By Philip R. Fischer, MD, DTM&H

Professor of Pediatrics, Department of Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, MN

Dr. Fischer reports no financial relationships relevant to this field of study.

SYNOPSIS: Although air travel has been linked to transmission of respiratory infections, the actual risk of becoming infected during air travel is low. The risk is greatest, though, when seated within about two seats/rows of a contagious individual. Walking around the cabin increases risk.

SOURCE: Hertzberg VS, Weiss H, Elon L, et al; FlyHealthy Research Team. Behaviors, movements, and transmission of droplet-mediated respiratory diseases during transcontinental airline flights. Proc Natl Acad Sci U S A 2018;115:3623-3627.

Commercial airline passengers take billions of trips each year, and it is well-documented that air travel can serve as a conduit for the spread of infections. Researchers have studied pathogens and aircraft to better understand the risks for infection, but the actual behaviors of crew members and airline passengers had not been carefully observed previously. Thus, Hertzberg and colleagues documented careful observations of the behaviors during flight that potentially could enhance the spread of infection.

The researchers made observations on 10 separate transcontinental flights in the United States involving single-aisle aircraft (with three seats on each side of the aisle). Flight durations varied between 211 and 313 minutes. Seven of the 10 flights were full, and the others had two, three, and 17 empty seats. Observations involved 1,540 passengers and 41 crew members in the economy class seating area.

Overall, 38% of passengers remained in their seats for the entire flight, another 38% left their seats once, 13% left their seats twice, and 11% left their seats more than twice. For those who left their seats, they were up for a median of 5.4 minutes. Seat location was linked to movement away from the seat, with 80% of people seated at the aisle moving, 62% of those in middle seats moving, and only 43% of those in window seats getting up to move around. Only half the passengers used a lavatory during the flight. The average crew member spent about one-third of the flight in contact with passengers and two-thirds of the flight in the galley area.

Moving around the aircraft increased contact with other passengers. Passengers who left their seats had a median of 44 other contacts with people and a total of 47 person-minutes of contact with someone not seated near them. Crew members had 206 person-minutes of contact with other crew members and 1,149 person-minutes of contact with passengers. Passengers in aisle seats had much more contact with other passengers (64 contacts with other people) than did passengers seated in window seats (12 contacts with other people).

Based on this information about passenger and crew behavior and knowing that respiratory pathogens are transmitted via droplets over distances less than one meter by cough, sneeze, and breathing, Hertzberg and coworkers then constructed a dynamic network model simulating transmission of influenza. Assuming a high rate of influenza transmission and an index case seated mid-plane in an aisle seat, up to 11 passengers likely would become infected, with the rest of the passengers having less than a 3% chance of having close contact with the index case. An infected coughing crew member likely would share the infection with four or five passengers.

Finally, the investigators gathered 228 air and surface (such as seat belt buckle) samples during the flights they studied. Eight of the 10 studied flights occurred during influenza season, and samples were tested by polymerase chain reaction (PCR) for 18 common respiratory viruses. No sample had evidence of harboring a respiratory pathogen.

The authors pointed out the extremely low risk of infection being transmitted on the 10 flights they happened to observe. By their modeling, even with a highly contagious individual moving around from an aisle seat in the middle of the plane, less than 10% of fellow passengers would risk becoming infected.

However, there are reports of higher infection rates with air travel. Astutely, the authors pointed out that transmission of infection also can occur in waiting areas and during the boarding and deplaning processes. In addition, pathogens that spread via aerosol might reach passengers more than a meter away from an infected index case. And, some of the reported outbreaks were associated with flights of longer duration than the three- to six-hour flights in this study.

COMMENTARY

These new data remind us that passenger behaviors influence the risk of becoming infected during air travel. Although most infections don’t extend more than two seats beyond an infected index passenger, movement around the aircraft can expand the risk of spread.

Anecdotally, I reviewed this paper during a trans-Atlantic flight; I did not notice anyone coughing or sneezing within two seats or two rows of me. However, standing 6’4”, I tend to move from my aisle seat much more than the typical passenger reported by Hertzberg’s group, and I tend to spend more time out of my seat stretching my legs than did the average studied passenger. These new data offer clues as to how my mobility exposed me to more potentially infected co-travelers and might explain why I developed a viral upper respiratory infection shortly after arriving at my destination.

Passenger movement during flight increases contact with different passengers. Modeling data also suggest that passenger movement alters the displacement of aerosolized particles in ways that might increase the spread of aerosolized pathogens.1

Beyond passenger behavior, other factors also influence whether a passenger will become infected during a long flight. Cabin ventilation systems on commercial aircraft use particulate filters and fully exchange cabin air approximately 15 times per hour.2 This is effective in decreasing the risk of transmission of infection and explains why most cases are from passengers sitting in close proximity to contagious individuals or related to passenger movement around the cabin. However, ventilation systems are not always activated during flight delays prior to take-off, and spread of influenza has been reported with a three-hour on-ground delay when a plane’s ventilation system was not activated.3

Hertzberg’s data suggest that it is relatively uncommon for sick people to travel. However, with approximately 3 billion air passengers each year (averaging one flight for each two people on the planet each year), there will be times when germs are flying with airplane passengers. What should be done when someone with a respiratory infection is traveling? “Cover the cough” is always good advice, and the use of surgical-type face masks is relatively more common in Asia than elsewhere, for protection against both air pollution and infection.

As noted by Hertzberg, aircraft cabin hard surfaces are disinfected at least daily; it is not clear that wiping down seats and trays by passengers alters the transmission of infection. Since most transmission occurs within a meter of an infected person, asymptomatic passengers might try to move away from coughing travelers when other seats are available.

REFERENCES

  1. Han Z, To GN, et al. Effect of human movement on airborne disease transmission in an airplane cabin: Study using numerical modeling and quantitative risk analysis. BMC Infect Dis 2014;14:434.
  2. Leitmeyer K, Adlhoch C. Influenza transmission on aircraft: A systematic literature review. Epidemiology 2016;27:743-751.
  3. Moser MR, Bender TR, Margolis HS, et al. An outbreak of influenza aboard a commercial airliner. Am J Epidemiol 1979;110:1-6.