Assessing the International Spreading Risk Associated with the 2014 West African Ebola Outbreak
Background: The 2014 West African Ebola Outbreak is so far the largest and deadliest recorded in history. The mainly affected countries, Sierra Leone, Guinea, Liberia, have struggled to contain and to mitigate the outbreak. After more than one year from the rise in transmission, the latest WHO report finally showed the lowest total number of weekly cases since March 2014. Up to August 2nd, 2015, a total of 27898 confirmed, probable, and suspected EVD cases have been reported worldwide, of which 11296 died. Aside from most affected countries in West Africa, a total of 7 other countries have had imported cases, reaching 3 continents: Africa (Nigeria, Senegal and Mali), Europe (Italy, Spain and the UK) and North America (the USA).
Method: We used the Global Epidemic and Mobility Model to generate stochastic, individual based simulations of epidemic spread worldwide, yielding, among other measures, the incidence and seeding events at a daily resolution for 3,362 subpopulations in 220 countries. The mobility model integrates daily airline passenger traffic worldwide and the disease model includes the community, hospital, and burial transmission dynamic. We use a multimodel inference approach calibrated on data from the month of August 2014. The estimates obtained were used to generate monthly ensemble forecasts that provided quantitative estimates of the local transmission of Ebola virus disease in West Africa and the probability of international spread if the containment measures were not successful at curtailing the outbreak. Results were posted in near real-time to help public health authorities decision making. We also tested the impact of on-site intervention measures in the local spread and importation probability.
Results: We model the short-term growth rate of the disease in the affected West African countries and estimate the basic reproductive number. We simulated the international spreading of the outbreak and provide the estimate for the probability of Ebola virus disease case importation in countries across the world, including intervention scenarios.