A new study found that, at least on intercontinental flights, what may make all the difference between getting sick and avoiding possible infection is where you sit and who is close to you.
For most in the cabin, the chance of infection was less than 3%, the researchers said.
"Passengers should not be concerned about getting sick from somebody coughing, for instance, five rows behind them," said Vicki Stover Hertzberg, first author of the study and a professor at the Nell Hodgson Woodruff School of Nursing Emory University.
The study did not compare the spread of germs in a plane to, say, a movie theater. So what's the real skinny? "The results of this study have not prevented me from flying," Hertzberg said.
The World Health Organization says respiratory diseases like flu or SARS are mainly transmitted when a sick person sneezes, coughs or talks, causing germy droplets to be propelled into the air.
A 1996 study found
that a woman who traveled from Honolulu to Baltimore and back transmitted drug-resistant tuberculosis to at least six other passengers. Public health officials worried about in-flight spread of disease during epidemics of SARS and Ebola, both potentially deadly illnesses. However, hard evidence from scientific studies was lacking.
So what are the actual risks of spreading a respiratory illness aboard an airplane? That depends on whether there are sick people onboard -- and whether you come into contact with them.
Hertzberg and her co-researchers boarded 10 transcontinental flights to find out. The aircraft company Boeing funded the research but played no part in its design, she said.
Above the clouds, Hertzberg and her colleagues tracked passengers in the often-crowded economy section of single-aisle planes.
One flight had 17 empty seats, two flights had just a few empty spaces, and seven flights were fully occupied. Of 1,540 total passengers, only one person was seen coughing moderately. None of the crew members, 41 people in total, coughed at all.
The researchers monitored passengers moving about the cabin and found that half did not use the bathroom. In fact, nearly 38% of passengers never left their seats, while the same proportion left their seats just once; 13% left their seats twice, and 11% left more often than that.
Waiting for, using or leaving the lavatory were the most common movements of passengers, followed by checking the overhead bin.
On average, the flights lasted 238 minutes, and each crew member had about 67 minutes of contact with passengers. Based on these data, the researchers estimated that a single sick crew member could infect nearly five passengers per flight.
To punctuate their findings, Hertzberg and her co-researchers also collected 229 environmental samples before, during and after the flights and tested each for 18 common respiratory viruses, including various strains of flu. All of the samples, taken from the air and from hard surfaces, tested negative.
The researchers caution that on shorter hops, the amount of movement of passengers and crew would be much less, but on longer flights, there would be substantially more movement -- so the results cannot be applied to either shorter or longer flights.
"We know so much now about what people do on airplanes and for how long," Hertzberg said, though she noted that her study lacked data about the risk of disease spread through fomites: inanimate objects or materials likely to carry germs, such as clothing.
Dr. Marta Feldmesser, chief of infectious disease at Lenox Hill Hospital in New York, said that the methodology for the new study was "outstanding" but that research is limited.
The study cannot account for the effects of airflow within a plane and pre- or post-flight experiences, such as waiting in an airport, said Feldmesser, who was not involved in the research.
Another problem was that "almost no passengers or crew were ill" on the 10 researched flights. This was fortunate for the travelers but less so for the researchers, who did not have an opportunity to investigate viral levels either in the air or on surfaces.
Still, a point was made, Feldmesser said: Basic hand hygiene is important.
"People should keep their hands as clean as possible and avoid touching their face," she advised. The 80% or higher transmission rates among economy passengers who actually rubbed elbows "should remind people to be considerate of others and to either reschedule travel when ill or take additional measures to prevent transmission," she said. Safety would include wearing a mask or taking cough medicine.
After all, one person's mild sickness may be another person's severe disease, Feldmesser added.
Hertzberg agreed that travelers should always practice good hand hygiene on an airplane, she said, "and keep your hands away from your face." If you are sick, use good etiquette: "Cough or sneeze downward, into the crook of your elbow."