Journal Review: At least 23 may have acquired SARS in flight

Five SARS deaths linked to air travel

Olsen SJ, Chang HL, Vheung T, et al. Transmission of the severe acute respiratory syndrome (SARS) on aircraft. N Engl J Med 2003: 349:2,416-2,422.

Transmission of SARS people appears to have occurred on at least one aircraft after infected people flew during the symptomatic phase of illness last year, the authors reported.

March 15, 2003, a Boeing 737 carrying 120 people (112 passengers, six flight attendants, and two pilots) flew for three hours from Hong Kong to Beijing. One of the passengers was a symptomatic 72-year-old man in whom fever had developed on March 11. He was hospitalized on arrival in Beijing, where he was given a diagnosis of atypical pneumonia and died on March 20. Among the 22 patients/passengers who subsequently developed SARS, the date of onset of illness ranged from March 17 to March 23, a mean of four days after the flight. A total of five died as a result of their illness (three from Hong Kong, one from China, and one from Singapore).

The authors interviewed passengers and crew members at least 10 days after they had taken one of three flights that transported a patient or patients with SARS. All index patients met the criteria of the World Health Organization (WHO) for a probable case of SARS, and index or secondary cases were confirmed to be positive for SARS-CoV on reverse-transcriptase polymerase chain reaction or serologic testing.

Overall, a total of 304 of 681 passengers on the three flights (45%) were interviewed directly; 16 had laboratory-confirmed SARS, two had illness that met the WHO definition of a probable case of SARS and were interviewed, four were reported to have probable cases of SARS but were not interviewed, and one had a suspected case of SARS. The infection in these 23 patients was subsequently transmitted to at least 13 others — two died of SARS.

"We believe that the most plausible explanation for the development of SARS in the passengers and crew members [on the 737] is that they were infected while on board the aircraft, although other explanations are possible," the authors concluded. " . . . However, the clustering of the dates of onset of illness around four days after the flight is in keeping with the expected incubation period for SARS and points to the day of the flight as a likely time of transmission. Furthermore, we identified no alternative exposures before or after the flight through our interviews with the ill passengers."

The authors emphasized that the passengers who became infected were clustered in the few rows directly in front of or behind the ill passenger, rather than being randomly distributed throughout the aircraft. That is consistent with the pattern described in other cases in which a respiratory pathogen was transmitted on board an aircraft and lends further plausibility to the theory that transmission occurred during the flight.1,2

The risk to passengers was greatest if they were seated in the same row as the index patient or within three rows in front of him. The greater concentration of people who became infected in front of the index patient than behind him may point to the role of coughing in transmission, causing a combination of aerosol and small-droplet spread. Fomites also may have played a role, so hand hygiene strategies could be a prevention tool.

The risk of transmission of an infectious agent on an aircraft can vary widely, with variables including the duration of the flight, the stage of illness, the type of air-ventilation system in use, the size of the aircraft, and the number of infected people on board. In addition, so-called "super-spreaders may transmit the virus to large numbers of their contacts.

"Aircraft ventilation systems are believed to be highly efficient at keeping the air free of pathogens, which they do by exchanging the air in passenger cabins every three to four minutes and passing the circulated air through high-efficiency particulate-arresting (HEPA) filters designed to filter out all particles larger than 0.3 µm by 1 µm." the authors noted. "[However], the fact that only 45 % of the passengers were interviewed despite intensive investigation by three health departments over the course of two months highlights the difficulties faced by such investigations and raises the possibility that more transmission occurred than was recognized."

References

1. Kenyon TA, Valway SE, Ihle WW, et al. Transmission of multidrug-resistant Mycobacterium tuberculosis during a long airplane flight. N Engl J Med 1996; 334:933-938.

2. Moser MR, Bender TR, Margolis HS, et al. An outbreak of influenza aboard a commercial airliner. Am J Epidemiol 1979; 110:1-6.