LONDON (Reuters) - Three Ebola-infected travellers a month would be expected to get on international flights from the West African countries suffering epidemics of the deadly virus if there were no effective exit screening, scientists said on Tuesday.
The three countries, Guinea, Liberia and Sierra Leone, do all check departing air passengers for fever, although the test cannot spot sufferers in the period before they show symptoms, which can be up to 21 days.
The researchers, whose work was published in The Lancet medical journal on Tuesday, said exit screening was nevertheless one of the most effective ways of limiting Ebola's spread.
Using modelling based on 2014 global flight schedules and 2013 passenger itineraries, as well as current epidemic conditions and flight restrictions, the analysis showed that, on average, just under three (2.8) Ebola-infected travellers are projected to travel on an international flight every month.
Dr Kamran Khan of St Michael's Hospital in Toronto, Canada, who led the research, said the study showed it was far more effective and less disruptive to screen travellers from the affected countries in West Africa as they leave, rather than when they land, as the United States, Britain, France and some other countries have begun to do.
"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," Khan said.
Ebola is known to have killed more than 4,500 people in Liberia, Sierra Leone and Guinea. But with at least half the cases going unreported and a 70 percent fatality rate, by World Health Organization (WHO) estimates, the true toll in what is by far the worst outbreak on record is probably more than 12,000.
Cases of the haemorrhagic fever have already been imported into Nigeria, Senegal, Spain and the United States, and WHO officials have said it is "unavoidable" that Ebola cases will be seen in more countries.
Many medical experts have argued that the best place to prevent the spread of Ebola is at its source.
Khan said 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".
The study found that, of the almost 500,000 travellers who flew on commercial flights out of Conakry, Monrovia and Freetown international airports in 2013, more than half were destined for one of five countries: Ghana (17.5 percent), Senegal (14.4 percent), Britain (8.7 percent), France (7.1 percent) or Gambia (6.8 percent).
It also found that more than 60 percent of travellers in 2014 were likely to be heading for poor or middle-income countries, where the medical and public health resources to prevent a wider outbreak are likely to be more limited.
Reporting by Kate Kelland; Editing by Kevin Liffey