title: Dynamic denominators: the impact of seasonally varying population numbers on disease incidence estimates.
abstract: Reliable health metrics are crucial for accurately assessing disease burden and planning interventions. Many health indicators are measured through passive surveillance systems and are reliant on accurate estimates of denominators to transform case counts into incidence measures. These denominator estimates generally come from national censuses and use large area growth rates to estimate annual changes. Typically, they do not account for any seasonal fluctuations and thus assume a static denominator population. Many recent studies have highlighted the dynamic nature of human populations through quantitative analyses of mobile phone call data records and a range of other sources, emphasizing seasonal changes. In this study, we use mobile phone data to capture patterns of short-term human population movement and to map dynamism in population densities.
  
  
  
  We show how mobile phone data can be used to measure seasonal changes in health district population numbers, which are used as denominators for calculating district-level disease incidence. Using the example of malaria case reporting in Namibia we use 3.5 years of phone data to investigate the spatial and temporal effects of fluctuations in denominators caused by seasonal mobility on malaria incidence estimates.
  
  
  
  We show that even in a sparsely populated country with large distances between population centers, such as Namibia, populations are highly dynamic throughout the year. We highlight how seasonal mobility affects malaria incidence estimates, leading to differences of up to 30 % compared to estimates created using static population maps. These differences exhibit clear spatial patterns, with likely overestimation of incidence in the high-prevalence zones in the north of Namibia and underestimation in lower-risk areas when compared to using static populations.
  
  
  
  The results here highlight how health metrics that rely on static estimates of denominators from censuses may differ substantially once mobility and seasonal variations are taken into account. With respect to the setting of malaria in Namibia, the results indicate that Namibia may actually be closer to malaria elimination than previously thought. More broadly, the results highlight how dynamic populations are. In addition to affecting incidence estimates, these changes in population density will also have an impact on allocation of medical resources. Awareness of seasonal movements has the potential to improve the impact of interventions, such as vaccination campaigns or distributions of commodities like bed nets.
authors: WorldPop, Geography and Environment, University of Southampton, University Road, Southampton, SO17 1BJ UK ; Flowminder Foundation, Roslagsgatan 17, 113 55 Stockholm, Sweden.
authors: Elisabeth
authors: 0000-0002-5315-4041
authors: E
authors: Zu Erbach-Schoenberg
authors: WorldPop, Geography and Environment, University of Southampton, University Road, Southampton, SO17 1BJ UK ; Flowminder Foundation, Roslagsgatan 17, 113 55 Stockholm, Sweden.
authors: Victor A
authors: VA
authors: Alegana
authors: WorldPop, Geography and Environment, University of Southampton, University Road, Southampton, SO17 1BJ UK ; Flowminder Foundation, Roslagsgatan 17, 113 55 Stockholm, Sweden.
authors: Alessandro
authors: A
authors: Sorichetta
authors: Spatial Epidemiology Lab (SpELL), Université Libre de Bruxelles, Av. FD Roosevelt 50, 1050 Brussels, Belgium ; Department of Geography, Université de Namur, Rue de Bruxelles 61, 5000 Namur, Belgium.
authors: Catherine
authors: C
authors: Linard
authors: WorldPop, Geography and Environment, University of Southampton, University Road, Southampton, SO17 1BJ UK ; Clinton Health Access Initiative, Boston, MA USA.
authors: Christoper
authors: C
authors: Lourenço
authors: WorldPop, Geography and Environment, University of Southampton, University Road, Southampton, SO17 1BJ UK ; Flowminder Foundation, Roslagsgatan 17, 113 55 Stockholm, Sweden.
authors: Nick W
authors: NW
authors: Ruktanonchai
authors: Mobile Telecommunications Limited, Windhoek, Namibia.
authors: Bonita
authors: B
authors: Graupe
authors: WorldPop, Geography and Environment, University of Southampton, University Road, Southampton, SO17 1BJ UK ; Flowminder Foundation, Roslagsgatan 17, 113 55 Stockholm, Sweden.
authors: Tomas J
authors: TJ
authors: Bird
authors: WorldPop, Geography and Environment, University of Southampton, University Road, Southampton, SO17 1BJ UK ; Flowminder Foundation, Roslagsgatan 17, 113 55 Stockholm, Sweden.
authors: Carla
authors: C
authors: Pezzulo
authors: Flowminder Foundation, Roslagsgatan 17, 113 55 Stockholm, Sweden ; Center for Communicable Disease Dynamics and Department of Epidemiology, Harvard, Boston, MA USA ; Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ USA.
authors: Amy
authors: A
authors: Wesolowski
authors: WorldPop, Geography and Environment, University of Southampton, University Road, Southampton, SO17 1BJ UK ; Flowminder Foundation, Roslagsgatan 17, 113 55 Stockholm, Sweden ; Fogarty International Center, National Institutes of Health, Bethesda, MD 20892 USA.
authors: Andrew J
authors: AJ
authors: Tatem
mesh_terms: D040421:Cell Phone; D006801:Humans; D015994:Incidence; D008288:Malaria / Q000453:epidemiology*; D009276:Namibia; D011157:Population Dynamics / Q000706:statistics & numerical data; D011159:Population Surveillance / Q000379:methods*; D012621:Seasons; D014171:Transients and Migrants; D014195:Travel
keywords: Disease incidence; Health metrics; Malaria; Mobile phones; Seasonality; Surveillance