title: Impact of high-risk sex and focused interventions in heterosexual HIV epidemics: a systematic review of mathematical models.
abstract: BACKGROUND
  The core-group theory of sexually transmitted infections suggests that targeting prevention to high-risk groups (HRG) could be very effective. We aimed to quantify the contribution of heterosexual HRGs and the potential impact of focused interventions to HIV transmission in the wider community.
  
  
  METHODS
  We systematically identified studies published between 1980 and 2011. Studies were included if they used dynamical models of heterosexual HIV transmission, incorporated behavioural heterogeneity in risk, and provided at least one of the following primary estimates in the wider community (a) the population attributable fraction (PAF) of HIV infections due to HRGs, or (b) the number per capita or fraction of HIV infections averted, or change in HIV prevalence/incidence due to focused interventions.
  
  
  RESULTS
  Of 267 selected articles, 22 were included. Four studies measured the PAF, and 20 studies measured intervention impact across 265 scenarios. In low-prevalence epidemics (≤5% HIV prevalence), the estimated impact of sex-worker interventions in the absence of risk compensation included: 6-100% infections averted; 0.9-6.2 HIV infections averted per 100,000 adults; 11-94% and 4-47% relative reduction in prevalence and incidence respectively. In high-prevalence epidemics (>5% HIV prevalence), sex-worker interventions were estimated to avert 6.8-40% of HIV infections and up to 564 HIV infections per 100,000 adults, and reduce HIV prevalence and incidence by 13-27% and 2-14% respectively. In both types of epidemics, greater heterogeneity in HIV risk was associated with a larger impact on the fraction of HIV infections averted and relative reduction in HIV incidence.
  
  
  CONCLUSIONS
  Focused interventions, as estimated by mathematical models, have the potential to reduce HIV transmission in the wider community across low- and high-prevalence regions. However, considerable variability exists in estimated impact, suggesting that a targeted approach to HIV prevention should be tailored to local epidemiological context.
authors.affiliation: Department of Infectious Disease Epidemiology, Imperial College, London, United Kingdom. sharmistha.mishra08@imperial.ac.uk
authors.forename: Sharmistha
authors.initials: S
authors.lastname: Mishra
authors.forename: Richard
authors.initials: R
authors.lastname: Steen
authors.forename: Antonio
authors.initials: A
authors.lastname: Gerbase
authors.forename: Ying-Ru
authors.initials: YR
authors.lastname: Lo
authors.forename: Marie-Claude
authors.initials: MC
authors.lastname: Boily
mesh_terms: D058872:Epidemics / Q000517:prevention & control*; D015658:HIV Infections / Q000453:epidemiology* / Q000209:etiology / Q000517:prevention & control* / Q000635:transmission; D020010:Heterosexuality / Q000706:statistics & numerical data*; D006801:Humans; D015233:Models, Statistical; D012309:Risk-Taking; D047568:Unsafe Sex / Q000706:statistics & numerical data*