Jan 03, 2015 Volume 385 Number 9962 p1-88 e1-e3
Ebola: worldwide dissemination risk and response priorities
Benjamin J Cowling, Hongjie Yu
The scale of the current outbreak of Ebola virus disease in west Africa is staggering. Thousands of infections and deaths have been reported in recent months, and unless major changes occur in the situation, incidence of Ebola virus disease has been projected to continue to grow and cumulative incidence to exceed 20 000 by November.1 A humanitarian crisis that stretches far beyond the impact of Ebola virus infections is unfolding in Africa, devastating the health systems and economies in affected countries.2 In the present outbreak, most infections remain confined to west Africa, although four cases have been detected outside this region: three cases diagnosed in Dallas, USA (of which one infection was contracted in Liberia and two were associated with nosocomial transmission from the first case), and one case in Madrid, Spain, associated with nosocomial transmission (figuravergaee).
Among all reported cases in the 2014 outbreak to date, most infections have been contracted in three countries in west Africa: Guinea, Liberia, and Sierra Leone.
In The Lancet, Isaac Bogoch and colleagues3 report on the potential for international dissemination of Ebola virus disease. Their assessment of risk for different countries is an advance over previous work,4 which analysed flight networks and connectivity, but did not account for passenger flows and final destinations. Because of the assumptions of uniform risk across the population and constant prevalence of infection (whereas, in fact, risk within the population is not likely to be uniform and incidence is doubling every 15–30 days),1 the relative risks comparing different countries can be more valuable than the estimated absolute risks. Bogoch and colleagues report that the two countries at highest risk of receiving cases are Ghana and Senegal and, outside Africa, the risk for export to the UK or France combined was estimated to be about eight times higher than the risk for export to the USA (15•8 vs 2•0).3 In other words, for every case of Ebola virus disease exported to the USA, the authors predict that there will be roughly eight cases exported to the UK or France combined.
Bogoch and colleagues3 then studied the potential for exit and entry screening to reduce export of unidentified infections, concluding that exit screening would be a much more efficient approach than entry screening. We would like to add several points to this discussion. First, international support would be essential for implementation of exit screening in the three highly-affected resource-poor countries in west Africa. However, implementation of more stringent checks beyond what is already being done could be very challenging. The affected countries have many urgent priorities—resources including money, personnel, medical equipment, and supplies are urgently needed to expand capacity for detection, diagnosis, and treatment of patients with Ebola virus disease, and to implement isolation and contact tracing, which are currently the best available interventions to control the outbreak. Meanwhile, the outbreak is having a catastrophic effect on the local health-care systems, which were already fragile.2, 5 No announcements have been made yet about earmarked contributions from the international community to support exit screening.
Second, exit and entry 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),1 combined with rapid disease progression after onset, so that most exportations would be incubating infections missed at border screening points. Finally, a choice is posed between entry and exit screening in Bogoch and colleagues’ study,3 with exit screening shown to be more efficient than entry screening and the combination of entry and exit screening shown to have little incremental usefulness. However, some countries have implemented and will continue entry screening6, 7 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.3
There are several important near-future research needs. Perhaps most urgent is a better understanding of the effectiveness of existing treatment options, including convalescent serum. In the medium term, it is hoped that new vaccines and drugs will be available quickly for human clinical trials and in exposed populations.8 The WHO Ebola Response team has neatly summarised the transmission dynamics and epidemiological characteristics including the reproductive number, incubation period, and case fatality risk in the current Ebola virus outbreak,1 but one important unknown is the proportion of infections that are asymptomatic or mildly symptomatic. If mild infections do occur and are infectious, disease control outside west Africa might be increasingly challenging. However, this scenario is thought to be unlikely.9 One particularly pressing need is for the reassessment of appropriate procedures for infection control, and the potential for the virus to spread via small particle aerosols10 in addition to via contact with infected patients or their bodily fluids. Infection of health-care personnel in west Africa is often attributed to the scarcity of appropriate protective equipment and supplies, or inadequate administrative controls.11, 12 However, the nosocomial cases in Dallas and Madrid have raised the concern that present protocols might not be sufficient to protect health-care personnel fully against infection, particularly if cases are managed in health-care facilities that are not fully prepared.
BJC has received research funding from MedImmune and Sanofi Pasteur, and consults for Crucell. HY declares no competing interests. We thank Ren Xiang, Michael Ni, and Charles Yiu for technical assistance with the figure.
Assessment of the potential for international dissemination of Ebola virus via commercial air travel during the 2014 west African outbreak
Isaac I Bogoch, MD, Maria I Creatore, PhD, Martin S Cetron, MD, John S Brownstein, PhD, Nicki Pesik, MD, Jennifer Miniota, MSc, Theresa Tam, MD, Wei Hu, MSA, Adriano Nicolucci, MSA, Saad Ahmed, BSc, James W Yoon, MISt, Isha Berry, Prof Simon I Hay, DSc, Aranka Anema, PhD, Andrew J Tatem, PhD, Derek MacFadden, MD, Matthew German, MSc, Dr Kamran Khan, Open Access
Open access funded by Wellcome Trust
The WHO declared the 2014 west African Ebola epidemic a public health emergency of international concern in view of its potential for further international spread. Decision makers worldwide are in need of empirical data to inform and implement emergency response measures. Our aim was to assess the potential for Ebola virus to spread across international borders via commercial air travel and assess the relative efficiency of exit versus entry screening of travellers at commercial airports.
We analysed International Air Transport Association data for worldwide flight schedules between Sept 1, 2014, and Dec 31, 2014, and historic traveller flight itinerary data from 2013 to describe expected global population movements via commercial air travel out of Guinea, Liberia, and Sierra Leone. Coupled with Ebola virus surveillance data, we modelled the expected number of internationally exported Ebola virus infections, the potential effect of air travel restrictions, and the efficiency of airport-based traveller screening at international ports of entry and exit. We deemed individuals initiating travel from any domestic or international airport within these three countries to have possible exposure to Ebola virus. We deemed all other travellers to have no significant risk of exposure to Ebola virus.
Based on epidemic conditions and international flight restrictions to and from Guinea, Liberia, and Sierra Leone as of Sept 1, 2014 (reductions in passenger seats by 51% for Liberia, 66% for Guinea, and 85% for Sierra Leone), our model projects 2•8 travellers infected with Ebola virus departing the above three countries via commercial flights, on average, every month. 91 547 (64%) of all air travellers departing Guinea, Liberia, and Sierra Leone had expected destinations in low-income and lower-middle-income countries. Screening international travellers departing three airports would enable health assessments of all travellers at highest risk of exposure to Ebola virus infection.
Decision makers must carefully balance the potential harms from travel restrictions imposed on countries that have Ebola virus activity against any potential reductions in risk from Ebola virus importations. Exit screening of travellers at airports in Guinea, Liberia, and Sierra Leone would be the most efficient frontier at which to assess the health status of travellers at risk of Ebola virus exposure, however, this intervention might require international support to implement effectively.
Canadian Institutes of Health Research.
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