Introduction
Since the emergence of coronavirus disease 2019 (COVID-19) which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in December 2019, Wuhan, China, the epidemic spread around the world quickly. To date, the global COVID-19 pandemic has involved over 200 countries/regions, resulting in more than 7,000,000 confirmed cases and more than 400,000 deaths worldwide according to the real-time statistics of worldometer.
Many unknowns about COVID-19 remain, but we know clearly: it is both deadly and highly contagious.The estimated mean basic reproductive values (R0 ) of COVID-19 is around 2 to 3 in the early phase of outbreak, which was higher than that of SARS and MERS[1, 2]. Some scholars believe that the outbreak of any infectious disease follows such a rule: in the early stage severe cases are recognized first and then mild or asymptomatic cases are detected more frequently[3]. Asymptomatic carriers are hidden, difficult to be diagnosed and isolated in time, and easy to cause the accumulation of infection sources in the community[4]. So identification of asymptomatic
cases is very important for the control of COVID-19 epidemic.
Now we conducted a literature review, focusing on the age trend, nucleic acid test results, chest CT images, transmission modes, children asymptomatic cases, and the estimated proportion of asymptomatic infections of COVID-19. This review will be helpful to understand the unique features of the asymptomatic cases of COVID-19 and improve our epidemic prevention and control measures in the future.
Asymptomatic cases were diagnosed based on positive SARS-CoV-2 viral  nucleic acid test results but  without any clinically detectable  signs  or  self-perceived  symptoms  (e.g. fever,
sore throat, dry cough). According to the definition of CCDC (China Center for Disease Control), There are two types of asymptomatic persons: 1)  those who have no self- perceived symptoms or clinically detectable signs throughout the 14-day quarantine; and 2) those who are ”asymptomatic” during the incubation period[4][5]. In this article, we will mainly focus on the first type.