1399. Is it Just Bad Luck? Examining the Role of Random Chance in the Epidemiology of Ebola
Session: Poster Abstract Session: All Things Ebola
Saturday, October 10, 2015
Room: Poster Hall
Background: The ongoing West African Ebola epidemic is the largest Ebola outbreak to date, eclipsing all previous epidemics both in the magnitude and duration. This has fostered an interest in understanding why this epidemic is different – are there differences in the virus, the population, or other factors that make this epidemic particularly large? Or is the current epidemic consistent with the epidemiology of previous epidemics, with the outcome of the observed epidemic simply attributable to chance? We investigate whether the trajectory of the 2014 outbreak is consistent with Ebola epidemiology as it has historically been understood, or if the outbreak deviates substantially from the range of expected trajectories.

Methods: We use an agent-based model of Liberia and Sierra Leone, calibrated to the current epidemic to simulate 2,000 epidemics (1,000 for each country) of Ebola. Both the distribution of the number of cases of Ebola as well as the length of the epidemics are considered, the latter using Kaplan-Meier survival curves to measure the time until the end of an epidemic.

Results: Both Sierra Leone and Liberia show distinctly multi-modal distributions for epidemic size and the duration of the epidemic. In both countries over 20% of the epidemics end with very few cases, and over half of the epidemics are the relatively small, short-lived outbreaks that previously characterized the epidemiology of Ebola. The remainders are long-lived, severe outbreaks like the one currently ongoing in West Africa, with ~30% of epidemics larger and longer-lived than the current outbreak.

Conclusion: The same model parameters that produce the current outbreak also produce a large number of short, relatively small outbreaks consistent with the previous epidemiology of Ebola. As these simulations only vary based on random chance, the hypothesis that the current outbreak is merely the result of chance, rather than changes to the underlying virus or the structure of the West African population cannot be ruled out. Furthermore, these results suggest that while the current epidemic is undeniably serious, it is far from the worst-case scenario, and the occurrence of other large, long-lasting Ebola outbreaks should be planned for in the future.

Eric Lofgren, MSPH, PhD1, Caitlin Rivers, PhD2, Bryan Lewis, MPH, PhD1, Madhav V. Marathe, PhD2, Meredith Wilson, MPH2, Jiangzhuo Chen, PhD2 and Stephen Eubank, PhD1, (1)Network Dynamics and Simulation Science Lab, Virginia Tech, Blacksburg, VA, (2)Network Dynamics and Simulation Science Lab, Virginia Bioinformatics Institute, Blacksburg, VA


E. Lofgren, None

C. Rivers, None

B. Lewis, None

M. V. Marathe, None

M. Wilson, None

J. Chen, None

S. Eubank, None

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