UC-MSCs in the Treatment of Severe and Critical COVID-19 Patients

The goal of this randomized, controlled, open-labeled interventional clinical trial is to test the efficacy and safety of umbilical cord mesenchymal stem cells (UC-MSCs) in the treatment of severe and critical COVID-19 patients. The main questions it aims to answer are: 1. The efficacy of UC-MSCs in the treatment of severe and critical COVID-19 patients. 2. The safety of UC-MSCs in the treatment of severe and critical COVID-19 patients. 3. The potential immune mechanisms of UC-MSCs in the treatment of severe and critical COVID-19 patients. Participants will receive standard therapy, Paxlovid treatment or the UC-MSCs treatment. For the standard treatment, it will be conducted according the 9th edition of Chinese guidelines for COVID-19 infection. For UC-MSCs treatment, participants will be given UC-MSCs instillation at the first and fourth day after assignment. Blood examples will be taken at indicated time for arterial blood gas analysis and other tests. And participants should also objectively report their symptoms change and other information related to the treatment as listed the research protocol.