Paris - Screening air travellers on departure from Ebola-hit countries is far smarter than monitoring them when they arrive abroad, experts said on Tuesday.
Instead of relying on a shield at their own borders, countries should help Liberia, Guinea and Sierra Leone boost capacity to spot Ebola cases, they said.
"The best approach to minimise risks to the global community is to control the epidemic at its source," said Kamran Khan of St Michael's Hospital in Toronto, Canada.
"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."
The analysis, published in The Lancet, said screening on exit would entail monitoring in just three international airports - in Conakry, Monrovia and Freetown.
But screening on arrival required far greater resources to be similarly effective.
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Sixteen airports in 15 countries receive direct international flights from Liberia, Guinea and Sierra Leone, the study found.
These, in turn, provide connecting flights that serve 1 238 cities, where only one in 2 500 passengers would be from the three west African countries.
The team looked at 2014 flight schedules and patterns of passenger movement in 2013 to predict the risk of the virus hitching a ride in a passenger.
Statistically, without exit screening, three Ebola-infected travellers would depart on an international flight from the affected area every month.
The experts did not quantify the likely impact of exit screening measures, but said gains from additional testing on arrival would be "negligible".
"Ideally, we would assume that all symptomatic travellers would be identified through exit screening," Marisa Creatore of the Li Ka Shing Knowledge Institute of St Michael's Hospital in Toronto told AFP.
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"However... because Ebola virus has a long incubation period where people are not ill and do not display symptoms (average 8-10 days, up to 21 days), most infected travellers will not be symptomatic and possibly not even know they are infected.
"If they are not showing symptoms, then they will not be detectable with thermal imaging or other interventions either at exit or at entry."
European health ministers agreed last week to launch a review of exit screening measures in Liberia, Guinea and Sierra Leone.
Just over half of passengers from the three affected countries travel to just five destinations: Accra in Ghana (17.5%), Dakar in Senegal (14.4%), London (7.7%), Banjul in Gambia (6.8%) and Paris (6%), the new study said.
New York was 21st on the list with 0.5%.
More than 60% of people leaving the three countries travelled to low- or low-middle income countries, where entry screening may not be up to scratch.
"Given that these countries have limited medical and public health resources, they may have difficulty quickly identifying and effectively responding to imported Ebola cases," said 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."
The Ebola outbreak has claimed more than 4 500 lives in West Africa since the beginning of the year.
Many countries further afield have sharpened their own border controls, with fear spreading since three cases of domestic infection of nurses caring for Ebola patients - two in the United States and one in Spain.