Modelling the spread of Ebola using air-transport networks

THE Ebola virus is not contagious through the air, but air travel has played its part in spreading it within west Africa and beyond. Dirk Brockmann, a specialist in complex networks at Humboldt University in Berlin, has used air-transport data to quantify the probabilities of an infected passenger who has boarded a plane in one of the three most affected countries—Guinea, Liberia and Sierra Leone—exiting the network at a specific airport around the world. For every 100 infected passengers embarking in Guinea, Liberia and Sierra Leone, 84 would normally disembark at another African airport. Three would arrive in Britain and France; only one in America.

The model does not show the current, absolute risks of Ebola arriving in a country. It starts with the assumption that infected people are on the plane in the first place: the actual risks are therefore far lower. It also reflects a normal travel schedule, when in fact many countries and airlines have suspended or modified flights. But Mr Brockmann's work does underline two things. First, the risks of Ebola spreading are overwhelmingly focused in Africa itself. Second, if a Western country is truly intent on keeping Ebola out, it has to go further than banning direct flights from the worst-affected places and must also target global hubs. Airports in London and Paris play a big part in linking Sierra Leone and Guinea to the rest of the world. America’s single Ebola fatality may have disembarked in Dallas but the first leg of his journey from Monrovia, Liberia’s capital, was to Brussels.