At an oversight hearing in the U.S. House of Representatives yesterday, lawmakers grilled health officials over the response to the first domestic cases of Ebola and asked them to respond to the idea—which many Republicans now promote—of banning incoming flights from West Africa. When Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, explained that restrictions would only cause travelers to reroute through other countries, making them harder to track, Representative Henry Waxman (D–CA) came to his defense with a visual aid.
The map he presented, illustrating the relative flow of passengers out of Ebola-affected countries in West Africa to the rest of the world, came from an article published on 2 September in PLOS Currents: Outbreaks. One of its authors, physicist Alessandro Vespignani of Northeastern University in Boston, says he didn’t know his work had figured in the debate until a few colleagues alerted him yesterday. The map demonstrates the complexity of global travel flow, he says, but in the rest of the paper, “we are more quantitative than that.” The authors combined this flight information with equations describing likely transmission dynamics in 16 countries at highest risk of Ebola importation to predict the probability that each will see a new imported case. According to the group’s more recent predictions, the risk of another infected person arriving in United States by 31 October given the current reductions in air traffic is about 25%. The published work simulates how reductions in travel can reduce the spread of the disease—right now, their estimates assume an 80% overall reduction in travel—but Vespignani notes that cutting down on traffic is just “postponing the problem for a finite amount of time.” (The 80% drop amounts to a 3- to 4-week delay in a probable case, he says.) “This debate doesn’t have to divert the discussion from the real issue, which is to win the battle in Africa.”
Ira Longini, a biostatistician at the University of Florida in Gainesville and another author on the PLOS paper, says he thinks “it’s fine” for Congress to make use of the work in a hearing, but laments that “people are notoriously bad about interpreting probability.” The group’s plots show how the probability of new imported cases would drop if the flow of travelers decreased, but it doesn’t wade into the complex costs and benefits of a travel ban. “You can point at that plot and make the argument in either direction,” Longini says. The group plans to publish a new paper next week that looks at the impact of travel restrictions that some airlines and governments have already imposed. “They can point at that one next,” he says.
*The Ebola Files: Given the current Ebola outbreak, unprecedented in terms of number of people killed and rapid geographic spread, Science and Science Translational Medicine have made a collection of research and news articles on the viral disease freely available to researchers and the general public.