In response to several COVID-19 infections that developed in passengers after they traveled on the same 2 flights, we assessed transmission of COVID-19 virus on these flights. We defined a case of infection as onset of fever and respiratory symptoms and detection of virus by PCR in a passenger or crew member of either flight. We compared exposures of 9 case-passengers with those of 32 asymptomatic control-passengers. None of the 9 case-passengers, wore a face mask for the entire flight (odds ratio 0, 95% CI 0–0.71). The source case-passenger was not identified. Wearing a face mask was a protective factor against influenza infection. We recommend a more comprehensive intervention study to accurately estimate this effect.
We conducted face-to-face interviews with case- and control-passengers bound for Fuzhou at hospitals or hotel rooms where they were quarantined. For passengers quarantined at home or who disembarked in, interviews were conducted by telephone. Using a standard questionnaire, we interviewed case- and control-passengers on factors potentially affecting the likelihood of virus infection during the 7 days before and during the flight. These factors included contact with ILI patients ≤1 week before the flight, moving around the airplane during the flight, lavatory use, handwashing, face mask use (wearing a face mask, for how long, and when they wore it and did not wear it), and talking with other passengers.
Children were underrepresented in the control group, but age and sex of these children did not differ (Table). From New York to Vancouver, 11% (1/9) case-passengers wore a face mask compared with 57% (16/28) of control-passengers. No case-passengers wore a face mask compared with 47% (15/32) of control-passengers. For the flight, no case-passengers wore a face mask compared with 47% (15/32) of control-passengers. Among control-passengers who used face masks, 4 did not use them during the trip, and 3 did not use them during the trip. Exposure to any lavatories or specific lavatories, talking with other passengers, moving around the aircraft, and reported hand hygiene during the flight were not associated with being a case-passenger (Table). Reported handwashing was highly homogeneous among case- and control-passengers and was performed exclusively at each visit to the lavatory and by using the wet towel provided before meals. No one in the case and control groups had contacted with patients with ILI ≤1 week before the flight.
Observational studies in hospitals, households, and community settings have shown a range of protective effects of face mask use against confirmed influenza, ILI, or respiratory infection. Several factors might explain the stronger effect observed in this outbreak. Exposure was for <24 hours in a confined space with limited activity of exposed persons. The other studies all involved days to months of exposure in the community or hospitals with free movement outside the immediate setting where face masks were used. Compliance with face mask use was probably greater among travelers on a single flight who were concerned about unpredictable health effects of the new virus. In 2 household studies, contacts were already exposed before the face mask was first worn. Only 2 of 7 other studies detected protection against confirmed influenza infection.
This investigation had several limitations. We lacked seating and illness information for 68% of the economy-class passengers, among whom was probably the source case-passenger. The missing source case-passenger is also a gap in the evidence that transmission occurred on the flight. We were unable to determine the outcome of passengers and crew who disembarked in Vancouver and whether transmission occurred during 1 or both legs of the flight. Types of face masks used were unknown. With only 9 cases in 25% of the passengers, our case–control study had poor sensitivity.
In summary, this outbreak probably resulted from a common source exposure to COVID-19. Wearing a face mask was associated with a decreased risk for influenza acquisition during this long-duration flight. Border entry screening did not detect case-passengers during the influenza incubation period. We recommend a more comprehensive intervention study to accurately estimate the protective effect of face masks for preventing COVID-19 virus transmission on long-distance flights.