Who mixes with whom among men who have sex with men? Implications for modelling the HIV epidemic in southern India.

In India, the identity of men who have sex with men (MSM) is closely related to the role taken in anal sex (insertive, receptive or both), but little is known about sexual mixing between identity groups. Both role segregation (taking only the insertive or receptive role) and the extent of assortative (within-group) mixing are known to affect HIV epidemic size in other settings and populations. This study explores how different possible mixing scenarios, consistent with behavioural data collected in Bangalore, south India, affect both the HIV epidemic, and the impact of a targeted intervention. Deterministic models describing HIV transmission between three MSM identity groups (mostly insertive Panthis/Bisexuals, mostly receptive Kothis/Hijras and versatile Double Deckers), were parameterised with behavioural data from Bangalore. We extended previous models of MSM role segregation to allow each of the identity groups to have both insertive and receptive acts, in differing ratios, in line with field data. The models were used to explore four different mixing scenarios ranging from assortative (maximising within-group mixing) to disassortative (minimising within-group mixing). A simple model was used to obtain insights into the relationship between the degree of within-group mixing, R0 and equilibrium HIV prevalence under different mixing scenarios. A more complex, extended version of the model was used to compare the predicted HIV prevalence trends and impact of an HIV intervention when fitted to data from Bangalore. With the simple model, mixing scenarios with increased amounts of assortative (within-group) mixing tended to give rise to a higher R0 and increased the likelihood that an epidemic would occur. When the complex model was fit to HIV prevalence data, large differences in the level of assortative mixing were seen between the fits identified using different mixing scenarios, but little difference was projected in future HIV prevalence trends. An oral pre-exposure prophylaxis (PrEP) intervention was modelled, targeted at the different identity groups. For intervention strategies targeting the receptive or receptive and versatile MSM together, the overall impact was very similar for different mixing patterns. However, for PrEP scenarios targeting insertive or versatile MSM alone, the overall impact varied considerably for different mixing scenarios; more impact was achieved with greater levels of disassortative mixing.

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.

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.

A systematic review of published evidence on intervention impact on condom use in sub-Saharan Africa and Asia.

OBJECTIVE: There has been much debate about the value of condoms in HIV/STI programming. This should be informed by evidence about intervention impact on condom use, but there is limited compiled literature. This review aims to quantify intervention impact on condom use in sub-Saharan Africa and Asia, in different types of partnership. METHODS: A systematic review was conducted of papers published between 1998 and 2006 presenting evaluations of interventions involving condom promotion in sub-Saharan Africa and Asia. Data on reported postintervention levels of condom use, and various measures of changes in condom use, by partnership type, were compiled. RESULTS: A total of 1374 abstracts were identified. Sixty-two met the inclusion criteria (42 reporting significant increases in condom use): 44 from sub-Saharan Africa and 18 from Asia. Many (19) reported on condom use in commercial sex (15 significant), six on use with casual partners (three significant), 11 on use in marital/steady partnerships (nine significant), 14 on use by youths (eight significant) and 20 combined partnership types (11 significant). There is substantial evidence of interventions targeted at sex workers and clients achieving large increases in condom use. Far less evidence exists of intervention impact on condom use in casual relationships. In primary partnerships, postintervention condom use was low unless one partner was knowingly HIV-infected or at high-risk, or avoiding pregnancy. Evaluations of interventions targeting youths recorded limited increases in condom use. CONCLUSIONS: The findings illustrate the range of evidence about postintervention condom use in different partnerships, and how patterns of use are influenced by partnership type and perceptions of risk. Where possible, intervention studies should also assess biological endpoints, since prevention of infection is the measure of most interest in the evaluation of condom promotion interventions.

What is the achievable effectiveness of the India AIDS Initiative intervention among female sex workers under target coverage? Model projections from southern India.

BACKGROUND: The India AIDS Initiative (Avahan) prevention programme funded by the Bill and Melinda Gates Foundation aims to reduce HIV prevalence in high risk groups such as female and male sex workers and their clients, to limit HIV transmission in the general population. OBJECTIVES: To assess the potential effectiveness of the Avahan intervention at the level of coverage targeted, in different epidemiological settings in India. METHODS: A deterministic compartmental model of the transmission dynamics of HIV and two sexually transmitted infections, and sensitivity analysis techniques, were used, in combination with available behavioural and epidemiological data from Mysore and Bagalkot districts in the Indian state of Karnataka, to evaluate the syndromic sexually transmitted infection (STI) management (STI treatment), periodic presumptive treatment of STI (PPT), and condom components of the Avahan intervention targeted to female sex workers (FSW). RESULTS: If all components of the intervention reach target coverage (that is, PPT, STI treatment and condom use), the intervention is expected to prevent 22-35% of all new HIV infections in FSW and in the total population over 5 years in a low transmission setting like Mysore, and to be half as effective in high transmission settings such as Bagalkot. The results were sensitive to small variations in intervention coverage. The condom component alone is expected to prevent around 20% of all new HIV infections over 5 years in Mysore and around 6% for the STI component alone; compared with 7%-14% for the PPT component alone. Multivariate sensitivity analyses suggested that interventions may be more effective in settings with low FSW HIV prevalence and small FSW populations, whereas HIV prevalence was most influenced by sexual behaviour and condom use parameters for FSW. CONCLUSION: The Avahan intervention is expected to be effective. However, to be able to demonstrate effectiveness empirically in the different settings, it is important to achieve target coverage or higher, which in the case of PPT could take a number of years to achieve. These preliminary model predictions need to be validated with more detailed mathematical models, as better data on sexual behaviour, condom use, STI and HIV trends over time, and intervention coverage data accumulate over the course of the programme.