Análisis de los resultados de los diarios de campo

2.1.1 Sexual behaviour

The MSM population in this model was defined as men and transgender women that have reported a sexual partnership with a man in the last 12 months. Characterising sexual behaviour among MSM in a diverse population such as the one of Lima is a complex task. As described in Chapter 1, behaviours and preferences of the subgroups are not always clear cut but follow a gradient with great variation within the population. The main determinants of HIV transmission risk among MSM and transgender women to be considered in a model are: sex distribution of sexual partners (majority of men or women), type of partners within each sex (stable, casual, commercial) and frequency of sex, condom use and sexual positioning with each partner type. Sexual positioning is a determinant of risk at the individual level as insertive and receptive anal sex imply different transmission probabilities but it will also have an impact on transmission at the population level. Versatility in particular can be an efficient means of STI transmission as it conveys both a greater risk of acquiring infection and a high probability of transmitting it [73]. For HIV this can be especially important during the acute phase of infection as the efficient period of transmission is concentrated over a short period [11]. Based on these considerations and balancing information on behaviour and identity to capture the most relevant determinants of transmission in a way that would be meaningful to programmatic activities, four interacting groups of individuals were defined: men who mostly have sex with women (MMSW), men who mostly have sex with men (MMSM), sex workers, and transgender women (including transsexuals and transvestites) at higher risk. These definitions broadly followed the classification commonly used in research studies in Peru[300]. Sex work was defined as the exchange of anal sex for money, drugs, gifts, or favours. The terms “bisexual”, “gay” or “homosexual” were avoided in the definitions of the groups as these can be attributed to a variety of sexual behaviours. Frequency and type of sex with other men was considered more relevant to this analysis, especially since women are not included in the model. Indeed, HIV

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prevalence in Lima among the general population is low, and therefore the risk of transmission to MSM and transgender women from other sources was not included.

The groups were further subdivided into mutually exclusive compartments according to their sexual positioning in anal sex (insertive, receptive, or versatile) resulting in a total of nine groups (Figure 2). Men who mostly have sex with women (MMSW) may sporadically form commercial or casual partnerships with other men; sexual roles for MMSW were restricted to insertive and versatile (which includes both insertive and receptive anal sex acts) as the exclusively receptive role is rarely reported among men who identify themselves as heterosexual. Men who mostly have sex with men represent men who generally self-identify as gay or homosexual. They might be in a stable partnership with another man, and occasionally form commercial partnerships with other men. They can be insertives, receptives or versatiles. Sex workers may have sex with men and women but their main source of risk is to report compensated sex with other men. They form commercial partnerships mainly with MMSW and MMSM, and occasionally with trans. Sex workers can be insertives, receptives or versatiles. Transgender women at higher risk represent a high risk sub-group of the transgender women population. They self-identify as “trans”, have a large number of partners in average, higher prevalence, and a high proportion reports compensated sex. This sub-group was assumed to always play the receptive role during anal sex, as this is the role reported by the large majority of trans [70]. Sero-adaptive behaviours including serosorting (which refers to having unprotected sex only with partners who are believed to have the same HIV serostatus) and strategic positioning (which involves choosing sexual role in function of sero-status or perceived risk) were not considered in the model as, to our knowledge, there is very little information available on the extent to which these practices are used in Peru. However, due to the low prevalence of testing among MSM, it could be expected that these strategies are also uncommon.

Studies describing the proportion of MSM reporting sex with a woman in the last year and/or the proportion of MSM self-identified as heterosexuals or bisexuals (for MMSW), as homosexuals/gay (for MMSM), and as transgender, transvestite or transsexual were reviewed to estimate the distribution of MSM into the different risk groups. The proportion of sex workers is the difference between the total and all the proportions of MMSW, MMSM, and transgender people. This estimate was then compared against reported proportions of MSM being involved in commercial sex during the last year [88,92,98,107,108,276]. Frequency of partnership formation was based on reported number of sexual partners from published studies and unpublished data from ongoing cohorts of transgender women (Trans-Amfar study)[70,301] and MSM (CPOS study)[302]. Stable, casual and commercial partnerships were defined by frequencies of sex acts. For the versatile group, the total

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number of sex acts was divided into a proportion of insertive or receptive acts. Condom use was estimated from reported condom use data at last sex act for different partnership types. If data were available for both the insertive and receptive partners, the probability of using a condom was determined by the receptive partner. Receptive partners have a higher probability of infection than insertive partners and condom use will have a higher impact on the receptive partner.

Figure 2. Model representation of sexual mixing and sexual positioning among MSM and transgender women.

MMSW: men that mostly have sex with women; MMSM: men that mostly have sex with men. Insertive and receptive men always take the insertive and receptive role during anal sex respectively. Versatile men take either the insertive or the receptive role during anal sex. Arrows indicate sexual partnerships being formed between individuals within groups – the width shows the number of partnerships and the direction illustrates sexual positioning (from insertive towards receptive). For each category: insertives will only form partnerships with receptives and versatiles (the latter in a receptive role), versatiles will form partnerships with insertives and both other versatiles and receptives depending on their role per sex act; receptives will only form partnerships with insertives or versatiles (the latter in an insertive role).

2.1.2 Natural history of infection

Following previous work, the natural course of HIV infection was represented as four phases of disease progression defined by duration and infectiousness (Figure 3). When an individual gets infected, he enters a phase of acute infection (short duration, high infectiousness) and progresses to a latent phase (long duration, low infectiousness), before entering a pre-AIDS phase (short duration, high infectiousness). The disease finally progresses to an AIDS phase (short duration, no infectiousness due to an interruption of sexual activity), followed by death [11,303,304,305]. A proportion of infected individuals receive antiretroviral treatment (ART) when their CD4 count is under 200 cells /mm3, and are assumed to stay on treatment for 12 years before they enter the pre- AIDS stage. ART reduces infectiousness and extends survival [306]. Late entry to treatment was assumed on the bases that Peru has not yet changed its guidelines to CD4<350 as recommended by the WHO and on data from HIV patients showing high heterogeneity in CD4 count at start with a mean count of 79 (IQR 32, 164) [307]. The relatively short survival on ART was based on the default value for survival used in spectrum for low and middle income countries which corresponds to 9.5

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years. We used a higher value than recommended by UNAIDS because survival in Latin American countries appeared higher [308] but kept it relatively low because full adherence to treatment and zero drop out were assumed in the model. ART has only been available for free in Peru since 2004 [309] and there is therefore little country-specific information on survival on ART. To our knowledge three studies have reported survival estimates. Corey et al. found high annual survival rates (on the order of 98%) which would result in life expectancy estimates of 55 years [310]. In the study by Muñoz et al. which looked at the effectiveness of an intervention to increase adherence to treatment in resource poor settings, life expectancy was much lower at 6.6 years for the control arm and 19.5 years for the intervention arm [311]. Bern et al. also found very low survival rates (6.6 years) but their study was not designed to study survival on ART [312]. As there is important heterogeneity in results and some point towards very low survival rates, using standard assumptions that are conservative about life expectancy on ART seemed to be the most reasonable option.

Figure 3. Progression of HIV infection

The introduction of ART in the model was simulated through a small coverage starting in 2002 and a linear increase up to a 2007 coverage level of 48%[313]. It then continues to increase and stabilises at 80% coverage, based on data from the UNAIDS epidemiological fact sheet until 2007 and from the UNGASS Country Progress reports thereafter. Figure 4 shows the estimated ART coverage in Peru from 2004.

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Figure 4. ART coverage in Peru.

Coverage estimates were obtained from the UNAIDS Epidemiological fact sheet up to 2007 and from the UNGASS Country progress reports thereafter. A trend line suggests it approximates a linear increase.