Types of transmission from COVID patients
Based on reports about the transmission of COVID-19, WHO has provided a brief overview of available evidence on transmission from symptomatic, pre-symptomatic and asymptomatic people infected with COVID-19.
A symptomatic COVID-19 case is defined as 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. Preliminary data suggests that people may be more contagious around the time of symptom onset as compared to later on in the disease.
Data from clinical and virologic provide evidence that shedding of the COVID-19 virus is highest in upper respiratory tract (nose and throat) early in the course of the disease, i.e. within the first 3 days from onset of symptoms.
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; but can be up to 14 days. During this period, which is also known as the “presymptomatic” period, some infected persons can be contagious. Therefore, transmission from a pre-symptomatic case can occur before symptom onset.
Please note that pre-symptomatic transmission still requires the virus to be spread via infectious droplets or through touching contaminated surfaces.
Asymptomatic transmission refers to transmission of the virus from a person, who does not develop symptoms. There have been a few reports of laboratory-confirmed cases who are truly asymptomatic, and to date, there has been no documented asymptomatic transmission.