12 Sep Ebola: Each Primary Infection Resulting In Spread To More Than One Additional Case
Medical Research: What are the main findings of the study?
1.We estimated the effective reproduction number of Ebola virus disease, i.e. average number of secondary cases produced by a single primary case at calendar time t (Rt), for the ongoing epidemic in West Africa from March to August 2014. Estimates of Rt for the Guinea, Sierra Leone and Liberia, countries that are experiencing sustained community transmission were consistently above 1.0 since June 2014.
2.Country-specific estimates of the reproduction number for Liberia and Sierra Leone lied between 1.0 and 2.0, reflecting continuous growth of cases in these countries
3. Effective reproduction number below 2 indicate that control could be attained by preventing over half of the secondary transmissions per primary case (e.g. by means of effective case isolation and contact tracing).
Medical Research: What recommendations do you have for future research as a result of this study?
Dr. Chowell-Puente: Our statistical analysis of the reproduction number of Ebola virus disease in West Africa has demonstrated that the continuous growth of cases from June to August 2014 signaled a major epidemic, which is in line with estimates of the Rt above 1.0. Our estimated reproduction numbers, broadly ranging from 1 to 2, are consistent with published estimates from prior outbreaks in Central Africa (e.g. the reproduction number for Ebola virus disease has been estimated at 1.83 for an outbreak in Congo in 1995 and 1.34 in Uganda in 2000 prior to the implementation of control intervention). Our estimates of Rt below 2 indicate that the outbreak could be brought under control if more than half of secondary transmissions per primary case are prevented. Uncontrolled cross-border transmission could fuel a major epidemic to take off in new geographical areas (e.g. as seen in Liberia). Close monitoring of this evolving epidemic should continue in order to assess the status of the outbreak in real time and guide control interventions in the region.
Early transmission dynamics of Ebola virus disease (EVD), West Africa, March to August 2014
H Nishiura ()1, G Chowell2,3
- Graduate School of Medicine, The University of Tokyo, Tokyo, Japan
- School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona, United States
- Division of International Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, Maryland, United States
New research from Arizona State University and the University of Tokyo that analyzes transmission rates of Ebola in West African countries shows how rapidly the disease is spreading.
Researchers Gerardo Chowell-Puente, associate professor in ASU’s School of Human Evolution and Social Change in the College of Liberal Arts and Sciences, and Hiroshi Nishiura of the University of Tokyo found that transmission rates for each single case of Ebola consistently showed at least one new case of the disease being transmitted. Country-specific analysis of transmission rates in Liberia and Sierra Leone showed on average between one and two new cases for every existing case.
“Our analysis of the reproduction numbers of Ebola cases shows continuous growth from June to August 2014 that signaled a major epidemic,” Nishiura said. “Uncontrolled cross-border transmission could fuel a major epidemic to take off in new geographical areas, as was seen in Liberia.”
Rates of transmission increased from June to July in Sierra Leone and Liberia, from 1.4 to 1.7 respectively for every existing case. The statistical analysis is detailed in the paper “Early transmission dynamics of Ebola virus disease, West Africa, March to August 2014,” published today in Eurosurveillance.
“Our findings suggest that control of the Ebola epidemic that has taken so many lives could be attained by preventing more than half of the secondary transmissions for each primary case. This could be attained by isolating those with Ebola and tracing each case to its source,” Chowell said.
The current epidemic is the largest and first regional outbreak of Ebola in Africa. The outbreak was not recognized until March of 2014 after starting around December of 2013, which facilitated the spread to Sierra Leone, Liberia and Nigeria. The World Health Organization declared the epidemic a Public Health Emergency of International Concern in August.
“Characterizing the distribution of secondary cases from a single case can help health care workers and officials understand Ebola transmission dynamics over time in affected countries and gauge the effect of interventions to control spread of the disease,” he added
Researchers analyzed case counts by the World Health Organization as of Aug. 26, 2014. Two groups of data were used: confirmed and probable cases; and the total number of reported cases (confirmed, probable and suspected cases).
Transmission rates are consistent with the rate of spread from prior outbreaks in Central Africa, researchers found.
“Close monitoring of this evolving epidemic should continue in order to assess the status of the outbreak in real time and guide control interventions in the region,” Chowell said
Julie Newberg , firstname.lastname@example.org Media Relations Arizona State University