Airline Travel Speeds Flu Spread
International air-travel volume was correlated with the introduction of new viral strains into the U.S.; domestic air-travel volume was correlated with their interregional spread.
Simulation studies have suggested that local mobility of people and virulence of a virus strain are important factors in the dissemination of the virus (see Journal Watch Infectious Diseases May 19 2006). These simulations have differed as to the importance of air travel, however. Now, using data from the U.S. Department of Transportation and the CDC, researchers in Boston have analyzed the relation between air-travel volume and pneumonia and influenza mortality in the U.S. for 1996 through 2005.
The time to transnational spread decreased from 24 and 26 days during the 1996/97 and 1997/98 flu seasons to 8 and 11 days during the 1999/2000 and 2000/01 seasons. In 2001/02, following the September 11 terrorist attacks, air-travel volume was greatly diminished in the U.S. During that season, transnational spread took 16 days. In addition, influenza mortality did not peak until March 2 13 days later than average. In France, where no flight restrictions were imposed, no such delay was seen. Overall, an inverse correlation was observed between U.S. domestic air-travel volume in November and time to transnational influenza spread (r2=0.60; P=0.014). Similarly, a strong inverse correlation was found between the timing of an influenza season and the volume of international air travel in September (r2=0.59; P=0.016). No significant relationships were found between climate and interregional influenza spread.
Comment: These findings provide the first empirical evidence that national and international spread of influenza is driven by airline travel, as predicted by simulations. If an influenza pandemic occurs, air-travel restrictions might help to delay introduction and spread of the virus, thus allowing the use of preventive measures. However, how much time could be gained, and whether the gain would be sufficient to decrease morbidity and mortality, are unknown.
Thomas Glück, MD
Published in Journal Watch Infectious Diseases September 27, 2006
Brownstein JS et al. Empirical evidence for the effect of airline travel on inter-regional influenza spread in the United States. PLoS Med 2006 Oct; 3:e401.
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