Clinical Trial to Assess the Safety and Efficacy of Intravenous Administration of Allogeneic Adult Mesenchymal Stem Cells of Expanded Adipose Tissue in Patients With Severe Pneumonia Due to COVID-19

Inclusion Criteria: 1. Age ≥ 18 2. Clinical diagnosis of Pneumonia, severe or critical, caused by COVID-19 infection 3. Life expectancy \> 48 hours. 4. Commitment to use a contraceptive method of proven efficacy in both men and women during the duration of the clinical trial. Exclusion Criteria: 1. Coinfection with other viruses or bacteria (HIV, tuberculosis, influenza virus, adenovirus or other respiratory infections, active infection by HBV or C). 2. History of multiple allergies, including allergy to Penicillin or other Blactams. 3. Pregnant and lactating women. 4. Patients with malignant tumors or hemopathies or any state of immunosuppression considered as severe. 5. Patients with autoimmune diseases. 6. Chronic heart failure with ejection fraction less than 30%. 7. Any other condition for which, in the opinion of the main investigator, the subject is considered not to be in compliance with the study.