This study has several limitations. As available data were recorded in days, and not in hours, it was not possible to determine the time intervals more precisely. As this was both with first and last date of the intervals, we expect no negative or positive bias. Secondly, the arrival date was used for date of exposure, while the actual exposure might have already taken place the day before at departure of the flight. This would imply an increase in delay and decrease the effectiveness of contact tracing. Also, we have no data if, and when contacts were actually reached and oseltamivir was administered. Since several steps were still required to reach the contacts after they were identified through passenger lists, this only would have lead to further delay in administrating prophylaxis. Further investigation into the timeliness of administration of prophylaxis among these contacts is initiated, to have insight in the delay of this last interval to facilitate future decisions on the effectiveness and necessity of contact tracing among flight passengers. Lastly, this study includes CT initiated at only one airport. CT procedures might be different at airports in other countries, which influences interval III. As this is not causing the main delay, we do not expect that in other countries CT would be much faster.