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Planes play big role in spreading flu

September 12, 2006 By Helen Pearson This article courtesy of Nature News.

Cancelling flights might delay future pandemic.

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. The reduced air travel did not, however, cut the total number of people who eventually died from the flu 39,132 people died that winter, close to the usual number of 40,000.

November hotspot

The researchers found that the number of people travelling in November is the best predictor for the rate at which the flu spreads in any given year, probably because so many people travel during Thanksgiving holiday, just when influenza gets going for the winter season. The results are reported in PLoS Medicine1.

Air travel helps the virus spread because infected people carry it with them as they hop from one area of the globe to another. In addition, there is some evidence that it spreads more readily among people cooped up together on a plane.

The study supports the idea that cancelling flights at the start of a future flu pandemic might also delay its spread. This is of particular interest at the moment because of fears that the H5N1 influenza virus in bird flocks will start spreading between people and explode into a pandemic.

A week's head start

A delay of just one or two weeks might allow public-health officials time to distribute antiviral drugs or vaccines and isolate those already infected. "It will only make a difference if you were racing to put a control measure in place," says Ira Longini, an expert in modelling the spread of disease at Fred Hutchinson Cancer Research Center in Seattle, Washington.

Having a plan in place to instigate these measures is far more important than having a plan to cancel air travel, notes Longini. He wrote one of two epidemiological studies last year that showed it would be possible to avert a pandemic if such measures were introduced within weeks of the first infection (see ' Drugs could head off a flu pandemic - but only if we respond fast enough').

Because restricting air travel would have huge social and economic repercussions, it is not currently a major part of international plans for a flu pandemic. But it might make good sense, Longini says, to cancel flights from a region if an outbreak was just emerging there and had been recognized in good time.

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  1. Brownstein J. S., PLoS Medicine, 3. e401 (2006).


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