Air travel and disease

Helen Pearson:

Can air travel help influenza to whip around the world? A study after the terrorist attacks of 11 September 2001 has shown that the answer is yes: a steep drop in air travel delayed the spread of winter flu across the United States.

This suggests that grounding flights might slow the spread of a future flu pandemic, and perhaps buy precious time for vaccinations and quarantine.

Several earlier studies have suggested that air travel might play such a role, but these have mostly been based on computer models.

John Brownstein of the Children's Hospital Boston and his colleagues tested the idea using data collected from real life. They studied statistics on influenza death in many US cites collected between 1996 and 2005 by the Centers for Disease Control and Prevention (CDC), along with estimates of the numbers of passengers who travelled by plane within the United States and from there to another country.

When studying their data, they noticed something odd: the flu spread far more slowly during the winter of 2001-2002. They realised that far fewer people travelled by air after 9/11 because of temporary flight restrictions and troubled nerves. "We were in the unique position of observing a natural experiment," says co-author Kenneth Mandl, also at the Children's Hospital (see 'A novel opportunity').

The 27% drop in passenger numbers on international flights delayed the normal peak of flu deaths by nearly two weeks, from February to March. And the fall in domestic air travel meant that the disease took 16 days longer to spread throughout the country.

Of course, influenza is just one of many diseases that can be spread around the world through air travel.