RESEARCH Figure 3. Onset date of inﬂuenza-like illness in passengers traveling to Australia on ﬂight 2, May 23, 2009. White bar sections indicate a negative test result for pandemic (H1N1) 2009 virus; black bar section indicates a positive test result for pandemic (H1N1) 2009; gray bar sections indicate ILI with no test given. ILI, inﬂuenza-like illness. Discussion Of 2 long-haul ﬂights entering Australia within the ﬁrst month after declaration of a level 5 alert for pandemic (H1N1) 2009, a total of 45 (6%) of 738 passengers on 2 aircraft were identiﬁed as having the potential to spread an ILI into the local community. Follow-up conﬁrmed 9 passengers with pandemic (H1N1) 2009 infection; 8 of these were from ﬂight 1. Flight 1, originating from a destination with documented widespread community transmission of pandemic (H1N1) 2009, had the greatest potential for introducing the pandemic virus into the Australian community, with 2% of its tested passengers being conﬁrmed positive for pandemic (H1N1) 2009. Spread of the virus from a region known to have sustained community transmission was to be expected and formed part of the case deﬁnition in Australia during the early phases of the pandemic (13). However, ﬂight 2 originated in Singapore, where the ﬁrst recorded case of the disease was on May 26, 2009 (14), 3 days after the aircraft arrived in Australia, which suggests that a targeted approach to aircraft screening that relies on country-speciﬁc information is not completely reliable. Transmission of ILIs on board these aircraft clustered closely with a passenger who was symptomatic during the ﬂight or may have been in contact with an infectious passenger for >15 minutes during the ﬂight. This ﬁnding is similar to transmission of pandemic (H1N1) 2009 noted on a long-haul ﬂight to New Zealand in 2009 (6). Recent studies on the transmission of pandemic (H1N1) 2009 in ferrets demonstrated preference for aerosol and droplet transmission of the virus (15,16). A similar study investigating the disease in a tour group in China indicated droplet transmission from coughing or talking with the index case-patient as being the main mode of transmission (17). The cabin in the A380-800 (ﬂight 1) allows for a 10% wider seat in economy class than does the 747-400 (ﬂight 2) (18), and modern ventilation systems in aircraft circulate air around bands of seat rows rather than the through length of the aircraft (19). However, neither of these measures are enough to prevent droplet transmission from either talking (≈1 meter) or spread of smaller aerosol droplets (7,8). Vigilance by health authorities and cooperation by the public assisted in detecting many ILIs that were not associated with pandemic (H1N1) 2009. These ILIs could be caused by different viruses, as seen by Follin et al. (20). Follin et al. reported that, although 5% of the 70 passengers examined in their study had pandemic (H1N1) 2009, rhinovirus, coronavirus, inﬂuenza B, and parainﬂuenza were also detected. Contact tracing and implementation of public health intervention measures after in-ﬂight exposure to disease is time and resource intensive (21). Southern Hemisphere estimates of the serial interval for pandemic (H1N1) 2009 varied from 1.5 days to 2.9 days (22,23), yet practicalities associated with disease diagnosis and contact tracing meant that quarantine dates began 1–5 days after ﬂight arrival, thus minimizing opportunities to halt transmission by social isolation or chemoprophylaxis. Although compliance with the current practice of following up all passengers in the same row as and within 2 rows either side of the index passenger (11) was similar to a recent survey from Switzerland of air travelers in Europe (24), the increased risk of contracting disease as found in the current study would suggest that further limiting of the zone required for contacting exposed passengers could assist in efﬁcient yet effective public health outcomes. Furthermore, use of risk assessment of different diseases would enable implementation of a public health response that would be proportionate to potential disease severity. Pandemic (H1N1) 2009 and other ILIs can be spread to a community by passengers who were symptomatic before boarding the aircraft. Four of 9 of passengers in whom pandemic (H1N1) 2009 was diagnosed displayed symptoms preﬂight. This ﬁnding is similar to that in a recent study looking at the travel patterns of patients with pandemic (H1N1) 2009 reported from Singapore, where 25% of patients had symptoms before boarding their ﬂights (14). Modeling predicting the global dynamics of disease spread and evidence obtained during the grounding of ﬂights in the United States after September 11, 2001, demonstrated that travel restrictions can delay the intercity spread of inﬂuenza (25). Further modeling has shown that the intervention by preventing symptomatic passengers from boarding ﬂights, particularly at airports considered major hubs, assisted in delaying inﬂuenza spread by up to 2 weeks (26). The control measure of exit screening, combined with the potential value of deterring passengers 1192 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 7, July 2011