Explaining seasonal fluctuations of measles in Niger using nighttime lights imagery

Abstract

Measles epidemics in West Africa cause a significant proportion of vaccine-preventable childhood mortality. Epidemics are strongly seasonal, but the drivers of these fluctuations are poorly understood, which limits the predictability of outbreaks and the dynamic response to immunization. We show that measles seasonality can be explained by spatiotemporal changes in population density, which we measure by quantifying anthropogenic light from satellite imagery. We find that measles transmission and population density are highly correlated for three cities in Niger. With dynamic epidemic models, we demonstrate that measures of population density are essential for predicting epidemic progression at the city level and improving intervention strategies. In addition to epidemiological applications, the ability to measure fine-scale changes in population density has implications for public health, crisis management, and economic development.

Fig. 1

(A) Map of Africa, Niger in gray. (B) Three cities of Niger included in this study. (C) Average weekly annual rainfall for Niger (dark gray) and national weekly average of annual measles cases, 1995–2004 (light gray). Shading gives 95% confidence intervals. (D) Relative transmission rates (number of infections per product of susceptible and infectious individuals per 2 weeks) for Niamey, Maradi, Zinder by calendar day 1 to 365 (x axis) ⁽¹⁾. Gray area indicates rainy season. (E) Relative brightness (cubic smoothing spline, df = 3) by calendar day 1 to 365 (x axis) for each city. Gray area indicates rainy season; dashed line indicates mean of brightness for each city (table S1). (F) Brightness against relative transmission rate for each city. Box indicates interquartile range, whiskers extend 1.5 times the interquartile range. Width of boxes correlates to number of observations.

Fig. 2

(A) Pixels of Niamey designating communes by color, consistent for panels (A) to (C). Black polygons outline communes. (B) (Plot) Brightness (cubic smoothing spline, df = 3) for each commune from calendar day 200 (x axis). Red arrow indicates start of epidemic in commune 1. (Panels above and vertical lines) Colors indicate relative brightness of each pixel in Niamey at the peak of the epidemic in commune 1 (left), the onset of ORV (center), and the peak of rainy season (right). Mean of each pixel is set to zero. Black polygons outline communes. (C) Weekly reported measles cases by commune from calendar day 200. Dashed line represents timing of ORV. (Inset) Maximum brightness value of each commune against total measles cases. (D) Points show reported measles cases, shading gives central 95% of predicted measles incidence from 25000 model simulations from nighttime lights–informed model (red), no migration model (blue), and constant migration model (gray). Dashed line indicates timing of ORV. The x axis spans the duration of the epidemic: day 307 of 2003 to day 153 of 2004; the y axis is the number of cases on a natural log scale.