It is a strategic decision by the company, coupled with low incidence of ARDS cases following widespread vaccination efforts.

This is a phase 1/2a study including 2 parts, phase 1 and phase 2a. The phase 1 part is an open-label, single-arm, dose-escalating study to evaluate the safety and explore the dose limiting toxicity and maximum tolerated dose of a human umbilical cord derived mesenchymal stem cell product (BX-U001) in severe COVID-19 pneumonia patients with acute respiratory distress syndrome (ARDS). Qualified subjects after the screening will be divided into low, medium, or high dose groups to receive a single intravenous infusion of BX-U001 at the dose of 0.5×10^6, 1.0×10^6, or 1.5×10^6 cells/kg of body weight, respectively. The Phase 2a part is a randomized, placebo-controlled, double-blind clinical trial examining the safety and biological effects of BX-U001 at the appropriate dose selected from phase 1 for severe COVID-19 pneumonia patients with the same inclusion/exclusion criteria as the phase 1 part.

Inclusion Criteria:

1. Male or female, aged between 18 and 80;
2. Laboratory (RT-PCR, Gene Sequencing or Antibody) confirmed cases of severe COVID-19 pneumonia with mild to moderate ARDS (Berlin definition);
3. Patients are intubated;
4. Patients who voluntarily adhere to the research procedures and ensure good compliance during the research period;
5. Patients who fully understand the research nature of this study and sign written informed consent.

Exclusion Criteria:

1. Subjects who have received investigational drug (except for Remdesivir) for the treatment of COVID-19 within 30 days before screening;
2. Subjects who are pregnant, breastfeeding or whose urinary pregnancy test is positive before participation in the study; subjects who are pregnant, breastfeeding, have a birth plan, or are unwillingness to use contraception during the study period and within 12 months of infusion; except for subjects who have sterilization surgery or menopause during the study period;
3. Within 3 days before screening/randomization, subjects who have used high-dose corticosteroids that is equivalent to methylprednisolone >240 mg/day or irregular use of systemic corticosteroids to treat other diseases that could affect the efficacy evaluated by the investigator;
4. Subjects receiving extracorporeal membrane oxygenation (ECMO) support.
5. Subjects who are allergic to low-molecular-weight heparin calcium or human albumin;
6. Subjects with ongoing malignant tumors.