Researchers have long speculated that air travel plays an important role in spreading influenza and other infectious diseases. The terrible events of September 11, 2001 provided a unique, one-time opportunity to test that hypothesis.

An analysis of influenza patterns indicates that the sharp, worldwide dip in air travel in the months following September 11, 2001 significantly slowed the spread of the flu and delayed the onset of the 2001-2002 U.S. flu season.

Influenza mortality tends to peak on or around February 17 of each year. But in the flu season after the attacks, the peak did not occur until March 2, 2002, according to researchers at Children's Hospital in Boston. Their findings, published in the September 12, 2006 issue of the online journal PLoS Medicine, suggest that limiting airline traffic could buy critical time during a flu or other major pandemic.

For obvious reasons, previous investigations of the effect of air travel on influenza spread have relied on simulations rather than on actual data.

"The post-September 11th flight ban was a natural experiment on the effect of flight restrictions on disease spread," says John Brownstein, Ph.D., the paper's lead author and a faculty member of the Children's Hospital Informatics Program (CHIP) at the Harvard-MIT Health Sciences and Technology program. "For the first time we've been able to show, using real data, that air travel spreads the flu, suggesting that reducing the number of air passengers might ameliorate a flu pandemic."

The spread of avian flu in Asia and Europe, including some likely cases of person-to-person transmission, has intensified debate over whether flight restrictions should be imposed to curb emerging flu pandemics. Both the World Health Organization (WHO) and the United States government are considering such restrictions.