Study design, participants, and main outcome measures
This study was a retrospective cohort investigation of the risk of transmission of influenza during a commercial airline flight. The cohort consisted of all passengers seated in the rear section of the aircraft, with a further susceptible subset identified on the basis of interviewing and testing for pandemic A/H1N1. The main outcome measure was the incidence of in-flight infection with pandemic A/H1N1. Secondary outcome measures were the sensitivity and specificity of symptoms of influenza for identifying laboratory confirmed cases and the completeness and timeliness of contact tracing.
Initial public health response
The initial public health response focused on controlling transmission from the high school group. Nasopharyngeal swabs were obtained from those with symptoms of influenza. After identification of influenza A in members of the group, the decision was made to trace all passengers on the flight and manage them to contain the spread of influenza (with the recommended protocol including nasopharyngeal swabs from those with symptoms, home isolation or quarantine, and antiviral post-exposure prophylaxis or treatment as appropriate). A flight manifest was obtained from the airline and arrival cards from immigration authorities in Auckland to identify passengers and their onward travel plans. Details were circulated to NZ public health units in the respective districts or international destinations in receiving countries.
A follow-up investigation began during the week of 4 May 2009. All members of the high school group were interviewed with a standard questionnaire covering illness and history of symptoms before, during, and after the flight. Follow-up serological specimens were also collected from the student group 16-23 days after the flight.
The affected students were seated in the rear section of the aircraft, so this population became the focus of the retrospective cohort study. We used the seating plan and passenger lists to construct a record of all passengers in this section. We retrieved the following data on these passengers from public health units involved in the initial response: symptoms during and after the flight, results of any laboratory testing for pandemic A/H1N1, timeliness and types of public health management. We re-interviewed passengers who reported symptoms, and those for whom no symptom history was recorded, by using a standard questionnaire. For assessment of the public health response, we defined passengers as having been “in transit” if they departed for another international destination within 24 hours of arrival or as NZ residents or visitors according to information collected during immigration processing. We used EpiInfo version 3.5.1 to analyse data.
The airline followed up cabin crew by using the same protocol as we used for passengers. We did not use results from this group in this study because we wanted to link the presence or absence of in-flight infection to seating locations, which were not applicable to mobile flight attendants. No illnesses were reported among cabin crew.
We placed nasopharyngeal and throat swabs in viral transport media and tested them by a real time polymerase chain reaction matrix assay for influenza A using primers from the Centers for Disease Control and Prevention, Atlanta, USA. We used an ABI 7000 Sequence Detection System (Applied Biosystems) for amplification as follows: reverse transcription 50°C for 20 minutes, Taq inhibitor inactivation 95°C for two minutes followed by 45 cycles of polymerase chain reaction amplification, 95°C for 15 seconds, and 55°C for 50 seconds. We sent specimens positive for influenza A RNA to a WHO collaborating centre for reference and research on influenza (Melbourne, Australia). Confirmatory testing was done by sequencing of the matrix, haemagglutinin, and neuraminidase genes for comparison with published influenza A sequences in GenBank or by real time polymerase chain reaction using primers that discriminate pandemic A/H1N1 and seasonal influenza A/H1N1, A/H3N2, and B sequences.
Sera were treated with enzymes that destroy receptors and assayed by haemagglutination inhibition using 1% turkey red blood cells for the presence of antibodies against reference viruses A/Auckland/1/2009 (pandemic A/H1N1) and A/Brisbane/59/2007 (seasonal A/H1N1) at the WHO collaborating centre. Seroconversion was defined as a fourfold rise in titre of haemagglutination inhibition between the acute and the convalescent serum or, when only one sample was available, the presence of a titre of at least 80 against A/Auckland/1/2009.
We considered passengers to have influenza-like illness if they had any two of fever or feverishness, cough, sore throat, or rhinorrhoea. We considered them laboratory confirmed if the nasopharyngeal swab was positive for pandemic A/H1N1, serology detected specific antibodies to pandemic A/H1N1, or both.
We considered a passenger to be a post-flight case of influenza if they developed laboratory confirmed or suspected influenza within a plausible incubation period after the flight. Assuming an incubation period of 0.6 to 3.2 days (based on values reported for influenza A8
) and given a flight duration of 13 hours, this meant that illness from exposure on the flight could begin at any time after arrival in NZ to 3.2 days (77 hours) later.
We divided passengers into categories of pandemic A/H1N1 infection on the basis of their combination of symptoms, timing of symptoms, and laboratory results, as follows.
Laboratory confirmed symptomatic case during flight—Influenza-like illness starting within two weeks before or during the flight with at least one infectious symptom (cough, sneeze, rhinorrhoea) persisting during the flight and nasopharyngeal swab or serology positive for pandemic A/H1N1.
Suspected symptomatic case during flight—Influenza-like illness starting within two weeks before or during the flight with at least one infectious symptom (cough, sneeze, rhinorrhoea) persisting during the flight and pandemic A/H1N1 not excluded (laboratory investigation for pandemic A/H1N1 either not done or incomplete).
Immune case—Symptoms of influenza-like illness before the flight, no infectious symptoms related to this illness during the flight, and serology positive for pandemic A/H1N1.
Laboratory confirmed post-flight case—Influenza-like illness starting within 3.2 days of arrival in NZ and nasopharyngeal swab or serology positive for pandemic A/H1N1.
Suspected post-flight case—Influenza-like illness starting within 3.2 days of arrival in NZ and pandemic A/H1N1 not excluded (laboratory investigation for pandemic A/H1N1 either not done or incomplete).
Non-case—No symptoms of influenza during or after the flight, or symptoms of influenza during or after the flight and pandemic A/H1N1 excluded (laboratory investigations negative for pandemic A/H1N1).
Unknown status—Could not be contacted or insufficient information to assign to another category.
We considered a laboratory confirmed post-flight case to be a case of in-flight infection if they had no other plausible sources of pandemic A/H1N1 infection (before or after the flight). In addition, we defined pandemic A/H1N1 susceptible passengers as those who were seated in the rear section of the plane, excluding laboratory confirmed and suspected cases during the flight, immune cases, and those with unknown status. This susceptible group became the cohort for calculating the risk of in-flight infection with influenza.