The importance of context: model projections on how microbicide impact could be affected by the underlying epidemiologic and behavioral situation in 2 African settings

Assessing the utility of the tipping point ratio to monitor HIV treatment programmes in the era of universal access to ART

BACKGROUND

The epidemiological tipping point ratio (TPR) has been suggested as a useful indicator to monitor the scale-up of antiretroviral treatment (ART) programmes and determine when scale-up is sufficient to control the epidemic. TPR has been defined as the ratio of yearly number of new HIV infections to the yearly number of new ART initiations or to the yearly net increase in the number of people on ART. It has been used to rank the progress of treatment programmes across countries, with the objective of reaching a TPR value under 1. Our study aims to assess if TPR alone can be used as an indicator of ART success across settings by comparing the expected changes in HIV incidence and ART coverage when TPR is maintained constant over time. In particular, we focus on the effect of ART initiation timing (emphasis on ART being initiated early or late during HIV progression) on the interpretation of the TPR.

METHODS

We used a dynamic model of HIV transmission in South Africa representing ART rollout leading to universal treatment in 2017. The model is calibrated to HIV incidence, HIV prevalence and ART coverage in 2012 in South Africa, and 1000 simulations are selected for the base-case scenario. To measure the effect of TPR, we simulate TPR-preserving interventions, maintaining TPR (yearly number of new ART initiations denominator) at the value observed in 2019 (between 0.65 and 1.25) for 15 years. We compare ART coverage and HIV incidence across TPR values and across strategies in which ART access is prioritized differently. In a secondary analysis, we illustrate the sensitivity of new ART initiations to ART retention, and we compare both definitions of the TPR.

RESULTS

Our analysis shows that HIV incidence reduction is weakly correlated to TPR: the same reduction in HIV incidence (15%) can be achieved by implementing the same strategy with a wide range of TPR maintained (0.65-1.12). Assuming high retention in ART, TPR-preserving strategies prioritizing early ART initiation yield greater reduction in HIV incidence than strategies where most individuals initiate ART late. High ART coverage is associated with low HIV incidence and it can be reached with a TPR below or equal to one with strategies favoring early ART initiation. Low ART retention over time results in higher HIV incidence even if TPR is maintained low. If ART retention is low, strategies prioritizing late ART initiation are associated with lower HIV incidence than strategies where ART is initiated early. Maintaining a fixed TPR value based on the net increase in people on ART gives higher HIV incidence reduction and requires fast ART scale-up.

CONCLUSION

Our analysis suggests that the TPR is not an adequate indicator of ART programme impact, without information on ART coverage and retention. Achieving early initiation and adherence to treatment to improve ART coverage might be as important as attaining a specific TPR target. Comparisons of TPR in different settings should account for differences in epidemic conditions.

Modelling the human immunodeficiency virus (HIV) epidemic: A review of the substance and role of models in South Africa

We review key mathematical models of the South African human immunodeficiency virus (HIV) epidemic from the early 1990s onwards. In our descriptions, we sometimes differentiate between the concepts of a model world and its mathematical or computational implementation. The model world is the conceptual realm in which we explicitly declare the rules – usually some simplification of ‘real world’ processes as we understand them. Computing details of informative scenarios in these model worlds is a task requiring specialist knowledge, but all other aspects of the modelling process, from describing the model world to identifying the scenarios and interpreting model outputs, should be understandable to anyone with an interest in the epidemic.

When are declines in condom use while using PrEP a concern? Modelling insights from a Hillbrow, South Africa case study

INTRODUCTION

Oral pre-exposure prophylaxis (PrEP) is a promising new prevention approach for those most at risk of HIV infection. However, there are concerns that behavioural disinhibition, specifically reductions in condom use, might limit PrEP's protective effect. This study uses the case of female sex workers (FSWs) in Johannesburg, South Africa, to assess whether decreased levels of condom use following the introduction of PrEP may limit HIV risk reduction.

METHOD

We developed a static model of HIV risk and compared HIV-risk estimates before and after the introduction of PrEP to determine the maximum tolerated reductions in condom use with regular partners and clients for HIV risk not to change. The model incorporated the effects of increased STI exposure owing to decreased condom use. Noting that condom use with regular partners is generally low, we also estimated the change in condom use tolerated with clients only, to still achieve 50 and 90% risk reduction on PrEP. The model was parameterized using data from Hillbrow, Johannesburg. Sensitivity analyses were performed to ascertain the robustness of our results.

RESULTS

Reductions in condom use could be tolerated by FSWs with lower baseline condom use (65%). For scenarios where 75% PrEP effectiveness is attained, 50% HIV-risk reduction on PrEP would be possible even with 100% reduction in condom use from consistent condom use as high as 70% with clients. Increased exposure to STIs through reductions in condom use had limited effect on the reductions in condom use tolerated for HIV risk not to increase on PrEP.

CONCLUSIONS

PrEP is likely to be of benefit in reducing HIV risk, even if reductions in condom use do occur. Efforts to promote consistent condom use will be critical for FSWs with high initial levels of condom use, but with challenges in adhering to PrEP.

How Much Do We Know about Drug Resistance Due to PrEP Use? Analysis of Experts' Opinion and Its Influence on the Projected Public Health Impact

BACKGROUND

Randomized controlled trials reported that pre-exposure prophylaxis (PrEP) with tenofovir and emtricitabine rarely selects for drug resistance. However, drug resistance due to PrEP is not completely understood. In daily practice, PrEP will not be used under the well-controlled conditions available in the trials, suggesting that widespread use of PrEP can result in increased drug resistance.

METHODS

We surveyed expert virologists with questions about biological assumptions regarding drug resistance due to PrEP use. The influence of these assumptions on the prevalence of drug resistance and the fraction of HIV transmitted resistance was studied with a mathematical model. For comparability, 50% PrEP-coverage of and 90% per-act efficacy of PrEP in preventing HIV acquisition are assumed in all simulations.

RESULTS

Virologists disagreed on the following: the time until resistance emergence (range: 20-180 days) in infected PrEP users with breakthrough HIV infections; the efficacy of PrEP against drug-resistant HIV (25%-90%); and the likelihood of resistance acquisition upon transmission (10%-75%). These differences translate into projections of 0.6%- 1% and 3.5%-6% infected individuals with detectable resistance 10 years after introducing PrEP, assuming 100% and 50% adherence, respectively. The rate of resistance emergence following breakthrough HIV infection and the rate of resistance reversion after PrEP use is discontinued, were the factors identified as most influential on the expected resistance associated with PrEP. Importantly, 17-23% infected individuals could virologically fail treatment as a result of past PrEP use or transmitted resistance to PrEP with moderate adherence.

CONCLUSIONS

There is no broad consensus on quantification of key biological processes that underpin the emergence of PrEP-associated drug resistance. Despite this, the contribution of PrEP use to the prevalence of the detectable drug resistance is expected to be small. However, individuals who become infected despite the use of PrEP should be closely monitored due to higher risk of virological failure when initiating antiretroviral treatment in the future.

Data and methods to characterize the role of sex work and to inform sex work programs in generalized HIV epidemics: evidence to challenge assumptions

In the context of generalized human immunodeficiency virus (HIV) epidemics, there has been limited recent investment in HIV surveillance and prevention programming for key populations including female sex workers. Often implicit in the decision to limit investment in these epidemic settings are assumptions including that commercial sex is not significant to the sustained transmission of HIV, and HIV interventions designed to reach "all segments of society" will reach female sex workers and clients. Emerging empiric and model-based evidence is challenging these assumptions. This article highlights the frameworks and estimates used to characterize the role of sex work in HIV epidemics as well as the relevant empiric data landscape on sex work in generalized HIV epidemics and their strengths and limitations. Traditional approaches to estimate the contribution of sex work to HIV epidemics do not capture the potential for upstream and downstream sexual and vertical HIV transmission. Emerging approaches such as the transmission population attributable fraction from dynamic mathematical models can address this gap. To move forward, the HIV scientific community must begin by replacing assumptions about the epidemiology of generalized HIV epidemics with data and more appropriate methods of estimating the contribution of unprotected sex in the context of sex work.

Pre-exposure prophylaxis in Southern Africa: feasible or not?

INTRODUCTION

Southern and Eastern Africa bear the brunt of the AIDS epidemic, and current prevention interventions remain inadequate. Antiretroviral-based pre-exposure prophylaxis (PrEP) is gaining momentum as an effective prevention intervention.

DISCUSSION

Discussions have been started on how this strategy could be employed in Africa such that the populations most in need can be reached urgently for the greatest impact. This requires the selection of specific risk groups and service environments in which PrEP can be distributed safely and cost effectively while being mindful of any ethical issues.

CONCLUSIONS

Given the need for an integrated public health approach to this, a number of potential populations and opportunities for PrEP distribution exist and are discussed in this commentary.

Potential impact of existing interventions and of antiretroviral use in female sex workers on transmission of HIV in Burkina Faso: a modeling study

BACKGROUND

The impact and cost-effectiveness of antiretroviral treatment (ART) as prevention is likely to vary depending on the local context. Burkina Faso has a concentrated mature HIV epidemic where female sex workers (FSW) are thought to have driven HIV transmission.

METHODS

A dynamic HIV transmission model was developed using data from the Yerelon FSW cohort in Bobo-Dioulasso and population surveys. Compared with current ART provision [status quo (SQ)], the model estimated the proportion of HIV infections averted or incremental life-years gained per additional person-year of ART over 20 years for ART targeting different subgroups or expanding eligibility to all HIV-infected individuals compared with SQ.

RESULTS

Modeling suggests that condom use within commercial sex has averted 40% of past HIV infections. Continuing SQ averts 35%-47% of new infections over 20 years compared with no ART. Expanding ART eligibility to all HIV-infected individuals and increasing recruitment (80% per year) could avert a further 65% of new infections, whereas targeting full-time FSW or all FSWs achieved less impact but was more efficient in terms of life-years gained per 100 person-years of ART. Local HIV elimination is possible with expanded ART provision to FSWs but requires condom use within commercial sex to be maintained at high levels.

CONCLUSIONS

Increasing FSW recruitment onto ART could be a highly efficient method for reducing HIV transmission in concentrated epidemic settings but should not be undertaken at the expense of existing interventions for FSWs. Specialized clinics providing multiple interventions for FSWs should be a fundamental component of prevention in concentrated epidemics.

Preparing for microbicide introduction, rollout, and sustained access

Two topical vaginal microbicide candidates for HIV prevention are at an advanced stage of clinical testing, with efficacy results from three clinical trials expected within the next 2 years. Therefore, preparations for introducing and ensuring access to these products in the event that they are proven safe and effective now require increased attention. Microbicides are expected to fill an important global public health need for HIV prevention options for women. They have been developed almost exclusively with public and private funding through academic and nongovernmental institutions and minimal involvement of commercial pharmaceutical partners. Efficient and rapid introduction of a new public health technology requires a broad range of expertise and collaborations, some of which are new to the microbicide field as products are at last completing late-stage pivotal licensure studies. Strong leadership, political commitment, and considerable financial investments will be required to ensure successful distribution as well as uptake and continued access to this new product class. This paper highlights work conducted since 2000 by scientists, advocates, and public health officials to prepare for microbicide introduction, and discusses some of the needed actions to ensure that products will become readily accessible to the women who need them.

An action agenda for HIV and sex workers

The women, men, and transgender people who sell sex globally have disproportionate risks and burdens of HIV in countries of low, middle, and high income, and in concentrated and generalised epidemic contexts. The greatest HIV burdens continue to be in African female sex workers. Worldwide, sex workers still face reduced access to needed HIV prevention, treatment, and care services. Legal environments, policies, police practices, absence of funding for research and HIV programmes, human rights violations, and stigma and discrimination continue to challenge sex workers' abilities to protect themselves, their families, and their sexual partners from HIV. These realities must change to realise the benefits of advances in HIV prevention and treatment and to achieve global control of the HIV pandemic. Effective combination prevention and treatment approaches are feasible, can be tailored for cultural competence, can be cost-saving, and can help to address the unmet needs of sex workers and their communities in ways that uphold their human rights. To address HIV in sex workers will need sustained community engagement and empowerment, continued research, political will, structural and policy reform, and innovative programmes. But such actions can and must be achieved for sex worker communities everywhere.

Combination HIV prevention for female sex workers: what is the evidence?

Sex work occurs in many forms and sex workers of all genders have been affected by HIV epidemics worldwide. The determinants of HIV risk associated with sex work occur at several levels, including individual biological and behavioural, dyadic and network, and community and social environmental levels. Evidence indicates that effective HIV prevention packages for sex workers should include combinations of biomedical, behavioural, and structural interventions tailored to local contexts, and be led and implemented by sex worker communities. A model simulation based on the South African heterosexual epidemic suggests that condom promotion and distribution programmes in South Africa have already reduced HIV incidence in sex workers and their clients by more than 70%. Under optimistic model assumptions, oral pre-exposure prophylaxis together with test and treat programmes could further reduce HIV incidence in South African sex workers and their clients by up to 40% over a 10-year period. Combining these biomedical approaches with a prevention package, including behavioural and structural components as part of a community-driven approach, will help to reduce HIV infection in sex workers in different settings worldwide.

User-identified gel characteristics: a qualitative exploration of perceived product efficacy of topical vaginal microbicides

Research has demonstrated that certain vaginal gel products--microbicides containing antiretroviral drugs--may reduce HIV infection risk among women. But for vaginal gels to avert HIV and other sexually transmitted infections (STIs), at-risk women must be willing to use them as directed. These products must therefore be "acceptable" to women and an important component of acceptability is users' perception that the product will work to prevent infection. We sought to understand how women's perceptions of vaginal gel properties may shape their understanding of product efficacy for HIV and STI prevention. Sixteen women completed two in-depth qualitative interviews (k = 32) to identify the range and types of sensory perceptions they experienced when using two vaginal gels. We identified emergent themes and linkages between users' sensory perceptions and their beliefs about product efficacy. Users' predictions about product efficacy for preventing infection corresponded to measurable physical properties, including gel volume, location in the vagina, coating behavior, sensation of the gel in the vagina, leakage, and gel changes during coital acts. Although the women described similar sensory experiences (e.g., gel leaked from the vagina), they interpreted these experiences to have varying implications for product efficacy (e.g., leakage was predicted to increase or decrease efficacy). To improve microbicide acceptability, gel developers should investigate and deliberately incorporate properties that influence users' perceptions of efficacy. When a microbicide is approved for use, providers should educate users to anticipate and understand their sensory experiences; improving users' experience can maximize adherence and product effectiveness.

Cost-effectiveness of tenofovir gel in urban South Africa: model projections of HIV impact and threshold product prices

BACKGROUND

There is urgent need for effective HIV prevention methods that women can initiate. The CAPRISA 004 trial showed that a tenofovir-based vaginal microbicide had significant impact on HIV incidence among women. This study uses the trial findings to estimate the population-level impact of the gel on HIV and HSV-2 transmission, and price thresholds at which widespread product introduction would be as cost-effective as male circumcision in urban South Africa.

METHODS

The estimated 'per sex-act' HIV and HSV-2 efficacies were imputed from CAPRISA 004. A dynamic HIV/STI transmission model, parameterised and fitted to Gauteng (HIV prevalence of 16.9% in 2008), South Africa, was used to estimate the impact of gel use over 15 years. Uptake was assumed to increase linearly to 30% over 10 years, with gel use in 72% of sex-acts. Full economic programme and averted HIV treatment costs were modelled. Cost per DALY averted is estimated and a microbicide price that equalises its cost-effectiveness to that of male circumcision is estimated.

RESULTS

Using plausible assumptions about product introduction, we predict that tenofovir gel use could lead to a 12.5% and 4.9% reduction in HIV and HSV-2 incidence respectively, by year 15. Microbicide introduction is predicted to be highly cost-effective (under $300 per DALY averted), though the dose price would need to be just $0.12 to be equally cost-effective as male circumcision. A single dose or highly effective (83% HIV efficacy per sex-act) regimen would allow for more realistic threshold prices ($0.25 and $0.33 per dose, respectively).

CONCLUSIONS

These findings show that an effective coitally-dependent microbicide could reduce HIV incidence by 12.5% in this setting, if current condom use is maintained. For microbicides to be in the range of the most cost-effective HIV prevention interventions, product costs will need to decrease substantially.

Designing preclinical perceptibility measures to evaluate topical vaginal gel formulations: relating user sensory perceptions and experiences to formulation properties

Abstract The effectiveness of any biomedical prevention technology relies on both biological efficacy and behavioral adherence. Microbicide trials have been hampered by low adherence, limiting the ability to draw meaningful conclusions about product effectiveness. Central to this problem may be an inadequate conceptualization of how product properties themselves impact user experience and adherence. Our goal is to expand the current microbicide development framework to include product "perceptibility," the objective measurement of user sensory perceptions (i.e., sensations) and experiences of formulation performance during use. For vaginal gels, a set of biophysical properties, including rheological properties and measures of spreading and retention, may critically impact user experiences. Project LINK sought to characterize the user experience in this regard, and to validate measures of user sensory perceptions and experiences (USPEs) using four prototype topical vaginal gel formulations designed for pericoital use. Perceptibility scales captured a range of USPEs during the product application process (five scales), ambulation after product insertion (six scales), and during sexual activity (eight scales). Comparative statistical analyses provided empirical support for hypothesized relationships between gel properties, spreading performance, and the user experience. Project LINK provides preliminary evidence for the utility of evaluating USPEs, introducing a paradigm shift in the field of microbicide formulation design. We propose that these user sensory perceptions and experiences initiate cognitive processes in users resulting in product choice and willingness-to-use. By understanding the impact of USPEs on that process, formulation development can optimize both drug delivery and adherence.

Analytic review of modeling studies of ARV Based PrEP interventions reveals strong influence of drug-resistance assumptions on the population-level effectiveness

Background

Four clinical trials have shown that oral and topical pre-exposure prophylaxis (PrEP) based on tenofovir may be effective in preventing HIV transmission. The expected reduction in HIV transmission and the projected prevalence of drug resistance due to PrEP use vary significantly across modeling studies as a result of the broad spectrum of assumptions employed. Our goal is to quantify the influence of drug resistance assumptions on the predicted population-level impact of PrEP.

Methods

All modeling studies which evaluate the impact of oral or topical PrEP are reviewed and key assumptions regarding mechanisms of generation and spread of drug-resistant HIV are identified. A dynamic model of the HIV epidemic is developed to assess and compare the impact of oral PrEP using resistance assumptions extracted from published studies. The benefits and risks associated with ten years of PrEP use are evaluated under identical epidemic, behavioral and intervention conditions in terms of cumulative fractions of new HIV infections prevented, resistance prevalence among those infected with HIV, and fractions of infections in which resistance is transmitted.

Results

Published models demonstrate enormous variability in resistance-generating assumptions and uncertainty in parameter values. Depending on which resistance parameterization is used, a resistance prevalence between 2% and 44% may be expected if 50% efficacious oral PrEP is used consistently by 50% of the population over ten years. We estimated that resistance may be responsible for up to a 10% reduction or up to a 30% contribution to the fraction of prevented infections predicted in different studies.

Conclusions

Resistance assumptions used in published studies have a strong influence on the projected impact of PrEP. Modelers and virologists should collaborate toward clarifying the set of resistance assumptions biologically relevant to the PrEP products which are already in use or soon to be added to the arsenal against HIV.

Impact of Pill Sharing on Drug Resistance Due to a Wide-Scale Oral Prep Intervention in Generalized Epidemics

BACKGROUND

The first antiretroviral drug (Truvada) to be used as a pre-exposure prophylaxis (PrEP) in preventing HIV transmission is about to be approved. Behavioral studies suggest that a portion of users may share anti-retroviral drugs with sex partners, family, or friends. Pill sharing will decrease PrEP efficacy and adherence level, and potentially create an environment favorable for the development of drug resistance. We aim to evaluate the potential impact of pill sharing on the PrEP effectiveness and on the rates of drug-resistance development in heterosexual populations.

METHODS

A transmission dynamic model was used to assess the population-level impact of oral PrEP. The fractions of new HIV infections prevented (CPF), drug resistance prevalence and the proportion of new infections in which drug-resistant HIV is transmitted (TDR) are evaluated over fixed time periods. The influence of different factors on CPF and TDR is studied through simulations, using epidemic parameters representative of the countries in Sub-Saharan Africa.

RESULTS

Without pill sharing, a 70% efficacious PrEP used consistently by 60% of uninfected individuals prevents 52.8% (95% CI 49.4%-56.4%) of all new HIV infections over ten years with drug-resistant HIV transmitted in 2.2% of the new infections. Absolute CPF may vary by 9% if up to 20% of the users share PrEP while the level of TDR and total resistance prevalence may increase by up to 6-fold due to pill sharing in some intervention scenarios.

CONCLUSION

Pill sharing may increase the PrEP coverage level achieved in the population but it also affects the PrEP efficacy for the users who do not follow the prescribed schedule. More importantly, it creates a pool of untracked users who remain unreached by the effort to avoid sub-optimal PrEP usage by infected people. This increases substantially the potential risk of drug resistance in the population.

Impact of high-risk sex and focused interventions in heterosexual HIV epidemics: a systematic review of mathematical models

Background

The core-group theory of sexually transmitted infections suggests that targeting prevention to high-risk groups (HRG) could be very effective. We aimed to quantify the contribution of heterosexual HRGs and the potential impact of focused interventions to HIV transmission in the wider community.

Methods

We systematically identified studies published between 1980 and 2011. Studies were included if they used dynamical models of heterosexual HIV transmission, incorporated behavioural heterogeneity in risk, and provided at least one of the following primary estimates in the wider community (a) the population attributable fraction (PAF) of HIV infections due to HRGs, or (b) the number per capita or fraction of HIV infections averted, or change in HIV prevalence/incidence due to focused interventions.

Findings

Of 267 selected articles, 22 were included. Four studies measured the PAF, and 20 studies measured intervention impact across 265 scenarios. In low-prevalence epidemics (≤5% HIV prevalence), the estimated impact of sex-worker interventions in the absence of risk compensation included: 6–100% infections averted; 0.9–6.2 HIV infections averted per 100,000 adults; 11–94% and 4–47% relative reduction in prevalence and incidence respectively. In high-prevalence epidemics (>5% HIV prevalence), sex-worker interventions were estimated to avert 6.8–40% of HIV infections and up to 564 HIV infections per 100,000 adults, and reduce HIV prevalence and incidence by 13–27% and 2–14% respectively. In both types of epidemics, greater heterogeneity in HIV risk was associated with a larger impact on the fraction of HIV infections averted and relative reduction in HIV incidence.

Conclusion

Focused interventions, as estimated by mathematical models, have the potential to reduce HIV transmission in the wider community across low- and high-prevalence regions. However, considerable variability exists in estimated impact, suggesting that a targeted approach to HIV prevention should be tailored to local epidemiological context.

How effective is effective enough? Opinions of potential end-users of microbicides from a rural South African community

To assess the views of potential end-users of a microbicide in KwaZulu-Natal regarding the characteristics that would justify further development, three focus group discussions were conducted in 2009 with 23 local staff members working on a microbicide clinical trial, 20 former trial participants and 14 Community Advisory Board members not enrolled in the trial, in an area with high HIV incidence and low consistent condom use. All participants agreed on the need for additional HIV prevention options that are as effective as possible and can be used by women. The majority of respondents stated that even a highly acceptable HIV prevention option with protection as low as 30% would still be an important addition to condoms for women; that a partially protective microbicide would have to be introduced as part of the existing prevention messages in order to continue promoting condom use; that there should eventually be a choice between antiretroviral (ARV) and non-ARV-based microbicides and a choice of how and where to access microbicides. Respondents also felt it would be important to make plans for access to a microbicide that can offer protection, even if partial, rather than wait to find out if alternative microbicides are equally or more effective. Potential end-users in a high HIV prevalence area believe that a partially effective microbicide would be an important addition to the limited HIV prevention options for women. The significant challenges of introducing a partially protective HIV prevention option were recognised, but seen as ones worth facing, as well as an opportunity to lay the ground work for the introduction of more efficacious HIV prevention methods in the future.

HIV treatment as prevention: optimising the impact of expanded HIV treatment programmes

Until now, decisions about how to allocate ART have largely been based on maximising the therapeutic benefit of ART for patients. Since the results of the HPTN 052 study showed efficacy of antiretroviral therapy (ART) in preventing HIV transmission, there has been increased interest in the benefits of ART not only as treatment, but also in prevention. Resources for expanding ART in the short term may be limited, so the question is how to generate the most prevention benefit from realistic potential increases in the availability of ART. Although not a formal systematic review, here we review different ways in which access to ART could be expanded by prioritising access to particular groups based on clinical or behavioural factors. For each group we consider (i) the clinical and epidemiological benefits, (ii) the potential feasibility, acceptability, and equity, and (iii) the affordability and cost-effectiveness of prioritising ART access for that group. In re-evaluating the allocation of ART in light of the new data about ART preventing transmission, the goal should be to create policies that maximise epidemiological and clinical benefit while still being feasible, affordable, acceptable, and equitable.

HIV treatment as prevention: considerations in the design, conduct, and analysis of cluster randomized controlled trials of combination HIV prevention

The rigorous evaluation of the impact of combination HIV prevention packages at the population level will be critical for the future of HIV prevention. In this review, we discuss important considerations for the design and interpretation of cluster randomized controlled trials (C-RCTs) of combination prevention interventions. We focus on three large C-RCTs that will start soon and are designed to test the hypothesis that combination prevention packages, including expanded access to antiretroviral therapy, can substantially reduce HIV incidence. Using a general framework to integrate mathematical modelling analysis into the design, conduct, and analysis of C-RCTs will complement traditional statistical analyses and strengthen the evaluation of the interventions. Importantly, even with combination interventions, it may be challenging to substantially reduce HIV incidence over the 2- to 3-y duration of a C-RCT, unless interventions are scaled up rapidly and key populations are reached. Thus, we propose the innovative use of mathematical modelling to conduct interim analyses, when interim HIV incidence data are not available, to allow the ongoing trials to be modified or adapted to reduce the likelihood of inconclusive outcomes. The preplanned, interactive use of mathematical models during C-RCTs will also provide a valuable opportunity to validate and refine model projections.

Modeling the gender-specific impact of vaginal microbicides on HIV transmission

Vaginal microbicides (VMB) are currently among the few women-initiated biomedical interventions for preventing heterosexual transmission of HIV. In this paper we use a deterministic model of HIV transmission to assess the public-health benefits of a VMB intervention and evaluate its gender-specific impact over short (initial) and extended periods of time. We define two distinct quantitative benefit ratios (QBRs) based on infections prevented in men and women to create and study regions of male advantage in different parameter spaces. Our analysis exposes complicated temporal correlations between the QBRs and series of pre-intervention (e.g., HIV acquisition risks per act) and intervention parameters (e.g., VMB efficacy mechanisms, rates of resistance development and reversion) and indicates that different QBRs may often disagree on the gender distribution of the benefits from a VMB intervention. We also outline the strong influence of some modeling assumptions on the reported results and conclude that the assessment of VMB and other biomedical interventions must be based on more comprehensive analyses than calculations of infections prevented over a fixed period of time.

What do mathematical models tell us about the emergence and spread of drug-resistant HIV?

PURPOSE OF REVIEW

To discuss recent HIV epidemic models examining the transmission of antiretroviral (ARV) drug resistance.

RECENT FINDINGS

A relatively small number of recent transmission models have investigated ARV resistance in the context of therapeutic, combined ART (cART); ARV-vaginal microbicides (ARV-VMB); and oral pre-exposure prophylaxis (PrEP). Models of cART use have highlighted potential concerns about future resistance transmission, particularly in resource-constrained settings, and have emphasized the benefits of viral load monitoring in limiting resistance spread. PrEP models have concluded that inadvertent use by HIV-infected individuals could increase resistance prevalence, and that risk compensation by PrEP users could limit their beneficial effects on HIV transmission. ARV-VMB models have demonstrated that whereas resistance can reduce prophylactic effectiveness in preventing HIV acquisition of female ARV-VMB users, it may concomitantly benefit users' male partners if the resistant strains that female users acquire are less transmissible than wild-type strains. The models have examined the balance between these two factors at the population level.

SUMMARY

Recent HIV transmission models have adopted a wide assortment of structures and assumptions to explore drug resistance in the context of different ARV interventions in various settings. There is a need for future work emphasizing the simultaneous effects of multiple ARV interventions, as well as the public health impact of resistance, not just its prevalence.

The role of the natural epidemic dynamics and migration in explaining the course of the HIV epidemic in rural Uganda: a modelling study

BACKGROUND

Human immunodeficiency virus (HIV) prevalence in Uganda fell during the 1990s and has risen since 2000. The changing trends since 2000 may be due to changing risky sexual behaviour, but other factors may also contribute. We explore the possible impact that two factors may have on cohort trends: natural epidemic dynamics and local migration. We simultaneously fit a mathematical model to the contrasting prevalence trends by age/gender in a southwest Ugandan cohort, which has never been done before.

METHODS

We fit a model to HIV trends, by age groups and gender, assuming: (i) neither migration nor intentional behaviour change, (ii) migration changes, (iii) intentional behaviour changes and (iv) both change. The model fits were assessed through sum of squares goodness of fits. We also explored the impact of the natural course of the epidemic on average partner acquisition rates, under the condition of no intentional behaviour change over time.

RESULTS

Without allowing intentional change in partner acquisition rates, an excellent fit was achievable to prevalence trends by gender. However, all of the contrasting trends by age/gender could not be replicated simultaneously. Adding intentional behaviour change improved the fit to some groups, but not all. Adding both intentional behaviour change and migration further improved the fit.

CONCLUSIONS

While some of the increasing HIV prevalence in Uganda since 2000 may be due to increased risky behaviour, some of the observed epidemiologic trends would likely occur without any intentional change in behaviour. Average population-level behaviour can change due to preferential mortality among higher risk takers, without individuals changing their behaviour.

Microbicidas Metodo de Prevencion en VIH/SIDA Controlado por Mujeres

HIV/AIDS continues to expand throughout the world, and in recent times has had a severe impact on the number of women living with HIV. Access to methods of prevention for HIV/AIDS around the world is limited and for many women may not be feasible. Examples of such methods are sexual abstinence, which though recognized as an effective method of prevention is difficult for many women to achieve. Mutual fidelity is another option, but many women cannot control the sexual behavior of their partners. Finally, the male condom continues to be an available method of HIV prevention for sexually active people. However, in many places, gender inequality, social norms, and economic disparities severely limit women's capacity to negotiate the use of a condom with their partner. For this reason, an urgent need exists to develop a product for the prevention of HIV/AIDS that can be handled by women. Microbicides, products that can reduce HIV risk when applied intravaginally, hold promise for stopping the advance of HIV/AIDS, especially when considering that women can make their own decisions about whether to use them when other methods of prevention are not available. The objective of the current literature review was to understand the scientific advances related to microbicides in the prevention of HIV/AIDS in women through the analysis of available literature in this area. Interest in this topic emerged from the need to contribute to women's health and HIV/AIDS prevention. To achieve this review's objective, a search was carried out in multiple databases, including OVID, PUBMED, PROQUEST, and CINAHL, as well as published materials from organizations related to this area such as UNAIDS and CONASIDA, and literature available from the Internet. Upon completion of the literature review, it was concluded that microbicides are an effective method for preventing HIV/AIDS for women as well as their partners. Although they have a high level of acceptance among the female population, the majority of microbicides are still in clinical studies, requiring further evaluation for their safe use in humans. There are 3 microbicides in the final phases of clinical studies that will soon be available on the market. One of these was found to be 60% effective in protecting against the transmission of HIV and was used by women in 50% of their sexual activity, which resulted in avoiding approximately 2.5 million new HIV infections in men, women, and children over a 3-year period.

Evaluation design for large-scale HIV prevention programmes: the case of Avahan, the India AIDS initiative

BACKGROUND

Closing the HIV prevention gap to prevent HIV infections requires rapid, worldwide rollout of large-scale national programmes. Evaluating such programmes is challenging and complex, requiring clarity of evaluation purpose and evidential approaches substantively different to those employed for pilots and small programmes.

OBJECTIVES

This paper describes the evaluation design for the implementation phase of Avahan, the India AIDS initiative, a large HIV prevention programme funded by the Bill and Melinda Gates Foundation. Avahan, which began in December 2003, has a 10-year charter to impact the Indian epidemic and its response by implementing an HIV prevention programme targeting core and bridge groups in 83 districts of six Indian states, transferring the programme to the Government of India, and disseminating programme learning.

METHODS

The foundation commissioned an external process to design Avahan's evaluation framework. An independent advisory group oversees and guides course corrections in the execution of this framework.

RESULTS

Avahan's evaluation framework comprises: trend and synthetic analysis of data from core, bridge and household biobehavioural surveys in a subset of intervention districts, denominator estimates and programme monitoring from all intervention districts, and government's antenatal surveillance (two sites per district in all districts); bespoke transmission dynamics modelling to estimate infections averted (subset of districts); cost effectiveness studies (subset of districts). In addition, there are other knowledge-building and quality-monitoring activities.

CONCLUSION

Rather than a small set of monofocal outcome measures, scaled programmes require nuanced evaluations that approximate programmatic scale by collecting data with different levels of geographical scope, synthesize multiple data and methods to arrive at a composite picture, and can cope with continuous environmental and programme evolution.

Estimating the number of HIV infections averted: an approach and its issues

Objective:

To propose a methodology to estimate the number of new HIV infections averted. Knowledge of HIV infection has increased tremendously and modelling tools to project current epidemics into the future have greatly improved. Different types of models can be used to estimate HIV infections averted, although the number of new HIV infections averted cannot be measured directly.

Method:

Using cohort-component population projections, a disease modelling-based approach was used to compare the observed epidemiology of a disease after programme initiation with an expected epidemiology from past trends before programme initiation. The concept of modelling infections averted in a disease modelling-based approach involves a comparison between an “expected” or baseline epidemic with an “estimated” one. A hypothetical example was featured in order to demonstrate the proposed methodology. Using both the Estimation and Projection Package (EPP) and the Spectrum demographic modelling program, the underlying annual incidence levels implied by both the baseline and estimated epidemics were examined.

Results:

The difference between baseline and estimated incidence levels is interpreted as “infections averted”. Strengths and limitations of the approach are discussed.

Conclusions:

In this study an expected epidemiological approach was compared to one based on observation. Once sufficient data become available, the validation of various country data including HIV prevalence, mortality, and behaviour must be done. Additional information related to behaviour change may be critical to further support arguments for a change in disease trend. It is therefore important to use all available data, consequently strengthening findings from a disease modelling-based approach on HIV infections averted.

Modelling the cost effectiveness of rapid point of care diagnostic tests for the control of HIV and other sexually transmitted infections among female sex workers

BACKGROUND

In sub-Saharan Africa, gonococcal and chlamydial infections are usually managed using the syndromic approach. However, many infections are asymptomatic in women, and the syndromic algorithm has poor sensitivity and specificity for infections caused by Neisseria gonorrhoeae (Ng) and Chlamydia trachomatis (Ct). Because of this, rapid point of care (POC) tests for Ct/Ng could improve sexually transmitted infection (STI) management in women. This study uses mathematical modelling to estimate the incremental cost effectiveness of using POC tests to diagnose Ng/Ct instead of the current syndromic approach used by the SIDA2 HIV/STI prevention project for female sex workers in Cotonou, Benin.

METHODS

A dynamic mathematical model was used with data from Cotonou to estimate the HIV impact of the existing SIDA2 project (1995-8), and to project how impact would change if POC tests had been used. As observed in test evaluations, the POC tests were assumed to have high specificity, but a range of sensitivities. The incremental economic cost effectiveness of using POC tests was modelled using data on intervention costs and an evaluation of an Ng POC test in Cotonou in 2004. All costs were in 2004 US dollars.

RESULTS

The model estimated the STI treatment aspect of the intervention averted 18 553 Ng/Ct and 359 HIV infections over 4 years when the syndromic approach was used. In contrast, if Ng/Ct had been diagnosed with a 70-80% sensitive and 95% specific POC test then 24-31% fewer clinic attenders would have been treated, 40-60% more Ng/Ct and HIV infections would have been averted, and the incremental cost effectiveness of using them would have been 107-151 dollars per HIV infection averted if the POC tests cost 2 dollars and 58-81 dollars if they cost 1 dollar.

CONCLUSIONS

POC tests can be a cost effective strategy for substantially increasing the impact on HIV transmission, and decreasing the degree of inappropriate treatment of STI treatment interventions that use syndromic management to diagnose Ng/Ct.