Inferring Timing of Infection Using Within-host SARS-CoV-2 Infection Dynamics Model: Are “Imported Cases” Truly Imported?

In countries/communities at risk of future outbreaks of COVID-19, ascertaining whether cases are imported or the result of local secondary transmission is important for government to shape appropriate public health strategies. In this study, we propose a novel approach to identify the timing of infection, whereby we developed a within-host model to capture viral load dynamics post-symptom onset. We submit our approach allow us to differentiate imported cases from local secondary cases. To illustrate our method, we use the initial reported cases in Singapore, where the first reported 18 cases were considered imported, as these individuals had recent travel history to Wuhan, China, which is a hotspot of COVID-19 outbreak. With additional information regarding day of entrance in Singapore, we were able to infer whether these were infected locally or prior to arriving in Singapore. Of all the cases, we identified 6 as likely evidence of ongoing secondary transmission within Singapore. In an early phase of outbreaks, collecting viral load data over time from cases from symptom onset is highly recommended.