A Multicenter, Randomized, Double-Blind, Placebo-Controlled Clinical Study to Evaluate the Safety and Efficacy of Uterine Blood Mesenchymal Stem Cells Injection for the Treatment of Severe Pneumonia Caused by Viruses

Inclusion Criteria: Patients must meet all of the following criteria to be eligible for this trial: 1. Aged between 18 and 85 years, regardless of gender; 2. Meet the 2018 primary care diagnostic criteria for severe community-acquired pneumonia (CAP) in adults in China; (Diagnosed according to the diagnostic criteria in the "2018 Guidelines for the Diagnosis and Treatment of Community-Acquired Pneumonia in Adults in Primary Care in China." Severe pneumonia can be diagnosed with one of the following major criteria or ≥3 minor criteria. Major criteria: ① Requires endotracheal intubation for mechanical ventilation; ② Septic shock requiring vasoactive medication after aggressive fluid resuscitation. Minor criteria: ① Respiratory rate ≥30 breaths/min; ② Oxygenation index ≤250 mmHg (1 mmHg=0.133 kPa); ③ Multilobar infiltrates; ④ Altered consciousness and/or disorientation; ⑤ Blood urea nitrogen ≥7.14 mmol/L; ⑥ Systolic blood pressure requiring aggressive fluid resuscitation.) 3. Viral infection confirmed by nucleic acid testing from patient specimens (oropharyngeal swabs, nasopharyngeal swabs, nasopharyngeal aspirates, tracheal aspirates, sputum, and other respiratory specimens, blood specimens), excluding SARS-CoV-2; 4. Compliance with the policies and institutional arrangements for pathogen control by the Health Commission of the region (country) where the experimental center is located; 5. Voluntarily sign and provide written informed consent. Exclusion Criteria: • Patients who meet any of the following criteria are not eligible for this trial: 1. Clearly diagnosed infections caused by non-viral pathogens, including tuberculosis, bacterial pneumonia, mycoplasma pneumonia, chlamydial pneumonia, or other atypical pathogens causing pneumonia, as defined in the "Clinical Laboratory Manual"; 2. Severe pneumonia of unknown viral infection, i.e., the viral pathogen infecting the patient is not well documented and recognized in current literature; 3. History of cancer or clinical examination confirming premalignant lesions at screening; 4. Use of extracorporeal membrane oxygenation (ECMO) at screening; 5. History of acute cerebral infarction within 3 months before screening, or history of deep vein thrombosis or pulmonary embolism at screening; 6. Active immunosuppression including: a) Chemotherapy within the last 4 weeks; B) Continuous use of corticosteroid treatment (equivalent to prednisone ≥1mg/kg·d) for nearly 4 weeks; C) Treatment with immunosuppressants (cyclophosphamide, azathioprine, methotrexate, cyclosporin) within 4 weeks; d) Absolute neutrophil count \<0.5×10\^9/L; 7. Severe cardiovascular disease within 6 months before screening, including unstable heart disease, myocardial infarction, NYHA Class III or IV heart failure, complete left bundle branch block, or second or third degree atrioventricular block; 8. Abnormal and clinically significant test results for hepatitis B surface antigen (HBsAg), hepatitis B core antibody (HBcAb), hepatitis C virus (HCV) antibody, human immunodeficiency virus (HIV) antibody, and syphilis treponemal (syphilis) antibody; 9. Severe dysfunction of the liver and kidneys; 10. History of poorly controlled mental illness; 11. Known or suspected allergy to the active or inactive ingredients of the study medication; 12. Pregnant or breastfeeding women and women of childbearing age who are not sterilized/refuse to use medically accepted effective contraception during the study period; 13. Men who are not sterilized/refuse to use medically accepted effective contraception during the study period; 14. Those who have participated in other clinical trials (excluding those who have not taken medication) or who have previously received stem cell therapy; 15. Other circumstances deemed unsuitable for enrollment by the investigator.