RESEARCH however, 6 passengers did not state exact date of disease onset (Figure 1). Self-reporting of symptoms from passengers did not distinguish between different causes of ILI (Table). Fever was reported from 4 of 8 passengers with conﬁrmed pandemic (H1N1) 2009, 7 of 16 passengers whose ILI was conﬁrmed negative for pandemic (H1N1) 2009, and 2 of 10 passengers with ILI who were not tested. Two or fewer symptoms, not including fever, were reported for 3 of the 8 passengers with conﬁrmed pandemic (H1N1) 2009, 7 for those testing negative for pandemic (H1N1) 2009, and 6 with ILI who were not tested. Location and Transmission Twenty (83%) of 24 passengers in whom an ILI developed postﬂight sat in aisle seats (Figure 2). This seat location increased the risk of contracting an ILI by 1.8×; however, it did not reach statistical signiﬁcance. Survey respondents were 1.3× more likely to sit in an aisle seat. Some clustering of cases was seen with the potential for spread of either ILI or pandemic (H1N1) 2009 from passengers who were infectious before and during the ﬂight (Figure 2). The passengers who were symptomatic for pandemic (H1N1) 2009 before the ﬂight departed were traveling as a family group; however, they were not connected to the persons who contracted the virus during the ﬂight. Likewise, 2 other clusters of nonpandemic ILI with either symptomatic persons boarding the ﬂight or showing symptoms postﬂight were in family groups and therefore would have had substantial contact before, during, and after the ﬂight. Disease was deemed to have spread during the ﬂight in passengers from seats 69H/69J/69K/70H to 68K and 71K, with all case-patients having positive test results for pandemic (H1N1) 2009; from 63C to 62D (neither passenger being tested); and from 84E/84F to 83G, with all case-patients testing negative for pandemic (H1N1) 2009. One cluster (seats 35B/36B/36D/37D) had no identiﬁable preﬂight index case-patient; however, 2 of the passengers were part of a family group. In the cabin section of rows 66–77 was a cluster of passengers with symptoms before boarding who were later found to have positive test results for pandemic (H1N1) 2009. ILI symptoms developed in 7 passengers after the ﬂight. In 2 of these passengers, pandemic (H1N1) 2009 was laboratory conﬁrmed; for 4, ILI was conﬁrmed as negative for pandemic (H1N1) 2009; and 1 person (seat 75G) was not tested. Similarly, in the cabin section of rows 50–64, symptoms developed in 2 passengers during the ﬂight or on the day of arrival (seats 52C and 58B); these passengers were found to be pandemic (H1N1) 2009 positive. Symptoms developed in 7 passengers after ﬂight arrival; 5 of whom had negative Figure 1. Onset date of inﬂuenza-like illness (ILI) in passengers traveling to Australia on ﬂight 1, May 24, 2009. Six other passengers did not state exact ILI onset date. White bar indicates a negative test result for pandemic (H1N1) 2009 virus; black indicates a positive test result for pandemic (H1N1) 2009; gray bars indicate ILI with no test given. test results for pandemic (H1N1) 2009, 2 (seats 54F and 62D) were not tested. Risk of Contracting Disease and Contact Tracing We examined the risk of contracting pandemic (H1N1) 2009 infection postﬂight to all susceptible passengers seated in the economy section of the aircraft. Health authorities contacted 145 passengers on ﬂight 1 for quarantine and prophylactic treatment after potential exposure to pandemic (H1N1) 2009 (Figure 2, large black boxes). Of these, 52 (35%) passengers responded to the surveys, of whom 8 (15%) went into isolation 1 day after ﬂight arrival, 17 (33%) by 2 days after ﬂight arrival, and the others >3 days after ﬂight arrival. Initially, we looked at passengers exposed to this disease who sat in the same row as or within 2 rows either side of passengers who had symptoms develop before or during the ﬂight. The increased risk of contracting pandemic (H1N1) 2009 by sitting in those seats was 1.4% (95% conﬁdence interval [CI] –0.5% to 3.4%). If the contact Table. Signs and symptoms reported by passengers on flight 1 with influenza-like illness, by pandemic (H1N1) 2009 testing status and results, Australia, May 2009* Sign or Tested symptom Positive, n = 8 Negative, n = 16 Fever 4 7 Cough 6 8 Sore throat 1 9 Headache 4 8 Runny nose 5 10 Muscle aches 3 6 Diarrhea 1 2 Lethargy 5 7 Vomiting 0 1 *Signs and symptoms are not mutually exclusive. Not tested, n = 10 2 3 6 3 5 3 1 2 0 1190 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 7, July 2011