Since it was identified in 1976 during simultaneous outbreaks in Sudan and the Democratic Republic of Congo, Ebola has concerned the public due to both its lethality and the manner of death. Historically, many outbreaks occur in remote locations that make it easier for public health and medical responders to isolate the outbreak. The scope of the current outbreak is unprecedented, with cases in urban areas in four countries. Curtailing this outbreak will require substantial efforts by the international community; WHO has drawn up a $100 million plan to do so.
Despite its high case fatality rate (hovering around 50% in this outbreak), Ebola is more difficult to spread than other feared diseases like SARS. People become infected through fluids from infected individuals. Fortunately, Ebola virus is only shed when people have symptoms, unlike influenza, for example. The incubation period for Ebola ranges from 2 to 21 days, which does allow for people to travel substantial distances after exposure and before they show symptoms. Given the dramatic increases in air travel connectivity, this Ebola outbreak does present some risk beyond the countries and continent currently affected.
At the time of writing, media sources are investigating potential imported Ebola cases in Europe, Asia, North America, and the Middle East, but none of the travelers have been confirmed as cases. The Disease Daily and BioDiaspora have teamed up to provide a brief threat assessment based on international air travel patterns. Led by infectious disease clinician and scientist Dr. Kamran Khan, BioDiaspora is a Toronto-based research group studying how infectious diseases are increasingly able to disperse worldwide through the movements of international travelers.
To explore global potential for Ebola case importation, we have examined historic air travel records to characterize the degree of connectivity between the countries at the center of the Ebola outbreak and the rest of the world. Global data from the International Air Transport Association (IATA) provides anonymized flight itineraries of all travelers who embarked on commercial flights, including scheduled charters, from any domestic or international airport within Guinea, Liberia, or Sierra Leone and who disembarked at an international destination in August 2012. IATA captures 93% of the world’s commercial air traffic data and uses market intelligence to produce estimates for the remainder. Each flight itinerary encompassed data on the city where each traveler initiated their trip along with connecting flights en route to their final destination.
In August of 2012, travelers departing from Guinea, Liberia, and Sierra Leone predominantly traveled to African destinations (55.2%). Most of the remaining travelers went to Europe (29.6%) and the Eastern Mediterranean (6.5%). The Americas received less than 5% and the United States only 2.5%. Also noteworthy, the total volume of international traffic from these three countries is quite low relative to other major countries. Finally, the countries at the center of the outbreak have announced a cross-border isolation zone.
Our interconnected world does provide opportunities for diseases to hop on a plane and travel great distances, but that possibility should be placed in the context of the burden of disease in a population (i.e. how likely a traveler will be infected) and the volume of travel between cities of the world.