The work, led by Toronto-based researcher Dr. Kamran Khan, predicts the world may see an exported case every 10 days or so via air travel. Ghana, Senegal, Britain and France top the list of countries at greatest risk of importing a case, based on current and historic flight patterns from the three affected countries.
Those arguing in favour of entry screening for or even banning travellers from the affected countries are likely to seize upon the findings and use them as fodder for their case.
But the authors — who include Dr. Martin Cetron, the head of the quarantine division at the US. Centers for Disease Control, and Dr. Theresa Tam, head of the Public Health Agency of Canada's health security infrastructure branch — advance a different argument.
They suggest that effective exit screening at the airports in the capital cities of Liberia, Guinea and Sierra Leone makes more sense than entry screening anywhere else. They say other countries should help with that work, given how stretched the affected countries' resources are.
And the authors stress that combating the outbreak at its source — in West Africa — is the only way to make the world safe from this scourge.
"The risk of international spread could increase significantly if the outbreak in West Africa persists and grows," said Khan, who specializes in using global flight data to predict the spread of infectious diseases.
He and his co-authors suggested the risks for all will become more substantial if someone infected with Ebola travels to a country with a weak public health system and triggers local spread there.
"Should a case get into a country that has very limited public health resources there could be a completely new epidemic somewhere else in the world," said Marisa Creatore, an epidemiologist at St. Michael's Hospital in Toronto, where Khan also practises.
"You start running and staying in the same spot."
The study, published in the journal The Lancet, uses flight data for the three countries from September through December last year, and compares it to the outbound travel from the countries this year. The research was funded by the Canadian Institutes of Health Information.
The three countries are not major contributors to international travel; their combined travel made up half of one per cent of all international air travel in 2013. And travel from those countries has reduced substantially this year, as many air carriers have cancelled flights to the countries. Flights from Liberia are down 51 per cent, from Guinea 66 per cent and Sierra Leone 85 per cent.
While nearly 30 per cent of travellers from these countries are bound for high-income countries, 64 per cent head to low-middle, and low income countries. Accra, Ghana, Dakar, Senegal and London, England are the top destinations for travellers from these countries.
On the list of top 20 destinations by country, Canada is 19th. But Khan has said only 1.5 per cent of all trips from the three countries in the course of a year end up in Canada. And there are no direct flights from those countries to Canada.
The authors note that on the issue of screening, checking people leaving the country for health status, exposure to Ebola patients and fever makes more sense than entry screening.
They note that with exit screening, officials can check everyone leaving the countries at three airports. But entry screening at all airports that receive flights from these countries requires 15 screening systems across 15 countries. And if passengers from these countries then board other planes for destinations that have no direct flights with these countries, the amount of work becomes much greater as the travellers join an enormous pool of international travellers.
The 1,238 cities worldwide without direct flights from the three West African countries would need to screen — or examine the flight itineraries of — 2,512 travellers to find one person whose trip originated in Guinea, Sierra Leone or Liberia. And many of those people would not have Ebola.
"Entry screening at international airports to which no direct flights are arriving from affected countries would be highly inefficient if border authorities are unable to easily identify which travellers originated from countries currently experiencing community-based Ebola virus transmission," they wrote.
A commentary that was published with the study makes an unexpected argument. The authors, from the University of Hong Kong, suggest that regardless of what the science on entry screening shows, countries may still want to do it. So they suggested perhaps the affected countries can abandon exit screening.
"However, 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," Benjamin Cowling and Hongjie Yu of the university's school of public health wrote.
But Khan insisted that if screening finds cases, it makes more sense to discover them before they leave West Africa. And he and his co-authors noted that while resource-rich countries may not have a problem covering the cost of entry screening, in some countries having to establish and maintain screening systems would take scarce resources from other important public health initiatives.
"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.
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