4 13-02-2020; 20-02-2020 to 30-12-2020 1Recruiting NCT04280224;https://clinicaltrials.gov/show/NCT04280224 NK cells treatment for novel coronavirus pneumonia NK cells 30 Experimental group: conventional treatment plus twice a week of NK cells (0.1–2×107 NK cells·kg−1 body weight)Control group: conventional treatment