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Research links air travel with timing of flu season

The attacks on the World Trade Center in New York and the Pentagon near Washington five years ago this week prompted a sharp fall-off in commercial aviation. The disruption gave scientists an unusual opportunity to study the impact of airline travel on the spread of influenza. The findings of a new study could be helpful to public health officials facing a possible future pandemic.

Other experts had used simulations to predict how the ups and downs of air travel would affect the spread of flu. But John Brownstein took advantage of the tremendous dropoff in air travel after the September 11, 2001, attacks, to see how accurate the predictions were.

"We actually had the opportunity, given the post 9/11 flight ban and subsequent depression of the airline travel market, to look at this relationship using real empirical data," says Brownstein, who notes there are two main findings from his study.

"The first is that the number of airline passengers flying within the U.S. determines how quickly influenza spread across the United States. The second is that the number of airline passengers flying into the U.S. from abroad determines the timing of new influenza viruses entering the country. So, to summarize, the more domestic travel, the faster the spread of influenza. And the more inbound international travel, the earlier the influenza season begins."

Although Brownstein and his colleagues at Harvard University and Boston Children's Hospital were looking at cases of the regular, seasonal flu, he says the same analysis should apply if avian flu mutates into a pandemic strain of virus, with possible policy implications for a response.

"Essentially, when we consider influenza epidemics, these are new viruses entering the country each year," he explains. "When we think about a pandemic, it's the same thing. So we have a new subtype in this case that's entering the U.S. And so, by limiting, for instance, international airline travel, we could potentially reduce the probability of that new strain entering the country."

Imposing restrictions on air travel would have economic costs and possible social and political consequences, and those would have to be weighed against any public health benefit. Interestingly, John Brownstein says there was no link between air travel and the death rates from the flu. It just affected the timing of the flu season.

John Brownstein's study was published Monday in the journal PLoS Medicine.

Incidentally, PLoS Medicine is what is known as an "open access" journal. The articles go through the same rigorous peer-review process as in other journals, but instead of charging readers a subscription fee, the authors pay a publication fee.

"It really allows results to be disseminated worldwide, and it doesn't impose the subscription restrictions of other journals," Brownstein says. "And these findings are not necessarily just applicable to the United States. They can also inform policy in other countries as well, so we think PLoS Medicine represents an opportunity to disseminate findings widely."

John Brownstein says he felt it was important to have his findings available, regardless of the ability of readers to pay for an expensive journal subscription.