Transmission of Inﬂuenza on International Flights Figure 2. Passenger conﬁguration on ﬂight 1 arriving in Sydney, New South Wales, Australia, on May 24, 2009. ILI, inﬂuenza- like illness. zone was modiﬁed to sitting in the same row as or within 2 rows either side of passengers who had preﬂight symptoms (rows 67–72) and not those in whom symptoms developed during the ﬂight, the risk of contracting pandemic (H1N1) 2009 postﬂight increased to 3.6% (95% CI –1.3% to 8.6%). No passengers were detected as acquiring pandemic (H1N1) 2009 from either of the passengers with symptoms that developed during the ﬂight; however, 2 passengers in the same section of the aircraft who responded to the survey indicated having had ILI symptoms but not being tested for pandemic (H1N1) 2009. The current zone for contact tracing is deﬁned by passengers in the same row as and within 2 rows either side of the index case-patient. A closer zone forming a square delimited by 2 seats in front, 2 seats behind, and 2 seats on either side of the index case-patient could be prescribed (Figure 2, small black boxes [rows 67–72]). When the risk of contracting pandemic (H1N1) 2009 postﬂight was calculated for economy passengers sitting in the 2 × 2 square around the preﬂight symptomatic index case-patients in seats 69H–70H, the risk of becoming ill postﬂight increased to 7.7% (95% CI –2.6% to 17.9%). Flight 2 On May 23, 2009, a Boeing 747-400 arrived in Sydney from Singapore carrying 293 passengers. Of the 131 (45%) passengers responding to a survey regarding the potential for contracting an ILI during their ﬂight, 114 (87%) were Australian residents. Public health authorities were not alerted to the potential for passengers carrying pandemic (H1N1) 2009 virus on this ﬂight until 6 days after arrival. They were alerted after mandatory notiﬁcation of the virus infecting 1 passenger. Survey data showed that 1 passenger was identiﬁed as having symptoms consistent with an ILI before the ﬂight and that symptoms developed in 2 additional passengers on the ﬂight. The passenger who was symptomatic before the ﬂight was not tested for pandemic (H1N1) 2009; of the passengers whose symptoms developed during the ﬂight, 1 was not tested and 1 (adult in seat 33D) was tested 6 days after ﬂight arrival. At that time, the test returned a negative result. Six passengers were identiﬁed from the survey as having ILI symptoms within 7 days after ﬂight arrival (Figure 3). Of these, 1 passenger with symptom onset within 48 hours after ﬂight arrival (a child in seat 33D) was laboratory conﬁrmed to have pandemic (H1N1) 2009, 1 passenger (seat 24K) had negative test results for pandemic inﬂuenza A, and 4 passengers (seats 34A, 35B, 41G, and 63D) were not tested. The passenger in whom pandemic (H1N1) 2009 was conﬁrmed was a child companion sharing a seat on the aircraft with the passenger whose symptoms developed during the ﬂight but who tested negative for the virus. Some clustering of cases was seen, with a potential spread of disease from passengers who were symptomatic during the ﬂight (seats 62C–63D, 33D–34A, and 35B and the child sharing 33D). Transmission of disease between adult and child in 33D could have occurred before or during the ﬂight. With lack of a conﬁrmed index case-patient for pandemic (H1N1) 2009, the increased risk for disease was not calculated. No other passengers on this ﬂight had positive test results for pandemic (H1N1) 2009 infection after triangulation methods with the notiﬁable diseases database. Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 7, July 2011 1191