title: Demographics, epidemiology and the impact of vaccination campaigns in a measles-free world - Can elimination be maintained?
abstract: All six WHO regions currently have goals for measles elimination by 2020. Measles vaccination is delivered via routine immunization programmes, which in most sub-Saharan African countries reach children around 9months of age, and supplementary immunization activities (SIAs), which target a wider age range at multi-annual intervals. In the absence of endemic measles circulation, the proportion of individuals susceptible to measles will gradually increase through accumulation of new unvaccinated individuals in each birth cohort, increasing the risk of an epidemic. The impact of SIAs and the financial investment they require, depend on coverage and target age range.
  
  
  
  We evaluated the impact of target population age range for periodic SIAs, evaluating outcomes for two different levels of coverage, using a demographic and epidemiological model adapted to reflect populations in 4 sub-Saharan African countries.
  
  
  
  We found that a single SIA can maintain elimination over short time-scales, even with low routine coverage. However, maintaining elimination for more than a few years is difficult, even with large (high coverage/wide age range) recurrent SIAs, due to the build-up of susceptible individuals. Across the demographic and vaccination contexts investigated, expanding SIAs to target individuals over 10years did not significantly reduce outbreak risk.
  
  
  
  Elimination was not maintained in the contexts we evaluated without a second opportunity for vaccination. In the absence of an expanded routine program, SIAs provide a powerful option for providing this second dose. We show that a single high coverage SIA can deliver most key benefits in terms of maintaining elimination, with follow-up campaigns potentially requiring smaller investments. This makes post-campaign evaluation of coverage increasingly relevant to correctly assess future outbreak risk.
authors.affiliation: Department of Ecology and Evolutionary Biology, Princeton University, USA. Electronic address: j.prada.1@warwick.ac.uk.
authors.forename: J M
authors.initials: JM
authors.lastname: Prada
authors.affiliation: Department of Ecology and Evolutionary Biology, Princeton University, USA; Office of Population Research, WWS, Princeton University, USA; Fogarty International Center, National Institutes of Health, USA.
authors.forename: C J E
authors.initials: CJE
authors.lastname: Metcalf
authors.affiliation: Department of Ecology and Evolutionary Biology, Princeton University, USA.
authors.forename: S
authors.initials: S
authors.lastname: Takahashi
authors.affiliation: Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, USA.
authors.forename: J
authors.initials: J
authors.lastname: Lessler
authors.affiliation: WorldPop, Department of Geography and Environment, University of Southampton, UK; Flowminder Foundation, Stockholm, Sweden.
authors.forename: A J
authors.initials: AJ
authors.lastname: Tatem
authors.affiliation: Center for Infectious Disease Dynamics, Pennsylvania State University, USA.
authors.forename: M
authors.initials: M
authors.lastname: Ferrari
mesh_terms: D000293:Adolescent; D017954:Africa South of the Sahara / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D003710:Demography; D060740:Disease Eradication / Q000379:methods*; D005260:Female; D006801:Humans; D017589:Immunization Programs; D007223:Infant; D008297:Male; D008457:Measles / Q000453:epidemiology* / Q000517:prevention & control*; D015233:Models, Statistical
keywords: Epidemiology; Mathematical models; Measles; SIR; Vaccines