Every year, tens of thousands of children in sub-Saharan Africa die from measles. In Niger, one of the world's poorest countries, measles epidemics typically start at the onset of the dry season in October. This timing is remarkably consistent, but the size of the epidemic varies greatly from year to year. In some years, the epidemics fade out completely, even in the capital city (Niamey), home to around 700,000 people.
Mathematical models can be developed to explain the size and duration of measles epidemics. However, models based on data from developed countries like England fail to account for the pattern of epidemics in Niger. In particular, models based on English data (extrapolated to the very high birth rates in Niger) predict regular epidemics and few epidemic fade-outs.
Therefore, Matt Ferrari, Nita Bharti, Ottar Bjornstad, Bryan Grenfell and collaborators from Epicentre and the World Health Organization modeled the measles outbreaks in Niamey using data from the Niger Ministry of Health.
The new models show that:
The highly variable amplitude of epidemics in Niamey are driven by extremely strong seasonal variations in transmission. This seasonal forcing is several times greater than in developed countries like the U.K.
Large epidemics continue to flare up at (chaotic) intervals, even under high levels of vaccination, thanks to the strong seasonality.
» The researchers discuss their findings, and implications for vaccination strategies, in a paper in Nature
Written By: Ferrari MJ, Grais RF, Bharti N, Conlan AJK, Bjornstad ON, Wolfson LJ, Guerin PJ, Djibo A, & Grenfell BT.
Journal: 451: 679-684
Journal Reference: 451: 679-684
Paper Id: 10.1038/nature06509