Transmission of influenza on international flights, may 2009

Abstract

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.

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 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.

Figure 2

Passenger configuration on flight 1 arriving in Sydney, New South Wales, Australia, on May 24, 2009. ILI, influenza-like illness.

Figure 3

Onset date of influenza-like illness in passengers traveling to Australia on flight 2, May 23, 2009. White bar indicates a negative test result for pandemic (H1N1) 2009 virus; black bar indicates a positive test result for pandemic (H1N1) 2009; gray bars indicate ILI with no test given. ILI, influenza-like illness.