Last week Canada stopped issuing visas to travelers from the three West African countries battling Ebola, echoing a decision Australia made days earlier. The new policies are the biggest travel restrictions yet from wealthy countries. They are intended to keep the virus from spreading, and they probably won’t work.
Blocking most travel from Guinea, Liberia, and Sierra Leone, where a total of more than 13,000 people have been infected with Ebola since the outbreak began in March, would only modestly reduce how long it takes for the virus to reach new countries, according to mathematical simulations published in the journal Eurosurveillance. For example, stopping 71 percent of travelers from entering other nations in Africa from the three countries in which Ebola is widespread would delay a case from appearing elsewhere on the continent by only 30 days, according to the model.
Why don’t travel bans have a stronger effect? It’s easy to grasp when you understand the math of the epidemic. The number people with Ebola in West Africa is increasing exponentially, roughly doubling every three weeks. The percentage of people who could bring it somewhere else is “a linear fraction of this exponentially increasing number,” says Vittoria Colizza, one of the paper’s authors and a researcher at the Pierre Louis Institute of Epidemiology and Public Health in Paris. Travel restrictions make the fraction smaller but can’t outrun the pace at which the total number of potential carriers is increasing.
“It’s often thought that by stopping the travel, it’s going to stop the epidemic,” Colizza says. “It’s much more effective to control the exponential increase, because once you control that, the risk of importation is substantially reduced.” A drop in travel from Mexico during the 2009 H1N1 flu did little to slow the spread of the pandemic, earlier research from Colizza and colleagues suggests.
The model assumes that travel among Guinea, Liberia, or Sierra Leone and any given country can’t be stopped entirely. Particularly in rural Africa, borders are leaky, and any travel restrictions may contain some exceptions: Canada and Australia’s new rules don’t apply to their own citizens, who don’t need visas to enter their home countries. And some people will evade restrictions. “It is quite unrealistic to assume that it’s really completely 100 percent effective,” Colizza says.
Her research took into account the changes to travel patterns in place at the end of August, including countries that restricted migration and airlines that cancelled routes. The list of countries with some Ebola-related travel rules has grown since then. If new restrictions impede the world’s efforts to slow the spread of new cases in West Africa, they may heighten the risk to people in unaffected countries.
There’s no sign of that happening yet, says Robert Quigley, regional medical director of International SOS, a company that assists businesses and nongovernmental organizations with health care and security in far-flung places, including West Africa. “Those people who make it their living to help others in distress, you could put up a brick wall” and not discourage them, he says. The company also has clients in the mining, energy, and infrastructure sectors operating in the three countries, and he says travel restrictions haven’t affected them: “It’s business as usual. Business has to go on.”
The moves by Australia and Canada, Quigley says, are most likely aimed at preventing West African expatriates living in those countries from visiting home, potentially being exposed to Ebola, and returning with the virus. That might satisfy an instinct to tighten borders in developed nations that haven’t yet encountered Ebola. The math of this epidemic suggests that it may delay the virus’s spread but won’t prevent it.