#Thread on symptomatic, pre-symptomatic, asymptomatic #transmission. A symptomatic #COVID19 case is a patient who developed signs & symptoms compatible with #SARSCoV2 #coronavirus #infection. Symptomatic transmission refers to transmission from a person experiencing symptoms. 
Data from published epidemiology & virologic studies provide evidence that #COVID19 is PRIMARILY transmitted from symptomatic people to others in close contact through respiratory droplets, by direct contact with infected persons or by contact with contaminated objects & surfaces
This is supported by experiences shared by WHO technical partners & presentations by Ministries of Health. Data from studies that have collected repeated samples from patients show that #COVID19 shedding is highest in upper respiratory tract early in the course of the disease.
That is within the FIRST 3 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.
Pre-symptomatic transmission: Incubation period for #COVID19, time between exposure to the #SARSCoV2 #coronavirus (becoming infected) & symptom onset, is on average 5-6 days, but can be up to 14 days. During this “pre-symptomatic” period, some infected persons can be contagious.
#Transmission from a pre-symptomatic case can occur before symptom onset. In a small number of case reports and studies, pre-symptomatic 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 #COVID19 from 1-3 days before they develop symptoms. So it is possible that people infected with #SARSCoV2 #coronavirus could transmit the #virus before significant symptoms develop.
NOTE: Pre-symptomatic transmission still requires the #coronavirus #SARSCoV2 to be spread via infectious DROPLETS or through touching CONTAMINATED surfaces. #fomites
Asymptomatic transmission: an asymptomatic lab-confirmed case is a person infected with #COVID19 who does NOT develop symptoms. Asymptomatic transmission refers to transmission of the #SARSCoV2 #coronavirus from an infected person, who does not develop symptoms.
There are few reports of lab-confirmed #COVID19 cases who are truly asymptomatic, but to date, no documented asymptomatic transmission. This does not exclude the possibility that it may occur. Asymptomatic cases have been reported as part of contact tracing in some countries.
