Air Travel Plays Role in Influenza Transmission

Air travel has finally been shown to influence the spread of influenza, said Dr. John S. Brownstein of the Children's Hospital Informatics Program at the Harvard-MIT division of health sciences and technology, Boston, and his associates.

They analyzed interregional influenza spread across the United States, then correlated the pattern with data on airline travel. Previous studies that used computer simulations have suggested that air travel may play a role in the spread of annual influenza, but “we provide what is to our knowledge the first empirical evidence to confirm the effect airline volume [has] on long-range spread,” they said.

The investigators analyzed weekly influenza mortality data from the Centers for Disease Control and Prevention for nine influenza seasons between 1996–1997 and 2004–2005. They found strong correlations between monthly fluctuations in airline volume and two other measures: the rate of disease spread and the timing of seasonal influenza mortality.

The number of domestic passengers from November to January of a given season showed a strong inverse correlation with the time to the spread of influenza across the United States. “Although influenza activity is highest between January and March, initial regional seeding of infection may occur earlier,” Dr. Brownstein and his associates said (PLoS Med. 2006 October [Epubdoi:10.1371/journal.pmed.0030401]).

“Our results suggest that for a nonpandemic year, travel during the Thanksgiving holiday may be central to the yearly national spread of influenza in the [United States],” they noted. The volume of international travel in all three months, but particularly in September, predicted the national seasonal peak in influenza mortality.

These findings suggest that airline passenger volume accounts for about 60% of the between-season variation in influenza spread and peak.

“Our results suggest that limiting domestic airline volume would have a measurable impact on the rate of spread of an influenza pandemic.” Restricting international flights into the United States also would reduce the probability of a pandemic strain reaching the country.