Epidemiology of the infection
Majority of the reports presented worldwide are asymptomatic cases,
which are typically exposed to SARS-CoV-2 during travel from Wuhan (12,
13), and living together(13, 14). Estimation of the asymptomatic ratio
of novel coronavirus (COVID-19) cases, a study in China revealed that
around 42% of evacuated citizens were asymptomatic. This figure could
reach 67% and unless RNA-PCR or other laboratory tests done, it was
impossible to identify as cases(15). One 17 years old young in Bahir Dar
Ethiopia on 04/May 2020, and another person in Southern Nations
Nationalities, Ethiopia found as SARS-CoV-2 positive were asymptomatic
cases (16). Many experts believe that unnoticed, asymptomatic cases of
coronavirus infection could be an important source of contagion. From a
total of 166 new infections in China in April 2020 more than half
130(78%) were identified as asymptomatic(17). Of course, this
proportion is not a real scientific indication based on the world health
organization report (18). Before becoming symptomatic, this person
exposed to two other colleagues who subsequently received a COVID-19
diagnosis but did not have contact with the primary patient from any
other sources. A report from Singapore described seven COVID-19 clusters
resulting from the presymptomatic transmission; presymptomatic primary
patients varied from persons with travel from high-incidence countries
to persons exposed in the local community, a very challenging indication
of SARS-CoV-2 transmission during presymptomatic stage(19). A cluster of
cases who have travel history traced their contacts before signs and
symptoms. Based on tracing and diagnosis reports, individuals who had no
travel history also contracted the disease. This indicates that the
asymptomatic cases are the potential source of transmission of
SARS-CoV-2(14). Similarly, cases of presymptomatic transmission have
been reported from other countries before widespread community
transmission occurred. A report from Germany, documented infection of a
German businessman after exposure to a mildly symptomatic colleague
visiting from China (20). This ability of transmission of SARS-CoV-2,
while a person is asymptomatic, was ascertained by different virologic
and modeling studies in different areas (9). The incubation periods for
asymptomatic primary patients with distinct exposures ranged from 3 to
14 days; sometimes it will last to 21 days(21, 22), the basic
reproductive number(R 0) COVID-19 is also higher than the previous
SARS epidemics (23), but this number is higher than 6.5 in other studies
done using stochastic methods(24-28). Therefore, it can be concluded as
all primary patients experienced different periods of initial exposure
and asymptomatic close contact with secondary patients who have no other
known exposure risks.