1. Background

In December 2019, the pandemic caused by a novel kind of enveloped RNA betacoronavirus took place in Wuhan, Hubei Province, China. This virus was named as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)[1]. Also, it was declared as coronavirus disease 2019 (COVID-19) by the World Health Organization (WHO)[1]. Because of its extremely strong transmission, this virus has spread to the world in less than 3 months and infected more than 1 million patients [2]. COVID-19 has a higher mortality rate, ranging from 3% to 12%[2]. Also, it is characterized by the high incidence of critical patients, which was reported to about 20% of the total number of patients. Although the clinical characteristics of critical patients defined have been reported in many literatures [3, 4], which paid much attention on general characteristics of patients at early-stage, rather than the observation of the whole course of the development of the disease. Judging from both the general law of disease occurrence and development and clinical and laboratory indicators, it is urgent to identify patients with poor prognosis in the early stage and take timely intervention measures so as to prevent patients from turning into further deterioration of the disease. This can help reduce the mortality at the same time.