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There’s one more reason to hate flying economy class.

In a new study of how the H1N1 flu virus spreads through commercial air travel, researchers have found that not only can the number of in-flight infections be predicted based on just one passenger carrying the virus, but also that people travelling in economy class are much more likely to become infected than people flying first class.

The study, published in the current online edition of the journal BMC Medicine, was conducted by Sally Blower, director of the Center for Biomedical Modeling at the University of California in Los Angeles, along with Bradley Wagner and Brian Coburn, postdoctoral fellows in Dr. Blower’s research group.

“Importantly, we found that the number of infections that would occur on a flight was very dependant on which cabin the infected individual was sitting in,” Dr. Blower said in a release. “We found that many infections could occur if the infected individual was travelling in economy class, but relatively few if the individual was travelling in first class.”

Two things explain the greater likelihood of catching the virus in economy class, the researchers said.

“Primarily, it’s the more crowded conditions in economy,” Dr. Coburn said. “And unfortunately, there is a very high probability – 75 per cent – that if an infected person is on board, they will be in the economy cabin.”

Mathematical modelling techniques enabled the researchers to predict in-flight transmission rates. A five-hour flight would likely result in two to five infections, according to the study. Five to 10 people would be infected during an 11-hour flight, while seven to 17 infections would occur during a 17-hour flight.

To make these predictions, researchers used the Wells-Riley equation, which is designed to predict the size of outbreaks within buildings and other enclosed environments. The equation is based on the number of individuals exposed to an infectious pathogen, the respiratory rate of the infected person, the length of exposure to the infectious droplets and the concentration of infectious viral particles over time.

While previous research has shown that smallpox, measles, tuberculosis, seasonal influenza and severe acute respiratory syndrome can be transmitted during commercial flights, this is the first study to predict the number of H1N1 flu infections, the researchers said.

“Clearly it was air travel, by transporting infectious individuals from the epicentre in Mexico to other geographical locations, that significantly affected the spread of H1N1 during the outbreak last spring,” Dr. Coburn said. “However, until our study, it hadn’t been determined how important in-flight transmission could be.”

Given the results of the study, researchers said that air-travel restrictions could be one means of controlling influenza pandemics.

“Our results imply that one individual travelling by plane, by infecting other travellers on the same flight, could cause multiple simultaneous outbreaks in different geographical locations rather than causing only one outbreak,” Dr. Wagner said. “For that reason, quarantining passengers who travel in economy class on long-haul flights could potentially be an important control strategy this winter.”

But for those people with extra money to spend, fear not.

“There is no point in quarantining passengers in first class,” Dr. Wagner said.