Screening of air travelers departing from three West African cities offers a cheaper and simpler way to check the international spread of Ebola virus, compared with screening at points of entry, which would carry many added costs with few added benefits, according to new research.
Without effective exit screening in place, 2.8 infected travelers a month would leave the three countries – Sierra Leone, Liberia, and Guinea – where intense transmission of Ebola is currently occurring, predicted Dr. Isaac I. Bogoch of the University of Toronto and his colleagues, who published their findings Oct. 20 in the Lancet (2014 Oct. 20 [doi:10.1016/S0140-6736(14)61828-6]).
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Ghana, Senegal, France, and the United Kingdom were top destinations of travelers from outbreak countries.
The investigators used air travel records and viral surveillance data from the World Health Organization to calculate the number of infected travelers likely to board a flight from those countries every month if no exit screening were in place. All three countries currently do conduct screening; however, international support is needed to ensure that it continues, Dr. Bogoch said.
While exit screening will not catch latent cases of Ebola prior to the onset of fever, entry screening likely will not be more effective at doing so, the researchers found, as travel records show that a majority of air travel from the affected countries consists of shorter flights lasting an average of 2.7 hours.
Adding entrance screening would vastly expand the number of people needing to be screened and the number of airports in which screening would occur, from 3 (Conakry, Guinea; Monrovia, Liberia; and Freetown, Sierra Leone) to more than 1,238, if accounting for passengers arriving on indirect flights, according to the study.
Ghana and Senegal were the top two destinations of air travelers from the affected countries, accounting for 17.5% and 14.4% of travel. The United Kingdom received 8.7% and France, 7.1%. The United States received 2% of the volume of travelers from these countries.
Of concern, some 64% of travelers from the affected countries were en route to other low- and middle-income countries, Dr. Bogoch and his colleagues found, meaning the larger share of importation risk fell on “inadequately resourced medical and public health systems [that] might be unable to detect and adequately manage an imported case of Ebola virus disease.”
Outbreaks following imported cases have been stopped in Nigeria and Senegal. The World Health organization declared the Nigeria outbreak over on Oct. 20 and the Senegal outbreak over on Oct. 17.
In an editorial comment accompanying the study, Benjamin Cowling, Ph.D., of the University of Hong Kong, and Dr. Hongjie Yu of the Chinese Center for Disease Control and Prevention, Beijing, agreed that international support was “essential” for stringent exit screening, yet noted that no specific funding had been announced to support it (Lancet 2014 Oct. 20 [doi:10.1016/S0140-6736(14)61895-X]).
Regardless, Dr. Cowling and Dr. Yu argued, screening “might not have a substantial effect on export rates, because of the long incubation period of the disease (average 8-10 days; range 2-21 days), combined with rapid disease progression after onset, meaning that most exportations would be incubating infections missed at border screening points … In addition to any entry or exit screening, vigilance within countries is essential for early detection of imported cases.”
Dr. Bogoch’s study was funded by the Canadian Research Institutes of Health. Dr. Bogoch and two of his coauthors, Dr. Maria Creatore and Dr. Kamran Khan, reported ties to Bio.Diaspora, a company that models global infectious disease threats. Dr. Cowling disclosed financial relationships with MedImmune, Sanofi Pasteur, and Crucell.