As the COVID-19 outbreak continues to evolve, new facts about this novel virus are revealed every day. Here is the summary what has been reported about the transmission of the COVID-19 virus, and provide a brief overview of available evidence on transmission from symptomatic, presymptomatic and asymptomatic people infected with COVID-19.

Symptomatic transmission

By way of definition, a symptomatic COVID-19 case is a case who has developed signs and symptoms compatible with COVID-19 virus infection. Symptomatic transmission refers to transmission from a person while they are experiencing symptoms.

Data from published epidemiology and virologic studies provide evidence that COVID-19 is primarily transmitted from symptomatic people to others who are in close contact through respiratory droplets, by direct contact with infected persons, or by contact with contaminated objects and surfaces.

Data from clinical and virologic studies that have collected repeated biological samples from confirmed patients provide evidence that shedding of the COVID-19 virus is highest in the upper respiratory tract (nose and throat) early in the course of the disease. That is, within the first three days from onset of symptoms. Preliminary data suggests that people may be more contagious around the time of symptom onset as compared to later on in the disease.

Presymptomatic transmission

The incubation period for COVID-19, which is the time between exposure to the virus (becoming infected) and symptom onset, is on average 5-6 days, however can be up to 14 days. During this period, also known as the “presymptomatic” period, some infected persons can be contagious. Therefore, transmission from a presymptomatic case can occur before symptom onset.

In a small number of case reports and studies, the presymptomatic transmission has been documented through contact tracing efforts and enhanced investigation of clusters of confirmed cases. This is supported by data suggesting that some people can test positive for COVID-19 from 1-3 days before they develop symptoms.Thus, it is possible that people infected with COVID-19 could transmit the virus before significant symptoms develop. It is essential to recognize that presymptomatic transmission still requires the virus to be spread via infectious droplets or through touching contaminated surfaces.

Asymptomatic transmission

An asymptomatic laboratory-confirmed case is a person infected with COVID-19 who does not develop symptoms. Asymptomatic transmission refers to transmission of the virus from a person, who does not develop symptoms.

There are few reports of laboratory-confirmed cases which are genuinely asymptomatic, and to date, there has been no documented asymptomatic transmission. This does not exclude the possibility that it may occur. Asymptomatic cases have been reported as part of contact tracing efforts in some countries.

Transmission modes:

Community transmission:This transmission is evidenced by the inability to relate confirmed cases through chains of transmission, or by increasing positive tests through sentinel samples, that is, the routine systematic testing of respiratory samples from established laboratories.

Local transmission:This transmission indicates locations where the source of infection is within the reporting location or region.

Imported cases: This transmission indicates locations where all cases have been acquired from outside the location of reporting.

Under investigation transmission:This transmission indicates locations where type of transmission has not been determined for any cases.

Interrupted transmission:This transmission indicates locations where interruption of transmission has been demonstrated.