Viral pneumonia is an acute respiratory infectious disease caused by respiratory viruses. It is mainly caused by the invasion of respiratory viruses such as influenza virus and adenovirus into the lower respiratory tract. Every winter and spring is the epidemic season. Influenza virus and adenovirus have high infectivity and pathogenicity. The rapid progress of some patients may be caused by acute respiratory distress syndrome (ARDS) or multiple organ dysfunction And died of complications. The fatality rate of severe patients is 9.8% - 60%. Most of the dead cases were the elderly and the patients with basic diseases. In addition to the complications, the main cause of death is ARDS caused by virus infection, on the other hand, the basic immune function of the elderly is poor, so it is difficult to form an effective antiviral response in response to virus infection. The direct destruction of the alveolar epithelial barrier and the systemic inflammatory response induced by the infection, namely, the destruction of the alveolar capillary barrier by the inflammatory waterfall, are the important pathogenesis of ARDS. How to effectively regulate the inflammatory response, prevent the inflammatory exudation and edema of the lungs, improve oxygenation, and reduce organ damage; at the same time, how to effectively improve the basic immune function and enhance the anti-virus immune response of such patients, has become the key to the success of the treatment of patients with severe viral pneumonia.

Umbilical cord mesenchymal stem cells (UC-MSCs) can differentiate into the different germ layers and play an important role in immune regulation and damage repair regulation. Clinical trials have shown that MSC is safe and effective in the treatment of acute lung injury and pulmonary fibrosis, and it can improve the immune function of patients with viral infectious diseases. Hence, intravenous infusion of HUC-MSCs is attractive therapy against severe viral pneumonia.

This is a randomized, singlecenter, open lable, intervention controlled clinical trial. The participants (n = 40) will be randomly distributed into two groups. The routine treatment group (n = 20) will receive the treatment according to "Influenza diagnosis and treatment plan (2019 version)", the HUC-MSCs adjuvant Group (n = 20) will receive intravenous infusion of definitive HUC-MSCs (1×10\^6 cells/Kg × body weight(kg), which was selected by immunomodulatory assay through coculture with BV2 cell) on the basis of the routine treatment once at day 1 after joining. Follow-up duration is 90 days. The difference of 90 day mortality and average length of stay between the two groups will be observed and recorded. The changes of inflammatory index, viral load, oxygenation index and pulmonary imaging will be monitored at different time points after treatment in the two groups. The serious adverse events (SAEs) and adverse events (AEs) will be observed during the period.

The intent of this study is to explore the efficacy of HUC-MSCs in the treatment of severe viral pneumonia through improving the antiviral immune response of patients, reduce the lung inflammatory damage caused by the virus and the pulmonary interstitial fibrosis after the injury, and finally achieve the goal of reducing the mortality and improving the prognosis of severe patients, and to evaluate the safety.