Novel coronavirus disease 2019 (COVID-19) was declared a pandemic on March 11, 2020.1 Transmission chains have been difficult to identify in routine investigations. Notable cases in Italy2 and the United States3 had no known source of infection. These cases have raised the concern that there is transmission of the novel coronavirus (SARS-CoV-2) in asymptomatic individuals (who never express symptoms) or in pre-symptomatic individuals (prior to any symptoms).

Many public actions so far, including quarantine measures, body temperature measurement, and fever symptom surveillance, have prioritized identification of possible infected cases. However, these all depend on active expression of symptoms and are not able to identify asymptomatic transmission or pre-symptomatic transmission. Better understanding of transmission parameters, including the serial interval and incubation period, can also help determine the possible progression of the outbreak. The serial interval is defined as the interval between a primary case of COVID-19 developing symptoms and a secondary case developing symptoms, whereas the incubation period is the time lag between infection and the start of symptoms. The Chinese Center for Disease Control and Prevention (CDC) has estimated that the mean serial interval for COVID-19 is 7.5 days, which is slightly longer than the estimated incubation period of 5.2 days.

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