Close contacts on fl ights accounted for the highest proportion of all the close contacts, in part because of how the index cases were detected and the broad defi nition we used for close contacts. However, the attack rate was much lower than that for other close contacts; 1 index case infected only 0.19 close contacts on fl ights on average. This fi nding indicated that the possibility of transmission of pandemic (H1N1) 2009 virus on fl ights was low, and the yield of tracing and quarantining of close contacts on fl ights was limited. Tracing contacts of service persons at public places was more diffi cult than tracing other categories of contacts, and the lowest attack rate (0.2%) was recorded in this category. Despite extensive measures, on average, only 0.01 infected close contacts per index case-patient were identifi ed among service persons. Tracing the contacts of service persons at public places seems far less cost-effective. Criteria for close contacts on fl ights and those of service persons should be refi ned with respect to exposure duration and age of those exposed. Exposure to index case-patients for >12 hours was a signifi cant independent risk factor for infection in fl ight passenger contacts. This fi nding suggests that limiting the time of contact with persons with ILI on aircraft can reduce risk for transmission, and a long duration of exposure may be necessary for transmission to occur on aircraft.