Measles dynamics on a seasonal and population gradient: implications for vaccination programs in Niger

Background/Question/Methods

Public health and vaccine policy for measles in the Sahel is faced with both logistical and dynamical challenges. Practically, there are powerful constraints on distribution of vaccine and access to care that make the application of uniform strategies a challenge. Dynamically, strong seasonal forcing and high birth rates lead to locally unstable outbreak dynamics. Consequently, large-scale regional persistence of measles depends on spatial coupling among communities across huge areas and strong demographic and seasonal gradients. Understanding these spatial dynamics and their implications for regional persistence is key to developing effective public health policy. National reporting of measles provides weekly reports of observed cases aggregated into 38 regions across the country that vary greatly in population density and the magnitude of the annual rainy season. We have analyzed an 11-year time series of measles cases to estimate both the strength of seasonal forcing of measles transmission and the local coupling to the regional metapopulation across the country. Results/Conclusions The seasonal fluctuation of measles transmission rates is very consistent across the country of Niger. Despite the broad regional similarities in the underlying seasonality, the magnitude of outbreaks is highly variable across the country from year to year, indicating very weak coupling among districts. We show that the patterns of measles persistence and reintroduction following local extinction are indicative of very weak local coupling and an important role of immigration of measles cases from outside of Niger. We summarize these dynamics in a national level metapopulation model to investigate the impacts of various vaccination strategies on overall incidence and local variability in outbreak magnitude in this exciting example of a highly forces host-natural enemy metapopulation.