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Remodelling core group theory: the role of sustaining populations in HIV transmission
  1. Charlotte Watts,
  2. Cathy Zimmerman,
  3. Anna M Foss,
  4. Mazeda Hossain,
  5. Andrew Cox,
  6. Peter Vickerman
  1. Department of Global Health and Development, Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine (LSHTM), London, UK
  1. Correspondence to Dr Charlotte Watts, Department of Global Health and Development, Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine (LSHTM), 15-17 Tavistock Place, London, UK; charlotte.watts{at}


Background and objectives 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.

Conclusion 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.

  • Core group
  • epidemiological theory
  • HIV
  • STI
  • commercial sex
  • sustaining populations
  • mobility
  • prostitution
  • sexual behaviour

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  • Funding Support for this research was provided by the AIDS, Security & Conflict Initiative, convened by the Netherlands Institute of International Relations ‘Clingendael’ and the Social Science Research Council. Partial funding for this analysis also came from the Sigrid Rausing Trust.

  • Competing interests None.

  • Provenance and peer review Not commissioned; externally peer reviewed.