Accounting for imported cases in estimating the time-varying reproductive number of COVID-19

ABSTRACT Background Estimating the time-varying reproductive number, R t , is critical for monitoring transmissibility of an emerging infectious disease during outbreaks. When local transmission is effectively suppressed, imported cases could substantially impact transmission dynamics. Methods We developed methodology to estimate separately the R t for local cases and imported cases, since certain public health measures aim only to reduce onwards transmission from imported cases. We applied the framework to data on COVID-19 outbreaks in Hong Kong. Results We estimated that the R t for local cases decreased from above one in the early phase of outbreak to below one after tightening of public health measures. Assuming the same infectiousness of local and imported cases underestimated R t for local cases due to control measures targeting travelers. Conclusions When a considerable proportion of all cases are imported, the impact of imported cases in estimating R t is critical. The methodology described here can allow for differential infectiousness of local imported cases.