  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. 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.
