The dynamics of measles in sub-Saharan Africa

• Published in: Nature 2008-02, Vol.451 (7179), p.679-684
• Main Authors: Ferrari, Matthew J., Grais, Rebecca F., Bharti, Nita, Conlan, Andrew J. K., Bjørnstad, Ottar N., Wolfson, Lara J., Guerin, Philippe J., Djibo, Ali, Grenfell, Bryan T.
• Format: Article
• Language: English
• Subjects: Africa; Biological and medical sciences; Birth rate; Control; Disease Outbreaks - prevention & control; Disease transmission; Distribution; Epidemics; Epidemiology; Health aspects; Human viral diseases; Humanities and Social Sciences; Humans; Infectious diseases; Measles; Measles - epidemiology; Measles - prevention & control; Measles - transmission; Measles - virology; Measles Vaccine - administration & dosage; Measles Vaccine - immunology; Measles-mumps-rubella vaccine; Medical sciences; Metapopulations; multidisciplinary; Natural history; Niger - epidemiology; Outbreaks; Patient outcomes; Prevention; Rain; Science; Science (multidisciplinary); Seasonal variations; Seasons; Statistics; Stochastic Processes; Tropical medicine; Vaccination; Vaccines; Viral diseases; Viral diseases with cutaneous or mucosal lesions and viral diseases of the eye
• ISSN: 0028-0836; 1476-4687; 1476-4679
• DOI: 10.1038/nature06509

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. The ebb and flow of measles Measles is all but eradicated in many countries, but in sub-Saharan Africa and parts of Asia it remains a major killer. An epidemiological study of measles in Niger between 1986 and 2002, just before a major immunization programme began, shows that the epidemics were unexpectedly episodic, interspersed with periods of local extinction. Modelling points to seasonality in disease transmission as the cause for this variability. This has implications for the management of vaccination campaigns. In particular, as vaccine delivery ramps up towards the goal of eradication, an occasional violent seasonal epidemic can be expected, and vaccination can be optimized to minimize this instability. It is reported that measles epidemics in Niger are unexpectedly episodic, and it is shown through modelling that powerful seasonality in transmission generates high amplitude, chaotic epidemics, with potentially important consequences for vaccine-based control strategies.