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Ground the Planes During a Flu Pandemic? Studies Disagree

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Science  15 Sep 2006:
Vol. 313, Issue 5793, pp. 1555
DOI: 10.1126/science.313.5793.1555a

By scouring mortality data from 121 cities across the United States, Harvard researchers have found footprints of 9/11 that they say should guide policy during an influenza pandemic. The decline in air travel in the months after the terrorist attacks delayed the annual flu season in the United States by almost 2 weeks, they conclude—a finding that suggests that a flu pandemic, too, could be slowed down, perhaps by months. But researchers who have studied the same question using computer models—and found closing down airports to be less useful—are skeptical.


The decline in air travel after 9/11 delayed the U.S. flu season by almost 2 weeks, a new study says.


The 2003 outbreak of SARS drove home the widely held belief that global mobility helps spread infections; indeed, it's almost a cliché among researchers to say that the most important disease vector today is the Boeing 747. But air-travel restriction won't help slow a flu pandemic much, three model studies concluded earlier this year—especially when compared to the judicious use of vaccines, antiviral drugs, isolation, and quarantine.

In a paper published in July in Nature, for instance, Neil Ferguson of Imperial College London and his colleagues tested how the United States and the United Kingdom might best mitigate a pandemic's ravages. They found that unless they are 99% effective, border controls and internal travel restrictions won't slow viral spread by more than 2 or 3 weeks. Ben Cooper and his colleagues at the U.K. Health Protection Agency, who modeled air travel around the world in a June paper in PLoS Medicine, also found limits “of surprisingly little value.” The reason, says Ferguson, is that flu spreads extraordinarily rapidly.

But in the real world, the 27% reduction in international air-travel volume after 9/11 appears to have caused a 13-day delay in the 2001–02 influenza season—considerably more than the models would predict, say John Brownstein and Kenneth Mandl of Children's Hospital Boston and Harvard Medical School in a paper released on 11 September by PLoS Medicine. Analyzing data from 1996 to 2005, they also found a correlation between higher air-travel volumes in the fall and a slightly earlier flu season. Extrapolations suggest that a full-blown travel ban, as opposed to the post-9/11 slump, might delay a flu pandemic by as much as 2 months, says Brownstein—precious time to activate countermeasures and work on a vaccine.

The modelers aren't convinced, however. Ferguson says there is no proof that the relation between travel and timing of the flu season is causal, and he questions the team's use of a complex statistical measure to determine the timing of the peak. Although the study is “very nice,” the 9/11 effect “is an n of 1; it's intriguing, but you can't draw any conclusions,” says Ira Longini of the University of Washington, Seattle, who co-authored a paper in the Proceedings of the National Academy of Sciences in April that also concluded that travel bans had little value.

Brownstein suspects that some of the criticism may stem from the contradiction between his data and the models. “They are making assumptions about the relationship between air travel and the spread of influenza,” he says. “But this is empirical evidence.”

Although some countries' pandemic preparedness plans list travel bans as an option, Ferguson says most governments that have studied the idea seriously have rejected it. The World Health Organization's (WHO's) Global Influenza Preparedness Plan does not recommend travel bans because enforcement “is considered impractical,” but a footnote adds that they “could be considered as an emergency measure to avert or delay a pandemic.” WHO spokesperson Gregory Hartl says the new study is “very interesting” and “opens up the debate again.”

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