Flu Skies: How Influenza Might Spread On a Plane

A new study suggests the chances of contracting a sick passenger’s flu virus is surprisingly low

This digitally-colorized negative-stained transmission electron microscopic (TEM) image shows recreated 1918 influenza virions that were collected from supernatants of 1918-infected Madin-Darby Canine Kidney (MDCK) cells cultures 18 hours after infection. Provided by: CDC/ Dr. Terrence Tumpey, Photo Credit: Cynthia Goldsmith

In the classic 1995 disaster flick Outbreak, an Ebola-like virus mutates and becomes airborne. In one of the most sobering scenes of the movie, the pathogen is sucked up into the ventilation system of an airplane, infecting almost everyone on board. Such scenes of plane-contracted illness are not easy to shake, leaving the creeping feeling that flying is a game of cold-and-flu Russian roulette.

As George Dvorsky of Gizmodo reports, there has been surprisingly little research about the spread of respiratory diseases on planes, until now. A new study details just how rapidly flu spreads on commercial flights, suggesting that planes aren’t as germ-infested as many believe.

As David Shultz at Science reports, Georgia Institute of Technology biomathematician Howard Weiss and ten graduate students tackled the question of plane germ levels by boarding the coach section of ten transcontinental flights between 3.5 and 5 hours long during the North American flu season. Dvorsky reports that one researcher was stationed every five rows. They then logged the movements of every passenger and crew member on the flight. They also took 229 samples from the air and surface swabs around the plane.

All that spying on passengers gave the team a "blueprint" for how people move around an aircraft and interact with one another, writes Shultz, showing potential connections for the spread of disease. But it was unclear how fast the disease could spread. So the researchers fed the information into a computer model based on historical data of spread. In fact, Shultz reports, it was primarily based on a 1977 incident in which 38 out of 54 people became infected with flu-like symptoms after sitting in a grounded plane for five hours without air circulation. They then multiplied that transmission rate by four to create a worst-case scenario.

Even in that extreme case, a passenger with the flu will likely only infect 0.7 fellow passengers per flight. And only people within about three-feet of the infected passenger run the risk of getting sick. However, that is not the case with the cabin crew. The model shows that an infected flight attendant has the potential to spread a virus to 4.6 new people per flight. The research appears in the Proceedings of the National Academy of Sciences.

“This was the first study to quantify passenger movement, behaviors, and social contacts and to estimate transmission likelihood using a data-driven model,” Weiss tells Dvorsky. “The simulations provide compelling evidence that for influenza, if you are not seated within a meter of an infected passenger, and you practice careful hand hygiene, then you are unlikely to get infected during flight.”

As Cari Nierenberg at LiveScience reports, only those passengers in the same row or directly ahead or behind someone with the flu is in the danger zone. Even more surprising, of the 229 swabs the researchers took from areas like seatbelt buckles, tray tables and bathroom door handles, none of them showed the presence of 18 common respiratory viruses.

Not only has the study shed some light on how diseases may move around a plane, it also gives some insight to how people behave on planes as well. “We now know a lot about how passengers move around on flights,” co-author Vicki Hertzberg of Emory University says in a press release. “For instance, around 40 percent of passengers never leave their seats, another 40 percent get up once during the flight, and 20 percent get up two or more times. Proximity to the aisle was also associated with movement. About 80 percent of passengers in aisle seats got up during flights, in comparison to 60 percent of passengers in middle seats and 40 percent in window seats. Passengers who leave their seats are up for an average of five minutes.”

Ira Longini of the University of Florida, Gainesville, who was not involved in the study, tells Shultz that the way the researchers map the movement of people is smart, but should still be approached with caution. Since it does not actually track a real virus, the transmission rate is still a guess.

The study also only really applies to respiratory viruses like the flu, which are spread by droplet transmission via coughing or sneezing, explains Edsel Maurice Salvaña, molecular biologist at the National Institutes of Health at the University of the Philippines, reports Dvorsky. “It did not simulate more contagious viruses and bacteria like chicken pox, measles and tuberculosis which spread over bigger distances,” he says.

In fact, as Michaeleen Doucleff at NPR reports, tuberculosis can be spread within two rows of an infected person on a flight over eight hours. SARS can reach as far as three rows and possibly up to seven.

Based on this latest research, how can you avoid getting sick on a flight? Hertzberg tells Doucleff that she chooses a window seat. That’s because people in window seats avoid contact with people moving around the cabin who could cough or touch people in aisle seats. She also avoids moving around, reducing exposure to other people who may be sick.

Mark Gendreau, who specializes in aviation medicine at Lahey Medical Center in Peabody, Massachusetts but was not involved in the study, tells Doucleff that he recommends using a sanitizing gel (60 percent alcohol) on your hands before eating or drinking anything on a plane. It's a good idea even after washing your hands in the plane lavatory. In 2004, the Environmental Protection Agency found that water tested on 15 of 327 planes was contaminated with high levels of fecal bacteria. Since this time, new regulations have been enforced and, according to the EPA, the water is now safe for anyone without suppressed immune systems.

That said, flying with a little caution—and hand sanitizer—is likely still a good plan.

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