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.