Background:SARS-CoV-2-caused coronavirus disease (COVID-19) is posinga large casualty. The features of COVID-19patients withand without pneumonia,SARS-CoV-2 transmissibility in asymptomatic carriers, and factors predicting disease progression remain unknown. Methods: We collected information on clinical characteristics, exposure history, andlaboratory examinations of all laboratory-confirmed COVID-19 patients admitted to PLA General Hospital. Cox regression analysis was applied to identify prognostic factors. The last follow-up was February 18, 2020. Results:We characterized 55 consecutive COVID-19 patients. The mean incubation was 8.42(95% confidence interval [CI], 6.55-10.29) days. The mean SARS-CoV-2-positive duration from first positive test to clearance was 9.71(95%CI, 8.21-11.22) days. COVID-19 course was approximately 2 weeks. Asymptomatic carriers might transmit SARS-CoV-2. Compared with patients without pneumonia, those with pneumonia were 15 years older and had a higher rate of hypertension, higher frequencies of having a fever and cough, and higher levels of interleukin-6 (14.61 vs. 8.06pg/mL, P=0.040), B lymphocyte proportion (13.0% vs.10.0%, P=0.024), low account (<190/μL) of CD8+ T cells (33.3% vs. 0, P=0.019). Multivariate Cox regression analysis indicated that circulating interleukin-6 andlactate independently predicted COVID-19 progression, with a hazard ratio (95%CI) of 1.052 (1.000-1.107) and 1.082 (1.013-1.155), respectively. During disease course,T lymphocytes were generally lower,neutrophils higher, in pneumonia patients than in pneumonia-free patients. CD8+ lymphocytes did not increase at the 20th days after illness onset. Conclusion: The epidemiological features areimportant for COVID-19 prophylaxis. Circulating interleukin-6 and lactateare independent prognostic factors. CD8+ T cell exhaustion might be critical in the development of COVID-19.