Restricting the contact zone on international long-haul flights to two seats in front, two seats behind, and two seats on either side of passengers infected with influenza may increase the chance of identifying those exposed to the illness, Dr. A. Ruth Foxwell of Australian National University, Canberra, and her colleagues reported in a retrospective cohort study published June 15 in Emerging Infectious Diseases.
At the time of the 2009 H1N1 influenza outbreak, the standard for contact tracing issued by Australia’s health officials was defined as passengers sitting in the same row as or within two rows of a confirmed case-patient. However, rather than basing its policy on influenza studies, the policy was based on evidence obtained from Mycobacterium tuberculosis studies.
"The way that public health authorities were responding to the pandemic in Australia was based on really no evidence at all from influenza," coauthor Dr. Paul M. Kelly, also of Australian National University, said in an interview. "This is the first study that really demonstrated that those assumptions that people close by are at greater risk are, in fact, true."
"What we suggest is that, if we restrict the zone to just those six people, then you’d find most of the cases and, by doing that, you would very much relieve the cost in terms of people and time and in money to follow up the larger group," he continued.
When using the current contact-tracing standards, passengers sitting near those who had preflight symptoms were at 3.6% increased risk of contracting pandemic H1N1 flu. That increased to 7.7% increased risk of contracting pandemic H1N1 post flight when using the 2-by-2-seat square proposed in this study.
Contact tracing was estimated by "dividing the number of people with pandemic (H1N1) 2009 or an ILI [influenzalike illness] by the number of susceptible people in the contact zone," Dr. Kelly said. Only those sitting in the economy class were considered exposed to pandemic H1N1 because of the location of symptomatic passengers.
Dr. Kelly and his colleagues examined in-flight transmission of pandemic H1N1 and ILIs using two long-haul flights entering Australia the weekend of May 23-24, 2009. An ILI includes pandemic H1N1 flu and was defined as one or more symptom (cough, runny nose, sore throat, or fever) within 7-14 days before the flight or during the flight or 7 or fewer days after arrival, according to the study (Emerg. Inf. Dis. 2011;17:1189-94).
Surveys, distributed 3 months after flight arrival, elicited a response rate of 43%. Results showed that a total of 45 (6%) of 738 passengers on both aircraft developed an ILI during the first week after flight arrival. Follow-up confirmed that nine passengers were infected with pandemic H1N1 flu; eight of these were from the first flight.
Flight 1 (14 hours) embarked from Los Angeles, an area of considerable community transmission, and was found to have six passengers with confirmed pandemic H1N1 infection within 24 hours after arriving to Sydney. In total, 2 of 24 (8%) other passengers who developed ILI symptoms less than or equal to 7 days after arriving in Sydney were later confirmed to have pandemic H1N1 flu.
Some passengers traveling in family groups either contracted pandemic H1N1 or an ILI before, during, and after the flight. This may have been the source of potential spread of infection or illness.
Flight 2 (about 8 hours) embarked from Singapore, an area that had its first recorded case of pandemic H1N1 3 days after the aircraft landed in Sydney. It was found to have one confirmed case of pandemic H1N1 after arrival.
Within 7 days after arrival, six passengers developed ILI symptoms and one passenger, a child in seat 33D, was confirmed to have pandemic H1N1 flu. The child was sitting with an adult passenger who developed ILI symptoms during the flight but tested negative for the virus.
The results of this study may encourage public health officials to take more safety measures preflight and during flights, and recognize that a smaller risk zone is more effective in finding infected passengers, Dr. Kelly said.
Other concurrent conditions such as obesity and pregnancy were taken into consideration, and did not make passengers more susceptible to contracting an ILI, the investigators said.
Dr. Kelly received funding from Australia’s National Health and Medical Research Council, and colleague Dr. A. Ruth Foxwell received funding from the Australian Government Department of Health and Aging.