This study has several limitations, particularly with regard to estimating the number of infected travelers who would have passed through the airport during the screening period. Most cases of illness acquired overseas would probably not have been notifi ed, particularly those in patients with mild illness who did not see a doctor or who saw a doctor but did not receive a diagnosis. The estimated proportion of overseas-acquired cases was based on data from the fi rst 100 cases and would have decreased as the pandemic progressed. The net effect of these factors is unknown, but they would probably have increased the estimated number of undetected infected travelers passing through screening, thereby further reducing the estimated sensitivity of screening.