Transmission of pandemic A/H1N1 2009 influenza on passenger aircraft: retrospective cohort study.
OBJECTIVE
To assess the risk of transmission of pandemic A/H1N1 2009 influenza (pandemic A/H1N1) from an infected high school group to other passengers on an airline flight and the effectiveness of screening and follow-up of exposed passengers.


METHODS
Retrospective cohort investigation using a questionnaire administered to passengers and laboratory investigation of those with symptoms.


METHODS
Auckland, New Zealand, with national and international follow-up of passengers.


METHODS
Passengers seated in the rear section of a Boeing 747-400 long haul flight that arrived on 25 April 2009, including a group of 24 students and teachers and 97 (out of 102) other passengers in the same section of the plane who agreed to be interviewed.


METHODS
Laboratory confirmed pandemic A/H1N1 infection in susceptible passengers within 3.2 days of arrival; sensitivity and specificity of influenza symptoms for confirmed infection; and completeness and timeliness of contact tracing.


RESULTS
Nine members of the school group were laboratory confirmed cases of pandemic A/H1N1 infection and had symptoms during the flight. Two other passengers developed confirmed pandemic A/H1N1 infection, 12 and 48 hours after the flight. They reported no other potential sources of infection. Their seating was within two rows of infected passengers, implying a risk of infection of about 3.5% for the 57 passengers in those rows. All but one of the confirmed pandemic A/H1N1 infected travellers reported cough, but more complex definitions of influenza cases had relatively low sensitivity. Rigorous follow-up by public health workers located 93% of passengers, but only 52% were contacted within 72 hours of arrival.


CONCLUSIONS
A low but measurable risk of transmission of pandemic A/H1N1 exists during modern commercial air travel. This risk is concentrated close to infected passengers with symptoms. Follow-up and screening of exposed passengers is slow and difficult once they have left the airport.
Baker
Michael G
MG
Department of Public Health, University of Otago, Box 7343 Wellington South 6242, New Zealand. michael.baker@otago.ac.nz
Thornley
Craig N
CN
Mills
Clair
C
Roberts
Sally
S
Perera
Shanika
S
Peters
Julia
J
Kelso
Anne
A
Barr
Ian
I
Wilson
Nick
N
D000401:Aircraft / Q000706:statistics & numerical data*; D016358:Contact Tracing; D004196:Disease Outbreaks / Q000706:statistics & numerical data*; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000453:epidemiology / Q000635:transmission*; D009520:New Zealand / Q000453:epidemiology; D012189:Retrospective Studies; D012307:Risk Factors; D012650:Self Disclosure; D011795:Surveys and Questionnaires