Available evidence indicates that the risk of transmission of M. tuberculosis on board aircraft is low and limited to persons in close contact with an infectious case for 8 hours or longer.
Research has shown that the risk of any communicable disease being transmitted on board aircraft is low (4). However, transmission probably occurs more frequently than reported because most diseases have an incubation period longer than the duration of air travel. Airborne and droplet-borne diseases that are potentially transmissible on board aircraft include TB, influenza, meningococcal disease, measles and SARS (5).
TB is an infectious disease, caused by mycobacteria of the M. tuberculosis complex and transmitted by exposure to tubercle bacilli in airborne droplet nuclei produced by a person with infectious TB during coughing, sneezing, singing or talking. When TB develops in the human body, it does so in two stages: firstly, the individual exposed to M. tuberculosis becomes infected and secondly, the infected individual develops the disease (active TB). However, only a small minority (<10%) of infected individuals will subsequently develop active disease and most of them will do so within five years. While the risk of progression to active TB disease is greatest within the first two years after infection, latent infection may persist for life.
No cases of TB disease have so far been reported among those known to have been infected with M. tuberculosis during air travel. From 1992 to 1994, the United States Centers for Disease Control and Prevention (CDC), together with state and local health departments, conducted contact investigations for seven index cases, involving one cabin-crew member and six passengers with infectious TB disease who had flown during this period. The concern was that the closed aircraft cabin environment may have facilitated transmission of M. tuberculosis (6–11). The total number of potentially exposed passengers and cabin crew exceeded 2600 on 191 flights involving nine different types of aircraft.
All index cases were identified as highly infectious, i.e. smears from spontaneous sputum specimens from all index cases were grossly positive for acid-fast bacilli (AFB) and all were culture-positive and had evidence of extensive pulmonary disease on chest radiography. In addition, one patient had biopsy- and culture-confirmed laryngeal TB, the most infectious form of TB.
Strains of M. tuberculosis resistant to isoniazid and rifampicin, i.e. multidrug-resistant TB (MDR-TB), were isolated in two of these episodes (6, 10). Organisms isolated from the other index cases were sensitive to all anti-TB drugs. Two passengers, who were flying to the United States for medical care, knew that they had active TB disease at the time of their flights but did not inform the airline of their status. In the other five instances, TB was diagnosed after the flights.
Investigation of close contacts found evidence of transmission of M. tuberculosis infection during a flight in only two of the seven episodes. In one event, transmission from a cabin flight attendant was detected in 2 of 212 crew members who had worked in close proximity with the index case during a 6-month period; both of those infected were exposed to the infectious source for at least 12 hours. In the other event, there was probable transmission from an infectious case to 4 passengers (seated in close proximity to the index case in the same cabin section), out of a total of 257 passengers tested on a flight longer than 8 hours (6, 10). These results suggest that the risk of infection with M. tuberculosis during air travel is similar to that associated with exposure during other activities in which prolonged contact with potentially infectious individuals may occur (e.g. train or bus travel, any gathering in enclosed spaces).
The average lifetime risk of untreated latent TB infection progressing to active disease at some time during life is <10% and not all persons infected as a result of exposure during air travel may receive effective preventive antibiotic treatment. Therefore, although no cases of TB disease have yet been reported among the infected contacts in the seven studies carried out by CDC, the possibility that future cases of TB disease due to TB infection acquired during air travel may occur cannot be excluded.
Subsequent published case reports of other instances of infectious TB in passengers on long-haul flights (12–16), reviewed in 2005 (17) and with a further case presently under investigation (18), have also suggested that the risk of transmission of infection on board appears to be low. According to an international airline analysis of in-flight TB on long-haul flights, 34 cases of infectious TB were notified to the airlines during a five-year period (2000–2004), giving an overall notification rate of 0.05 per 100 000 long-haul passengers (19).
There is currently little evidence concerning the transmissibility of drug-resistant strains of TB during air travel. However, in other settings, drug-resistant TB has not been found to be more transmissible than drug-sensitive strains, although the patients may remain infectious for longer (20–22). While the risk of acquiring infection may not differ, the consequences are significant in terms of complexity of treatment, outcomes and cost.
Further information is needed on the outcome of contact investigations.
The available evidence on the risk of transmission of TB during air travel and outcome data from passenger-contact investigations are limited. In order to strengthen the evidence base for operational decision-making and policy development, a coordinated international approach to research, data collection, analysis and dissemination is needed.
