Three people infected with Ebola predicted to fly from West Africa every month if no exit screening takes place

Dr Kamran Khan at St Michael's Hospital in Toronto, Canada, and colleagues analysed 2014 worldwide flight schedules and historic flight itineraries of passengers from 2013 to predict expected population movements out of Guinea, Liberia, and Sierra Leone. They also used WHO Ebola virus surveillance data to model the expected number of exported Ebola virus infections and to determine how useful air travel restrictions and airport departure and arrival screening might be in controlling the spread of the deadly virus.

The analysis, assuming no exit screening, showed that based on current epidemic conditions and international flight restrictions to and from Guinea, Liberia, and Sierra Leone, on average, just under three (2·8) travellers infected with Ebola virus are projected to travel on an international flight every month. Although airport screening is currently in place in the locations modelled, the research draws attention to the importance of ensuring that exit screening is implemented and maintained effectively in these locations.

"The risk of international spread could increase significantly if the outbreak in West Africa persists and grows," says Dr Khan. "Risks to the global community would further increase if Ebola virus were to spread to and within other countries with weak public health systems."

Of the almost 500,000 travellers who flew on commercial flights out of Guinea, Liberia, or Sierra Leone in 2013, over half had destinations in five countries: Ghana (17.5%), Senegal (14.4%), UK (8.7%), France (7.1%), and Gambia (6.8%). More than 60% of travellers in 2014 are expected to have final destinations in low or lower-middle income countries. "Given that these countries have limited medical and public health resources, they may have difficulty quickly identifying and effectively responding to imported Ebola cases," says Dr Khan.

The findings also indicate that it is far more efficient and less disruptive to screen travellers for Ebola as they leave affected countries in West Africa rather than screen the same travellers as they arrive at other airports around the world. The authors also point out that with effective exit screening of travellers in place, the additional utility of entry screening is very low given the short flight durations out of affected countries, compared with the much longer incubation period of Ebola virus.

Dr Khan explains, "Exit screening at the three international airports (Conakry, Monrovia, and Freetown) in Guinea, Liberia, and Sierra Leone should allow all travellers at highest risk of exposure to Ebola to be assessed with greater efficiency compared with entry screening the same passengers as they arrive in cities around the world. However, this will require international support to effectively implement and maintain."

He adds, "The best approach to minimise risks to the global community is to control the epidemic at its source. While screening travellers arriving at airports outside of West Africa may offer a sense of security, this would have at best marginal benefits, and could draw valuable resources away from more effective public health interventions. Furthermore, excessive constraints on air travel could have severe economic consequences that could destabilise the region and possibly disrupt critical supplies of essential health and humanitarian services. Decision-makers must carefully balance the potential harms that could result from travel restrictions against any reductions in the risk of international spread."

Writing in a linked Comment, Dr Hongjie Yu from the Chinese Center for Disease Control and Prevention, Beijing, China and Dr Benjamin Cowling from The University of Hong Kong, China, welcome the study, which they say, "is an advance over previous work, which analysed flight networks and connectivity, but did not account for passenger flows and final destinations." However, they add that, "Some countries have implemented and will continue entry screening for various reasons. Subject to entry screening already being implemented, exit screening from the affected countries might not have incremental utility, especially considering the other urgent priorities in the region. In addition to any entry or exit screening, vigilance within countries is essential for early detection of imported cases of Ebola virus disease."