title: When to control endemic infections by focusing on high-risk groups.
abstract: BACKGROUND
  For nontransmissible diseases, decisions between interventions focused on high-risk groups and unfocused interventions can be based on attributable-risk calculations. The assumptions of those calculations, however, are violated for infectious diseases.
  
  
  METHODS
  We used deterministic compartmental models of infection transmission having both high- and low-risk groups and both susceptible and infected states to examine intervention effects on endemic infection levels. High risk is generated by increased susceptibility or contagiousness-factors that can be reduced by interventions. Population effects of focused and unfocused interventions are compared at settings where these would be equal if there were no transmission.
  
  
  RESULTS
  In the most likely range of mixing between high- and low-risk groups, focused interventions have considerably larger effects than unfocused interventions. At all mixing levels, both interventions have greater effects on infectious than noninfectious diseases because a change in risk factor for one individual alters risk in others. Interventions on contagiousness in high-risk groups have greater effects than comparable interventions on susceptibility.
  
  
  CONCLUSIONS
  Risk assessment for infectious disease requires analysis of the population system that is circulating the infection. Vaccine trials on individuals will miss important effects that trials on transmission units will detect. Focusing HIV control on contagiousness factors in high-risk groups will be especially productive.
authors.affiliation: Department of Epidemiology, University of Michigan, Ann Arbor, MI, USA. jkoopman@umich.edu
authors.forename: James S
authors.initials: JS
authors.lastname: Koopman
authors.forename: Carl P
authors.initials: CP
authors.lastname: Simon
authors.forename: Chris P
authors.initials: CP
authors.lastname: Riolo
mesh_terms: D003140:Communicable Disease Control / Q000379:methods*; D003141:Communicable Diseases / Q000453:epidemiology / Q000635:transmission*; D004198:Disease Susceptibility; D019353:Endemic Diseases / Q000517:prevention & control*; D006801:Humans; D015994:Incidence; D015233:Models, Statistical; D018570:Risk Assessment; D013997:Time Factors