Randomized, double-blind, parallel, two-arms clinical trial to assess the efficacy and safety of 2 infusions of Wharton-Jelly mesenchymal stromal cells (day 1 and day 3, endovenously at 1E6cells/Kg per dose) in patients with moderate acute respiratory distress syndrome (ARDS) secondary to SARS-CoV-2 infection. Follow-up will be established on days 3, 5, 7, 14, 21, and 28. Long term follow-up will be performed at 3, 6 and 12 months.

This is a prospective, double-blind, randomized, parallel, placebo-controlled pilot clinical trial to assess the efficacy and safety of two infusions of Wharton Jelly mesenchymal stromal cells (WJ-MSC) in patients with moderate acute respiratory distress syndrome (ARDS) secondary to SARS-CoV-2 infection. The study will enroll 30 patients who after signing the informed consent will be checked for inclusion and exclusion criteria. Patients will then be randomized (1:1) to one of the 2 treatment arms: Treatment A WJ-MSC/WJ-MSC; Treatment B Placebo/ Placebo. The 2 infusions will be administered endovenously on day 1 (D1) and on D3. Thereafter, patients will be followed-up on days 3, 5, 7, 14, 21, and 28 Once the study is completed, controls will be established at 3 months, 6 months and 12 months as long-term follow-up.

The study treatments (A or B) will be added on top of the Standard of Care treatment prescribed by the attending physician. Each dose of MSC-WJ will consist of the intravenous administration of 1E6cells/Kg.

Recruitment will be competitive for the centers participating in the study. A Data Safety and Monitoring Board (DSMB) will be established to review safety and efficacy along the trial.

Inclusion Criteria:

1. Positive PCR fpr SARS-CoV-2
2. Intensive Care Unit admission for less than 3 days
3. Moderate acute respiratory distress (Berlin criteria definition with 100 mmHg < PaO2/FiO2 ≤ 200 mmHg)
4. Male or female, aged 18 to 70 years old
5. Signed informed consent by the patient or by a legal representative (in this case, can be obtained by phone, although it must be confirmed in writing later, accepted by email)

Exclusion Criteria:

1. Expected survival less than 3 days
2. Treatment with immunosuppressive drugs (tocilizumab, sarilumab) with corticosteroids being allowed
3. Neoplastic disease either active or without complete remission
4. Immunosuppressed patients (except treatment with corticosteroids for respiratory distress)
5. Pregnant or lactating women
6. Participation in another clinical trial with an experimental drug in the last 30 days
7. Other pathologies that, in medical judgment, contraindicate participation in the study