3 26-02-2020; 28-02-2020 to 31-05-2020 0Not recruiting ChiCTR2000030261;http://www.chictr.org.cn/showprojen.aspx?proj=49963 A study for the key technology of mesenchymal stem cells exosomes atomization in the treatment of novel coronavirus pneumonia (COVID-19) MSC exosomes 26 Experimental group: aerosol inhalation of exosomesControl group: blank