title: Remodelling core group theory: the role of sustaining populations in HIV transmission.
abstract: OBJECTIVE
  Core group theory describes the central role of groups with high rates of sexual partner change in HIV transmission. Research illustrates the heterogeneous and dynamic nature of commercial sex, and that some men involved in the organisation or policing of sex work regularly have sex with sex workers. These findings are used to explore gaps in core group theory.
  
  
  METHODS
  Evidence from developing countries on the duration that women sell and men buy sex was reviewed. Simple compartmental dynamic models were used to derive analytical expressions for the relative HIV equilibrium levels among sex workers and partners, incorporating partner change rates and duration in commercial sex settings. Simulations explored the degree to which HIV infection can be attributable to men with low partner change rates who remain in sex work settings for long periods, and their influence on the impact of HIV intervention.
  
  
  RESULTS
  Partner change rates and duration of time in a setting determine equilibrium HIV levels. Modelling projections suggest that men with low mobility can substantially contribute to HIV prevalence among sex workers, especially in settings with prevalences <50%. This effect may reduce the impact of sex-worker interventions on HIV incidence in certain scenarios by one-third. Reductions in impact diminish at higher sex-worker prevalences.
  
  
  CONCLUSIONS
  In commercial sex settings, patterns of HIV risk and transmission are influenced by both partner change rates and duration in a setting. The latter is not reflected in classic core group theory. Men who control the sex industry and regular clients may form an important 'sustaining population' that increases infection and undermines the impact of intervention. Intervention activities should include these groups, and examine the social organisation of sex work that underpins many of these relationships.
authors.affiliation: Department of Global Health and Development, Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine, 15-17 Tavistock Place, London, UK. charlotte.watts@lshtm.ac.uk
authors.forename: Charlotte
authors.initials: C
authors.lastname: Watts
authors.forename: Cathy
authors.initials: C
authors.lastname: Zimmerman
authors.forename: Anna M
authors.initials: AM
authors.lastname: Foss
authors.forename: Mazeda
authors.initials: M
authors.lastname: Hossain
authors.forename: Andrew
authors.initials: A
authors.lastname: Cox
authors.forename: Peter
authors.initials: P
authors.lastname: Vickerman
mesh_terms: D000349:Africa / Q000453:epidemiology; D001208:Asia / Q000453:epidemiology; D017280:Condoms / Q000706:statistics & numerical data; D005260:Female; D015658:HIV Infections / Q000453:epidemiology / Q000523:psychology / Q000635:transmission*; D006801:Humans; D008297:Male; D015995:Prevalence; D011477:Sex Work / Q000523:psychology / Q000706:statistics & numerical data*; D012747:Sexual Partners; D013997:Time Factors; D047568:Unsafe Sex / Q000523:psychology / Q000706:statistics & numerical data