This incident provided a unique opportunity to investigate the transmission of M. tuberculosis on aircraft. A high proportion of U.S. residents among the passengers, repeated notification, and national media attention led to a high response rate among contacts. Prompt notification by a state tuberculosis-control program allowed us to assess the development of tuberculous infection among the May 1994 passengers and flight crew prospectively.

Our investigation provides evidence of the transmission of M. tuberculosis from passenger to passenger and from passenger to flight crew aboard commercial aircraft. Although the possibility of transmission from the index patient to other passengers on flights 1, 2, and 3 cannot be excluded, the evidence is most compelling for flight 4. This includes evidence of recent transmission (i.e., skin-test conversions), an association between transmission and proximity to the index patient, and a dose–response effect. All but one of the contacts who had no risk factors for tuberculosis and had positive skin tests, including all those with conversions, were seated in the same section as the index patient. Those seated within two rows were at greatest risk. The skin-test conversions on flight 4 but not on flight 3, although the flights took place on the same day, suggest that prolonged exposure to aerosol droplets from the index patient played a part. The apparent absence of transmission on flight 1 may have been due to the varying infectiousness of the index patient, who had long-standing disease. She was more symptomatic in May than in April. The timing of the skin-test conversion of the child in Household 2 suggests that transmission occurred just before flights 3 and 4. The children in Household 1 may have escaped infection because the index patient was less infectious in April, because they were not in close proximity to her when she returned in May, or because of chance alone, since many household contacts of people with infectious tuberculosis are known to remain uninfected.8,9 Since there are no clinical data on the risks and benefits of preventive therapy that does not include isoniazid and rifampin, clinicians of the infected contacts had two options: to administer no preventive therapy and watch carefully for the appearance of signs and symptoms of tuberculosis,10 or to consider six months of preventive therapy with rifabutin, to which the isolate was fully susceptible.

These findings are consistent with previous reports of the transmission of other airborne pathogens on commercial aircraft, such as measles, influenza, and smallpox viruses.11-13 Our results are also consistent with the previous finding that the risk of transmission of M. tuberculosis from a flight-crew member with infectious tuberculosis to other crew members increased with the duration of in-flight exposure.1 Previous investigations involving closed environments, including naval ships, also showed an association between proximity to a person with infectious tuberculosis and transmission of M. tuberculosis. 14 In our investigation, the absence of passengers with skin-test conversions in other cabin sections of the aircraft on flight 4 is further evidence that M. tuberculosis was not transmitted through the aircraft's air-recirculation system.

Domestic air travel in the United States increased by 62 percent from 1980 through 1993 — from 275 million to 445 million passengers per year.15 Air travel from foreign countries to the United States increased by 182 percent, from 12.6 million passenger arrivals during 1975 to 35.5 million in 1991.15 The projections of the World Health Organization for the worldwide tuberculosis epidemic include 90 million new cases during the present decade.16 Increased air travel, the presence of tuberculosis worldwide, and immigration to the United States from countries with high rates of tuberculosis increase the probability that passengers on commercial aircraft will be exposed to persons with tuberculosis.15-17

After the national media reported this incident in July 1994, the CDC received unsolicited reports of another 30 airline passengers with tuberculosis, including 10 whose diagnosis was already known at the time of travel, who were on commercial flights from July through December 1994. Assuming approximately 260 million airline passengers during that period, the 30 passengers with tuberculosis are estimated to represent approximately 1 of every 9 million passengers.15 This probably underestimates the risk of exposure to tuberculosis on aircraft, since the reporting was unsolicited and probably incomplete. Assuming 300 passengers per international flight and 150 per domestic flight, however, as many as 10,000 passengers may have been exposed to M. tuberculosis on these flights, or approximately 1 of every 26,000 passengers who flew during that period. Furthermore, in our investigation less than 1 percent of all the contacts had skin-test conversions as a result of exposure to the index patient on the aircraft. Although limited by underreporting, these data suggest that passengers and flight crews have a relatively low risk of exposure to and transmission of M. tuberculosis on commercial aircraft in the United States.

In this investigation, the passenger with tuberculosis was a tourist from a region of the world where tuberculosis is highly endemic.7,16 Screening for active tuberculosis is required for immigrants and refugees applying for legal residency in the United States but not for tourists, visitors on business, or students.18 During 1993, 79 percent of the 21.4 million nonimmigrants who arrived in the United States were tourists.19 Screening such large numbers of nonimmigrants, even from selected countries with high rates of tuberculosis, would be impractical and very costly and, unless performed just before their flights, would not necessarily prevent exposure to persons with active tuberculosis.

Table 4. Table 4. Suggested Criteria and Procedures for Notifying Passengers and Flight Crews after Exposure to Tuberculosis on Commercial Aircraft.

To develop recommendations based on the available scientific evidence, in February 1995 officials of the CDC met with representatives from the Federal Aviation Administration, the Air Transport Association, the Council of State and Territorial Epidemiologists, and the National Tuberculosis Controllers Association, as well as medical consultants from major airline companies. In March 1995, the CDC summarized six investigations of possible transmission of M. tuberculosis on aircraft and provided guidance for notifying passengers and flight crews in the event of exposure to tuberculosis during travel on commercial aircraft.2 Four investigations found no conclusive evidence of the transmission of M. tuberculosis to other passengers.2,20,21 Table 4 shows suggested criteria and procedures for the notification of contacts that were distributed nationally to airline companies, state health departments, and tuberculosis-control programs in March 1995. The decision to notify passengers and crew members potentially exposed to tuberculosis should be guided by three criteria: the flight duration, the infectiousness of the index patient (e.g., whether he or she has smear-positive, cavitary pulmonary tuberculosis or laryngeal tuberculosis and whether there has been documented transmission to contacts), and seating proximity to the index patient, depending on the aircraft design.2 In cases in which the airline is informed first, it should provide the name of the passenger's physician to the state health department in the state where the patient resides or is being treated for tuberculosis so that the health department can make a determination of infectiousness. Applying these criteria to instances of exposure to tuberculosis on aircraft will make it easier to decide when to inform those who may potentially benefit from preventive therapy, while averting the expenditure of resources in circumstances in which the transmission of M. tuberculosis is highly unlikely. The top priority of tuberculosis-control programs is still to identify and ensure the complete treatment of all patients with active tuberculosis.23

These suggested procedures apply to all domestic and foreign airlines. They were developed in the context of tuberculosis control in the United States, however, and may not be directly applicable to countries where strategies of tuberculosis prevention and control are different. The Global Tuberculosis Program of the World Health Organization, in collaboration with the CDC, has suggested these procedures to carriers that are not U.S.-based through the International Airline Transportation Association.