1. Center for Infectious Disease Dynamics,
2. Department of Biology and,
3. Departments of Biology and Entomology, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
4. Epicentre, Paris 75011, France
5. DAMTP, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK
6. Fogarty International Center, National Institutes of Health, Bethesda, Maryland 20892, USA
7. World Health Organization, 20 Avenue Appia, CH-1211 Geneva 27, Switzerland
8. Direction Generale de la Sante Publique, Ministere de la Sante, BP 623, Niamey, Niger

Correspondence to: Matthew J. Ferrari¹ Correspondence and requests for materials should be addressed to M.J.F. (Email: mferrari@psu.edu).

Abstract

Although vaccination has almost eliminated measles in parts of the world, the disease remains a major killer in some high birth rate countries of the Sahel. On the basis of measles dynamics for industrialized countries, high birth rate regions should experience regular annual epidemics. Here, however, we show that measles epidemics in Niger are highly episodic, particularly in the capital Niamey. Models demonstrate that this variability arises from powerful seasonality in transmission—generating high amplitude epidemics—within the chaotic domain of deterministic dynamics. In practice, this leads to frequent stochastic fadeouts, interspersed with irregular, large epidemics. A metapopulation model illustrates how increased vaccine coverage, but still below the local elimination threshold, could lead to increasingly variable major outbreaks in highly seasonally forced contexts. Such erratic dynamics emphasize the importance both of control strategies that address build-up of susceptible individuals and efforts to mitigate the impact of large outbreaks when they occur.

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