Most previous investigations of the effect of air travel on influenza spread have relied on simulations of flu activity rather than actual data.

"The post-September 11th flight ban was a natural experiment on the effect of flight restrictions on disease spread," says John Brownstein, PhD, 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 (H5N1) 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.

Using data on influenza mortality from the Centers for Disease Control and Prevention (CDC), Brownstein and senior investigator, Kenneth Mandl, MD, MPH, a CHIP faculty member and an attending physician in Children's Department of Emergency Medicine, measured the rate of influenza spread across the U.S. during nine flu seasons, from 1996-97 to 2004-05.

During the first five flu seasons, flu mortality consistently peaked on or around February 17. But in the flu season after September 11, 2001, the peak was delayed until March 2, nearly two weeks later than average. In subsequent years, the peaks moved back toward February 17 as airline activity resumed its pre-9/11 levels.

In addition, analysis of laboratory surveillance data from the WHO and CDC showed that in the 2001-2002 flu season, it took 53 days for flu to spread across the U.S., 60 percent longer than the average time of 33 days.

By contrast, in France, where flight restrictions were not imposed, there was no delay in flu activity during the 2001-2002 flu season.

Brownstein and Mandl, both also of Harvard Medical School, then compared their data on flu spread with monthly estimates of passengers on domestic and international flights, obtained from the U.S. Department of Transportation.

For domestic flights, airline volume in November 2001 was an especially strong predictor of flu spread. With the Thanksgiving holiday, November is typically one of the busiest travel months of the year, but in 2001, many people kept close to home or sought other forms of travel. "Thanksgiving is when new flu strains often spread across the country," Brownstein notes.

For international flights, volume during September most strongly predicted the U.S. flu peak, suggesting that September is a key month for introduction of foreign flu strains. In September 2001, international flights fell 27 percent (from 4.9 to 3.5 million passengers), and peak flu mortality that winter was delayed by two weeks. In 2002, international travel was still down by 10 percent, and the U.S. peak was again delayed.

"When we first looked at our data we noticed that the 2001-2002 flu season was highly aberrant," Mandl recounts. "At first we thought it was a problem with the data, but then we realized we were seeing the shadow of September 11th cast upon the influenza season."

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