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How much could a microbicide’s sexually transmitted infection efficacy contribute to reducing HIV risk and the level of condom use needed to lower risk? Model estimates
  1. A M Foss1,
  2. P T Vickerman1,
  3. M Alary2,
  4. C H Watts1
  1. 1
    London School of Hygiene and Tropical Medicine, London, UK
  2. 2
    Centre Hospitalier affilié Universitaire de Québec, Québec, Canada
  1. Dr A Foss, Department of Public Health and Policy, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK; anna.foss{at}


Objective: This study explores the potential contribution of a microbicide’s sexually transmitted infection (STI) efficacy in reducing a female sex worker’s (FSW) risk of STI and HIV infection. The study then investigates whether the threshold for the reduction in condom use following microbicide introduction that can be tolerated without increasing HIV risk is affected by STI efficacy.

Methods: A dynamic model describing the transmission of a bacterial STI between FSW and their clients was coupled with a static HIV model. The model uses data from Cotonou, Benin (1998–9), for illustration, to estimate the change in risk following the introduction of 50% HIV efficacious microbicides of different STI efficacies, used in 50% of sex acts when a condom is not used. The condom migration thresholds were estimated. The degree to which the findings are influenced by STI prevalence was explored.

Results: For highly transmissible STI, there is a non-monotonic relationship between STI prevalence and microbicide impact on HIV with the relative reduction in HIV risk first increasing, due to the proportion of HIV risk attributable to the STI increasing, but then decreasing at high prevalences as the STI becomes harder to control. A less transmissible STI can still be impacted upon with a moderate/high STI efficacy microbicide even at high STI prevalences. This relationship is also reflected in the condom migration thresholds.

Conclusions: A microbicide’s STI efficacy may have a substantial impact on STI and HIV incidence among high-risk groups. The variation in the condom migration thresholds for different STI efficacies and STI prevalences may be difficult to measure accurately.

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  • Funding: The microbicide work is funded by the Global Campaign for Microbicides at the Program for Appropriate Technology in Health (PATH), via a grant from USAID, and by the UK Department for International Development (DFID) through the Microbicide Development Programme and the Research Programme Consortium for Research and Capacity Building in Sexual and Reproductive Health and HIV in Developing Countries.

  • Competing interests: None.

  • Contributors: AMF took the lead in the analysis and made substantial contributions to the conception, design of the analysis and interpretation of the results presented. PTV and CHW contributed to the conception and design of the analysis, and helped to interpret the findings. PTV and CHW provided substantial input into several drafts of the paper. MA helped with interpreting the data from Cotonou and commented on drafts of the paper. AMF, PTV and CHW were also members of the Health Economics and Financing Programme and the Knowledge Programme on HIV/AIDS and STI, which were funded by DFID. MA is a national researcher of the Fonds de la Recherché en Santé du Québec (grant no 8722).