transmission. Passenger details were obtained through collection of Health Declaration Cards and comparing the cards to ﬂight manifests obtained from the airlines. The deﬁnition of ILI was broad to capture as many persons as possible within the dataset. Passengers were asked to self-report development of any of the following signs or symptoms: fever, cough, sore throat, headache, runny nose, muscle aches, diarrhea, and lethargy. ILI was deﬁned as >1 symptom (cough, runny nose, sore throat, or fever) within 7–14 days before the ﬂight or during the ﬂight or <7 days after arrival. The time periods were put in place to help determine when passengers were most likely to have contracted their ILI. Passengers were excluded who indicated bacterial infection (antibiotic prescription from medical personnel) or regular health issues, such as migraines. All but 4 passengers reporting symptoms had conﬁrmation of ILI status from a qualiﬁed health professional. Self-identiﬁcation of other health conditions that were considered potential concurrent conditions for purpose of this study included obesity, diabetes mellitus, immunosuppression, asthma, chronic lung disease, and pregnancy. Seat location, concurrent condition status, and contraction of disease were compared. Ethics approval was given by the Australian Government Department of Health and Ageing Ethics Committee and the Australian National University Human Research Ethics Committee. Data Collection Surveys were distributed to passengers 3 months after ﬂight arrival. The survey asked about inﬂuenza-like symptoms, symptom onset time, concurrent conditions, antiviral prophylaxis and treatment, isolation or quarantine dates, other potential exposure to ILI before and after the ﬂight, contact with health professionals after the ﬂight, and details of testing for pandemic (H1N1) 2009 virus. Two reminders were sent to improve the response rate of the study. As a triangulation method, all passenger names, passenger sex, disease onset dates, and postal codes were cross-checked against those of passengers with known pandemic (H1N1) 2009 cases that were notiﬁed to national authorities for 1 month after ﬂight arrival to verify information received from the survey responses and identify additional cases. Travel details were veriﬁed for pandemic (H1N1) 2009 case-patients identiﬁed through national notiﬁcation. Contact tracing through public health authorities also identiﬁed ILI case-patients who had negative laboratory test results for pandemic (H1N1) 2009 virus. Data Analysis The increased risk of passengers contracting either laboratory-conﬁrmed pandemic (H1N1) 2009 or an Transmission of Inﬂuenza on International Flights ILI (including pandemic [H1N1] 2009) was separately estimated by dividing the number of persons with the illness by the number of susceptible persons in the contact zones as described. For pandemic (H1N1) 2009, only passengers sitting in the economy class were considered exposed because of the location of persons displaying symptoms preﬂight or during the ﬂight and the sectional layout of the aircraft. Results Of the 738 passengers on the 2 ﬂights, 319 (43%) responded to a questionnaire; 143 (18%) of passengers could not be contacted. Cross-checking of mandatory notiﬁcations of pandemic (H1N1) 2009 against all passengers on both ﬂights found 2 additional pandemic (H1N1) 2009 infection cases while also conﬁrming symptom data on survey responses. Contact tracing by public health authorities found 5 additional passengers with ILI who had negative test results for pandemic (H1N1) 2009. No passengers who had positive test results for pandemic (H1N1) 2009 had underlying health conditions considered to make them more susceptible to inﬂuenza; however, 5 of 32 passengers reporting ILI postﬂight had >1 potential concurrent conditions (1 each of obesity, diabetes mellitus, pregnancy, asthma, and chronic lung disease). Limited analysis demonstrated that the concurrent condition did not make these persons more susceptible than other passengers to contracting an ILI. Flight 1 Flight 1, an Airbus A380, embarked from Los Angeles and arrived in Sydney on May 24, 2009, carrying 445 passengers. Of the 188 (42%) passengers who responded to a survey, 169 (90%) were Australian residents. Response rate varied with class of travel, with 11 (79%) of 14 ﬁrst class passengers, 40 (56%) of 71 business class passengers, 19 (59%) of 32 premium economy class passengers, and 117 (36%) of 327 economy class passengers responding. Combined results from the survey and disease notiﬁcation data sources identiﬁed 8 passengers who had an ILI at the beginning of the 14-hour ﬂight. For 4 of these passengers, pandemic (H1N1) 2009 infection was later laboratory conﬁrmed; for 1, pandemic (H1N1) 2009 infection was conﬁrmed as negative; 3 passengers were not tested. ILI symptoms developed in 2 other passengers during the ﬂight, and pandemic (H1N1) 2009 was conﬁrmed in both. Twenty-four passengers were identiﬁed as developing ILI symptoms <7 days after arrival in Australia. Of these, 2 passengers had laboratory-conﬁrmed pandemic (H1N1) 2009 infection, 15 had illness conﬁrmed as negative for the pandemic virus, and 7 were not tested. Most passengers experienced onset of symptoms <3 days after ﬂight arrival; Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 7, July 2011 1189