RESEARCH Transmission of Inﬂuenza on Inter national Flights, May 2009 A. Ruth Foxwell, Leslee Roberts, Kamalini Lokuge, and Paul M. Kelly Understanding the dynamics of inﬂuenza transmission on international ﬂights is necessary for prioritizing public health response to pandemic incursions. A retrospective cohort study to ascertain in-ﬂight transmission of pandemic (H1N1) 2009 and inﬂuenza-like illness (ILI) was undertaken for 2 long-haul ﬂights entering Australia during May 2009. Combined results, including survey responses from 319 (43%) of 738 passengers, showed that 13 (2%) had an ILI in ﬂight and an ILI developed in 32 (5%) passengers during the ﬁrst week post arrival. Passengers were at 3.6% increased risk of contracting pandemic (H1N1) 2009 if they sat in the same row as or within 2 rows of persons who were symptomatic preﬂight. A closer exposed zone (2 seats in front, 2 seats behind, and 2 seats either side) increased the risk for postﬂight disease to 7.7%. Efﬁciency of contact tracing without compromising the effectiveness of the public health intervention might be improved by limiting the exposed zone. Tin Mexico and the United States, with rapid spread to he emergence of pandemic inﬂuenza A (H1N1) 2009 Europe, Asia, and the Paciﬁc, is testament to the ease of spread of infectious disease across the globe (1). The World Health Organization activated level 5 pandemic alert on April 29, 2009, when sustained community transmission of the pandemic virus was demonstrated in Mexico and the United States. In her address to the United Nations on May 4, 2009, Margaret Chan, Director-General of the World Health Organization, called for heightened vigilance Author afﬁliations: Department of Health and Ageing, Canberra, Australian Capital Territory, Australia (A.R. Foxwell, L. Roberts); Australian National University, Canberra (A.R. Foxwell, L. Roberts, K. Lokuge, P.M. Kelly); and ACT Government Health Directorate, Canberra (P.M. Kelly) DOI: 10.3201/eid1707.101135 to limit international spread of the virus (2). Australia’s response was rapid, with the introduction of a number of measures as outlined in the Australian Health Management Plan for Pandemic Inﬂuenza, 2008 (3). These measures included in-ﬂight messages to incoming passengers, use of health declaration cards by all incoming travelers, and mandatory reporting by the pilot on the health status of crew and passengers before landing (4). The novel virus was also listed as a quarantinable disease under Australia’s Quarantine Act 1908, which allows for the application of public health powers for intervention (5). Reports documenting spread of disease during airline ﬂight are limited (6–9). Speciﬁc policy stating that passengers sitting in the same row as and within 2 rows of a conﬁrmed case-patient should be treated as suspected of having that disease relies on studies of air travel where the index case-patient was infected with Mycobacterium tuberculosis (10–12). The aim of this study was to investigate the spread of pandemic (H1N1) 2009 infection from persons with conﬁrmed disease on ﬂights to Australia during May 2009. The spread of other inﬂuenza-like illness (ILI) was also documented. Methods Study Population A retrospective cohort study designed to determine exposure risk to known pandemic (H1N1) 2009 virus was undertaken for 2 long-haul ﬂights that entered Australia the weekend of May 23–24, 2009. Flight 1 was chosen after identiﬁcation of 6 passengers with conﬁrmed pandemic (H1N1) 2009 infection within 24 hours after ﬂight arrival from the United States. Flight 2 was chosen after identiﬁcation of a conﬁrmed case of pandemic (H1N1) 2009. This ﬂight came from an area that lacked community 1188 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 17, No. 7, July 2011