title: Modeling the obesity epidemic: social contagion and its implications for control.
abstract: BACKGROUND
  As an obesity epidemic has grown worldwide, a variety of intervention programs have been considered, but a scientific approach to comparatively assessing the control programs has still to be considered. The present study aims to describe an obesity epidemic by employing a simple mathematical model that accounts for both social contagion and non-contagious hazards of obesity, thereby comparing the effectiveness of different types of interventions.
  
  
  METHODS
  An epidemiological model is devised to describe the time- and age-dependent risk of obesity, the hazard of which is dealt with as both dependent on and independent of obesity prevalence, and parameterizing the model using empirically observed data. The equilibrium prevalence is investigated as our epidemiological outcome, assessing its sensitivity to different parameters that regulate the impact of intervention programs and qualitatively comparing the effectiveness. We compare the effectiveness of different types of interventions, including those directed to never-obese individuals (i.e. primary prevention) and toward obese and ex-obese individuals (i.e. secondary prevention).
  
  
  RESULTS
  The optimal choice of intervention programs considerably varies with the transmission coefficient of obesity, and a limited transmissibility led us to favour preventing weight gain among never-obese individuals. An abrupt decline in the prevalence is expected when the hazards of obesity through contagious and non-contagious routes fall into a particular parameter space, with a high sensitivity to the transmission potential of obesity from person to person. When a combination of two control strategies can be selected, primary and secondary preventions yielded similar population impacts and the superiority of the effectiveness depends on the strength of the interventions at an individual level.
  
  
  CONCLUSIONS
  The optimality of intervention programs depends on the contagiousness of obesity. Filling associated data gaps of obesity transmission would help systematically understand the epidemiological dynamics and consider required control programs.
authors.affiliation: School of Public Health, The University of Hong Kong, Pokfulam, Hong Kong.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Ejima
authors.forename: Kazuyuki
authors.initials: K
authors.lastname: Aihara
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Nishiura
mesh_terms: D000367:Age Factors; D014943:Global Health; D006801:Humans; D008962:Models, Theoretical; D009765:Obesity / Q000453:epidemiology*; D012919:Social Behavior