Using mathematical modelling to estimate the impact of periodic presumptive treatment on the transmission of sexually transmitted infections and HIV among female sex workers
- Peter Vickerman1,
- Francis Ndowa2,
- Nigel O'Farrell3,
- Richard Steen2,
- Michel Alary4,
- Sinead Delany-Moretlwe5
- 1Health Policy Unit, London School of Hygiene and Tropical Medicine, London, UK
- 2World Health Organization, Geneva, Switzerland
- 3Pasteur Suite, Ealing Hospital, London, UK
- 4Population Health Research Unit, Centre Hospitalier Affilié Universitaire de Québec, and Laval University, Quebec City, Canada
- 5Reproductive Health and HIV Research Unit, Johannesburg, South Africa
- Correspondence to Peter Vickerman, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK;
Contributors PV and FN developed the research. All model analyses were undertaken by PV and the manuscript was drafted by PV. All other authors supplied data for the analysis, guided and commented on the model analyses and helped write the manuscript.
- Accepted 4 October 2009
- Published Online First 22 October 2009
Background In settings with poor sexually transmitted infection (STI) control in high-risk groups, periodic presumptive treatment (PPT) can quickly reduce the prevalence of genital ulcers, Neisseria gonorrhoeae (NG) and Chlamydia trachomatis (CT). However, few studies have assessed the impact on HIV. Mathematical modelling is used to quantify the likely HIV impact of different PPT interventions.
Methods A mathematical model was developed to project the impact of PPT on STI/HIV transmission amongst a homogeneous population of female sex workers (FSWs) and their clients. Using data from Johannesburg, the impact of PPT interventions with different coverages and PPT frequencies was estimated. A sensitivity analysis explored how the projections were affected by different model parameters or if the intervention was undertaken elsewhere.
Results Substantial decreases in NG/CT prevalence are achieved among FSWs receiving PPT. Although less impact is achieved among all FSWs, large decreases in NG/CT prevalence (>50%) are possible with >30% coverage and supplying PPT every month. Higher PPT frequencies achieve little additional impact, whereas improving coverage increases impact until NG/CT becomes negligible. The impact on HIV incidence is smaller, longer to achieve, and depends heavily on the assumed NG/CT cofactors, whether they are additive, the assumed STI/HIV transmission probabilities and STI durations. Greater HIV impact can be achieved in settings with lower sexual activity (except at high coverage), less STI treatment or high prevalences of Haemophilus ducreyi.
Conclusions Despite the model's assumption of homogeneous risk behaviour probably resulting in optimistic projections, and uncertainty in STI cofactors and transmission probabilities, projections suggest PPT interventions with sufficient coverage (≥40%) and follow-up (≥2 years) could noticeably decrease the HIV incidence (>20%) among FSW populations with inadequate STI treatment.
Linked articles 40329.
Funding The analysis was funded by the World Health Organization (WHO). Peter Vickerman also received funding from the UK Department for International Development (DFID) funded Research Programme Consortium for Research and Capacity Building in Sexual and Reproductive Health and HIV in Developing Countries. The views expressed are those of the authors and cannot be taken to reflect the official opinion of the London School of Hygiene and Tropical Medicine, DFID or WHO.
Competing interests None.
Patient consent Obtained.
Provenance and peer review Not commissioned; externally peer reviewed.