title: The Western Africa ebola virus disease epidemic exhibits both global exponential and local polynomial growth rates.
abstract: BACKGROUND
  While many infectious disease epidemics are initially characterized by an exponential growth in time, we show that district-level Ebola virus disease (EVD) outbreaks in West Africa follow slower polynomial-based growth kinetics over several generations of the disease.
  
  
  METHODS
  We analyzed epidemic growth patterns at three different spatial scales (regional, national, and subnational) of the Ebola virus disease epidemic in Guinea, Sierra Leone and Liberia by compiling publicly available weekly time series of reported EVD case numbers from the patient database available from the World Health Organization website for the period 05-Jan to 17-Dec 2014.
  
  
  RESULTS
  We found significant differences in the growth patterns of EVD cases at the scale of the country, district, and other subnational administrative divisions. The national cumulative curves of EVD cases in Guinea, Sierra Leone, and Liberia show periods of approximate exponential growth. In contrast, local epidemics are asynchronous and exhibit slow growth patterns during 3 or more EVD generations, which can be better approximated by a polynomial than an exponential function.
  
  
  CONCLUSIONS
  The slower than expected growth pattern of local EVD outbreaks could result from a variety of factors, including behavior changes, success of control interventions, or intrinsic features of the disease such as a high level of clustering. Quantifying the contribution of each of these factors could help refine estimates of final epidemic size and the relative impact of different mitigation efforts in current and future EVD outbreaks.
authors: Division of Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, Maryland, USA; Mathematical, Computational & Modeling Sciences Center, School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona, USA.
authors: Gerardo
authors: G
authors: Chowell
authors: Division of International Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, Maryland, USA.
authors: Cécile
authors: C
authors: Viboud
authors: Department of Mathematics, Tulane University, New Orleans, Louisiana, USA.
authors: James M
authors: JM
authors: Hyman
authors: Department of Global Health, George Washington University, Milken Insittute School of Public Health, Washington, District of Columbia, USA.
authors: Lone
authors: L
authors: Simonsen
keywords: ebola