assess the effect of WJ-MSC, compared to placebo, in patients with SARS-CoV-2 related moderate to severe ARDS, on:~the duration of invasive mechanical ventilation during the hospital stay and maximum for 28 days~the evolution of organ failures during the hospital stay and maximum for 28 days~the duration of stay in intensive care unit, the mortality during intensive care unit, during hospitalization, on D28 and D90, and the respiratory morbidity.~the evolution of viral load between D0 and D28~the immediate or delayed tolerance following the WJ-MSCs injection
Mesenchymal stem cells (MSCs) are of potential help in acute respiratory distress syndrome (ARDS), due to their anti-inflammatory properties.~The investigators will analyze the effect of 3 iterative infusions of ex vivo expanded Wharton's Jelly MSCs (total dose 2.10^6/kg) in patients with ARDS due to COVID19, who require mechanical ventilation.