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.