title: High incidence is not high exposure: what proportion of prevention trial participants are exposed to HIV?
abstract: OBJECTIVE
  Randomized clinical trials of HIV prevention in high-risk populations of women often assume that all participants have similar exposure to HIV. However, a substantial fraction of women enrolled in the trial may have no or low exposure to HIV. Our objective was to estimate the proportion of women exposed to HIV throughout a hypothetical high-risk study population.
  
  
  METHODS
  A stochastic individual-based model was developed to simulate the sexual behavior and the risk of HIV acquisition for a cohort of sexually active HIV-uninfected women in high HIV prevalence settings. Key behavior and epidemic assumptions in the model were based on published studies on HIV transmission in South Africa. The prevalence of exposure, defined as the proportion of women who have sex with HIV-infected partner, and HIV incidence were evaluated.
  
  
  RESULTS
  Our model projects that in communities with HIV incidence rate of 1 per 100 person years, only 5-6% of women are exposed to HIV annually while in communities with an HIV incidence of 5 per 100 person years 20-25% of women are exposed to HIV. Approximately 70% of the new infections are acquired from partners with asymptomatic HIV.
  
  
  CONCLUSIONS
  Mathematical models suggest that a high proportion of women enrolled in HIV prevention trials may be unexposed to HIV even when incidence rates are high. The relationship between HIV exposure and other risk factors should be carefully analyzed when future clinical trials are planned.
authors.affiliation: Vaccine and Infectious Disease and Public Health Sciences Divisions, Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America; Department of Applied Mathematics, University of Washington, Seattle, Washington, United States of America.
authors.forename: Dobromir
authors.initials: D
authors.lastname: Dimitrov
authors.affiliation: Vaccine and Infectious Disease and Public Health Sciences Divisions, Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America; Department of Global Health, University of Washington, Seattle, Washington, United States of America.
authors.forename: Deborah
authors.initials: D
authors.lastname: Donnell
authors.affiliation: Vaccine and Infectious Disease and Public Health Sciences Divisions, Fred Hutchinson Cancer Research Center, Seattle, Washington, United States of America; Department of Biostatistics, University of Washington, Seattle, Washington, United States of America.
authors.forename: Elizabeth R
authors.initials: ER
authors.lastname: Brown
mesh_terms: D015331:Cohort Studies; D005260:Female; D015658:HIV Infections / Q000453:epidemiology* / Q000635:transmission; D006801:Humans; D015994:Incidence; D008297:Male; D008962:Models, Theoretical; D012307:Risk Factors; D012725:Sexual Behavior; D012747:Sexual Partners; D013997:Time Factors