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 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 (ScienceNOW, 2 May).
But in the real world, the 27% reduction in 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. And analyzing data from 1996 to 2005, the team found a consistent 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.
World Health Organization (WHO) spokesperson Gregory Hartl says the new study is "very interesting" and "opens up the debate again." The modelers aren't convinced, however. One--Neil Ferguson of Imperial College London--says there is no proof that the relation between travel and timing of the flu season is causal. In addition, 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.
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. 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."