Ebola: worldwide dissemination risk and response priorities
Authors: Benjamin J Cowling, Hongjie Yu
Journal: The Lancet (2014)

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.1WHO Ebola Response TeamEbola virus disease in west Africa—the first 9 months of the epidemic and forward projections.N Engl J Med. 2014; (published online Sept 23.)https://doi.org/10.1056/NEJMoa1411100Crossref Scopus (1206) Google Scholar 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.2Piot P Muyembe JJ Edmunds WJ Ebola in west Africa: from disease outbreak to humanitarian crisis.Lancet Infect Dis. 2014; (published online Oct 2.)http://dx.doi.org/10.1016/S1473-3099(14)70956-9PubMed Google Scholar 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 (figure). In The Lancet, Isaac Bogoch and colleagues3Bogoch II Creatore MI Cetron M et al.Assessment of the potential for international dissemination of Ebola virus via commercial air travel during the 2014 west African outbreak.Lancet. 2014; (published online Oct 21.)http://dx.doi.org/10.1016/S0140-6736(14)61828-6PubMed Google Scholar report on the potential for international dissemination of Ebola virus disease. Their assessment of risk for different countries is an advance over previous work,4Gomes MFC Piontti AP Rossi L et al.Assessing the international spreading risk associated with the 2014 west African Ebola outbreak.PLoS Currents Outbreaks. 2014; (published online Sept 2.)https://doi.org/10.1371/currents.outbreaks.cd818f63d40e24aef769dda7df9e0da5Crossref Google Scholar 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),1WHO Ebola Response TeamEbola virus disease in west Africa—the first 9 months of the epidemic and forward projections.N Engl J Med. 2014; (published online Sept 23.)https://doi.org/10.1056/NEJMoa1411100Crossref Scopus (1206) Google Scholar 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).3Bogoch II Creatore MI Cetron M et al.Assessment of the potential for international dissemination of Ebola virus via commercial air travel during the 2014 west African outbreak.Lancet. 2014; (published online Oct 21.)http://dx.doi.org/10.1016/S0140-6736(14)61828-6PubMed Google Scholar 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 colleagues3Bogoch II Creatore MI Cetron M et al.Assessment of the potential for international dissemination of Ebola virus via commercial air travel during the 2014 west African outbreak.Lancet. 2014; (published online Oct 21.)http://dx.doi.org/10.1016/S0140-6736(14)61828-6PubMed Google Scholar 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.2Piot P Muyembe JJ Edmunds WJ Ebola in west Africa: from disease outbreak to humanitarian crisis.Lancet Infect Dis. 2014; (published online Oct 2.)http://dx.doi.org/10.1016/S1473-3099(14)70956-9PubMed Google Scholar, 5The LancetEbola: a failure of international collective action.Lancet. 2014; 384: 637Summary Full Text Full Text PDF PubMed Scopus (21) Google Scholar 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),1WHO Ebola Response TeamEbola virus disease in west Africa—the first 9 months of the epidemic and forward projections.N Engl J Med. 2014; (published online Sept 23.)https://doi.org/10.1056/NEJMoa1411100Crossref Scopus (1206) Google Scholar 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,3Bogoch II Creatore MI Cetron M et al.Assessment of the potential for international dissemination of Ebola virus via commercial air travel during the 2014 west African outbreak.Lancet. 2014; (published online Oct 21.)http://dx.doi.org/10.1016/S0140-6736(14)61828-6PubMed Google Scholar 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 screening6Centers for Disease Control and PreventionPress release: enhanced Ebola screening to start at five US airports and new tracking program for all people entering US from Ebola-affected countries.http://www.cdc.gov/media/releases/2014/p1008-ebola-screening.htmlDate: 2014Google Scholar, 7Booth R UK to introduce Ebola screening as death of Briton reported in Macedonia.http://www.theguardian.com/world/2014/oct/09/uk-screening-airports-ebola-symptomsDate: 2014Google Scholar 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.3Bogoch II Creatore MI Cetron M et al.Assessment of the potential for international dissemination of Ebola virus via commercial air travel during the 2014 west African outbreak.Lancet. 2014; (published online Oct 21.)http://dx.doi.org/10.1016/S0140-6736(14)61828-6PubMed Google Scholar 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.8Kanapathipillai R Restrepo AM Fast P et al.Ebola vaccine—an urgent international priority.N Engl J Med. 2014; (published online Oct 7.)https://doi.org/10.1056/NEJMp1412166Crossref Scopus (94) Google Scholar 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,1WHO Ebola Response TeamEbola virus disease in west Africa—the first 9 months of the epidemic and forward projections.N Engl J Med. 2014; (published online Sept 23.)https://doi.org/10.1056/NEJMoa1411100Crossref Scopus (1206) Google Scholar 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.9Baxter AG Symptomless infection with Ebola virus.Lancet. 2000; 355: 2178-2179Summary Full Text Full Text PDF PubMed Scopus (12) Google Scholar 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 aerosols10MacIntyre CR Chughtai AA Seale H Richards GA Davidson PM Respiratory protection for healthcare workers treating Ebola virus disease (EVD): are facemasks sufficient to meet occupational health and safety obligations?.Int J Nurs Stud. 2014; 51: 1421-1426Summary Full Text Full Text PDF PubMed Scopus (68) Google Scholar 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.11Feldmann H Geisbert TW Ebola haemorrhagic fever.Lancet. 2011; 377: 849-862Summary Full Text Full Text PDF PubMed Scopus (1006) Google Scholar, 12Fowler RA Fletcher T Fischer 2nd, WA et al.Caring for critically ill patients with ebola virus disease. Perspectives from west Africa.Am J Respir Crit Care Med. 2014; 190: 733-737Crossref PubMed Scopus (178) Google Scholar 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. This online publication has been corrected. The corrected version first appeared at thelancet.com on November 3, 2014 This online publication has been corrected. The corrected version first appeared at thelancet.com on November 3, 2014 Assessment of the potential for international dissemination of Ebola virus via commercial air travel during the 2014 west African outbreakDecision 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. Full-Text PDF Open AccessDepartment of ErrorCowling BJ, Yu H. Ebola: worldwide dissemination risk and response priorities. Lancet 2015; 385: 7–9—This Comment has been made Open Access, and is now distributed under the terms of CC BY-NC-SA. This correction has been made to the online version as of Nov 3, and the printed Comment is correct. Full-Text PDF