Post-9/11 Drop in Air Travel Delayed Flu Season, Study Finds

By Rick Weiss
Washington Post Staff Writer
Tuesday, September 12, 2006

The decline in U.S. air travel that followed the Sept. 11, 2001, attacks delayed the onset of that year's annual flu outbreak and slowed its spread around the country, according to a new analysis that could help health officials decide whether to impose flight restrictions in the event of a global flu pandemic.

The delay was modest -- flu deaths peaked about two weeks later than usual that winter -- and by the time the season was over, the same number of people had died from the disease as in a typical year.

But pandemic flu is expected to be much more deadly than conventional flu, so having a few weeks of extra time to deliver protective drugs or vaccines could make a big difference in total mortality, said the scientists who conducted the study.

"Flu spreads exponentially, and the more that's out there, the faster it spreads," said Kenneth D. Mandl, a physician with the Children's Hospital Informatics Program in Boston, who led the study with Harvard epidemiologist John S. Brownstein. "So anything you can do early on to reduce cases could have fairly large downstream positive effects."

The researchers and others also emphasized, however, that there are many reasons to think twice about imposing travel restrictions to stem the spread of a communicable disease, including economic impact, social disruption and even the ability to ship drugs to where they are needed most.

"When you look at the big picture, do you really want to shut down air travel?" asked Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases and a key contributor to the U.S. pandemic preparedness plan. "What is that going to mean for the just-in-time economy, where you rely every day on shipments of medicines for hospitals?

"That's not to diminish the elegance of the study," Fauci continued. "You just have to be careful about how you interpret it."

The new analysis comes amid warnings that a pandemic of especially virulent influenza could emerge at any time, and as officials at the World Health Organization and the U.S. Department of Health and Human Services continue to debate the value of travel restrictions and other forms of quarantine should such an outbreak occur.

Plans developed by both those organizations call for travel restrictions only as a last resort. In part that is because researchers seeking to analyze the approach's effectiveness have had little to work with beyond computer models, whose relevance remains uncertain because they rely on many untested assumptions.

The Boston study -- published in the October issue of the journal PLoS Medicine -- reduces that uncertainty by taking advantage of an unanticipated trove of empirical data: annual statistics on the spread of conventional flu, gathered by the Centers for Disease Control and Prevention.

It was in the course of a routine analysis of the data concerning 1996 through 2005 that the Boston team noticed that the 2001-02 flu season did not fit the usual pattern.

While the national peak for flu deaths is almost always within two days of Feb. 17, the 2001-02 peak was not until March 2. And the amount of time it took the flu to spread around the country -- based on reporting from nine national regions -- was 68 percent longer than usual, or 16 days instead of the 8 to 11 days in the previous two years.

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