The contribution of steady and casual partnerships to the incidence of HIV infection among homosexual men in Amsterdam

Is concurrency driving HIV transmission in sub-Saharan African sexual networks? The significance of sexual partnership typology

Recently, there has been debate about the role of concurrent partnerships in driving the transmission of HIV, particularly in Southern Africa, where HIV prevalence is up to 25 % in many heterosexual populations and where evidence from sexual behavior surveys also suggests high levels of male concurrency. While mathematical modeling studies have shown that concurrency has the potential to enhance the speed at which HIV spreads in a population, empirical studies up to now have failed to provide conclusive evidence supportive of these effects. Here we discuss some reasons for the apparent discrepancy between theoretical and empirical studies. We propose that studying the impact of concurrency on HIV transmission should be differentiated by taking more insight from social and behavioral studies on sexual partnerships into account. We also suggest that a more rigorous definition is needed for when a factor is considered a driving force for HIV epidemic spread. We illustrate this with a modeling example.

Antiviral agents and HIV prevention: controversies, conflicts, and consensus

Antiviral agents can be used to prevent HIV transmission before exposure as preexposure prophylaxis (PrEP), after exposure as postexposure prophylaxis, and as treatment of infected people for secondary prevention. Considerable research has shed new light on antiviral agents for PrEP and for prevention of secondary HIV transmission. While promising results have emerged from several PrEP trials, the challenges of poor adherence among HIV-negative clients and possible increase in sexual risk behaviors remain a concern. In addition, a broader pipeline of antiviral agents for PrEP that focuses on genital tract pharmacology and safety and resistance issues must be developed. Antiretroviral drugs have also been used to prevent HIV transmission from HIV-infected patients to their HIV-discordant sexual partners. The HIV Prevention Trials Network 052 trial demonstrated nearly complete prevention of HIV transmission by early treatment of infection, but the generalizability of the results to other risk groups - including intravenous drug users and MSM - has not been determined. Most importantly, the best strategy for use of antiretroviral agents to reduce the spread of HIV at either the individual level or the population level has not been developed, and remains the ultimate goal of this area of investigation.

Dynamic concurrent partnership networks incorporating demography

We introduce a population model that incorporates From a mathematical point of view we deal with continuous-time Markov chains at the individual level, with the interaction between individuals captured by a global variable describing opportunities for new partnerships. We show that for large time a stationary distribution is attained and we deduce various statistical features of that distribution, with particular attention for concurrency, i.e. the overlap in time of multiple partnerships of one and the same individual. Our ultimate motivation is to model the spread of sexually transmitted infections in the population, for which the present paper serves as a prelude.

Relationship factors associated with gay male couples' concordance on aspects of their sexual agreements: establishment, type, and adherence

Factors associated with gay male couples' concordance on aspects of sexual agreements remain understudied. The present study examined which relationship factors, self-reports of UAI, and patterns of HIV testing may be associated with men who were concordant about having a sexual agreement, the same type of sexual agreement, and adhering to their sexual agreement with their main partner. Various recruitment strategies were used to collect dyadic data from 142 gay male couples. Concordance on aspects of sexual agreements varied within the sample. Results indicated that relationship satisfaction was significantly associated with couples who were concordant about having and adhering to their sexual agreement. Predictability and faith of trusting a partner, and value in one's sexual agreement were also positively associated with couples' adhering to their sexual agreement. More research is needed to better understand how relationship dynamics, including sexual agreements, affect HIV risk among gay male couples in the U.S.

Determinants of sexual network structure and their impact on cumulative network measures

There are four major quantities that are measured in sexual behavior surveys that are thought to be especially relevant for the performance of sexual network models in terms of disease transmission. These are (i) the cumulative distribution of lifetime number of partners, (ii) the distribution of partnership durations, (iii) the distribution of gap lengths between partnerships, and (iv) the number of recent partners. Fitting a network model to these quantities as measured in sexual behavior surveys is expected to result in a good description of Chlamydia trachomatis transmission in terms of the heterogeneity of the distribution of infection in the population. Here we present a simulation model of a sexual contact network, in which we explored the role of behavioral heterogeneity of simulated individuals on the ability of the model to reproduce population-level sexual survey data from the Netherlands and UK. We find that a high level of heterogeneity in the ability of individuals to acquire and maintain (additional) partners strongly facilitates the ability of the model to accurately simulate the powerlaw-like distribution of the lifetime number of partners, and the age at which these partnerships were accumulated, as surveyed in actual sexual contact networks. Other sexual network features, such as the gap length between partnerships and the partnership duration, could-at the current level of detail of sexual survey data against which they were compared-be accurately modeled by a constant value (for transitional concurrency) and by exponential distributions (for partnership duration). Furthermore, we observe that epidemiological measures on disease prevalence in survey data can be used as a powerful tool for building accurate sexual contact networks, as these measures provide information on the level of mixing between individuals of different levels of sexual activity in the population, a parameter that is hard to acquire through surveying individuals.

The cost-effectiveness of preexposure prophylaxis for HIV prevention in the United States in men who have sex with men

BACKGROUND

A recent randomized, controlled trial showed that daily oral preexposure chemoprophylaxis (PrEP) was effective for HIV prevention in men who have sex with men (MSM). The Centers for Disease Control and Prevention recently provided interim guidance for PrEP in MSM at high risk for HIV. Previous studies did not reach a consistent estimate of its cost-effectiveness.

OBJECTIVE

To estimate the effectiveness and cost-effectiveness of PrEP in MSM in the United States.

DESIGN

Dynamic model of HIV transmission and progression combined with a detailed economic analysis.

DATA SOURCES

Published literature.

TARGET POPULATION

MSM aged 13 to 64 years in the United States.

TIME HORIZON

Lifetime.

PERSPECTIVE

Societal.

INTERVENTION

PrEP was evaluated in both the general MSM population and in high-risk MSM and was assumed to reduce infection risk by 44% on the basis of clinical trial results.

OUTCOME MEASURES

New HIV infections, discounted quality-adjusted life-years (QALYs) and costs, and incremental cost-effectiveness ratios.

RESULTS OF BASE-CASE ANALYSIS

Initiating PrEP in 20% of MSM in the United States would reduce new HIV infections by an estimated 13% and result in a gain of 550,166 QALYs over 20 years at a cost of $172,091 per QALY gained. Initiating PrEP in a larger proportion of MSM would prevent more infections but at an increasing cost per QALY gained (up to $216,480 if all MSM receive PrEP). Preexposure chemoprophylaxis in only high-risk MSM can improve cost-effectiveness. For MSM with an average of 5 partners per year, PrEP costs approximately $50,000 per QALY gained. Providing PrEP to all high-risk MSM for 20 years would cost $75 billion more in health care-related costs than the status quo and $600,000 per HIV infection prevented, compared with incremental costs of $95 billion and $2 million per infection prevented for 20% coverage of all MSM.

RESULTS OF SENSITIVITY ANALYSIS

PrEP in the general MSM population would cost less than $100,000 per QALY gained if the daily cost of antiretroviral drugs for PrEP was less than $15 or if PrEP efficacy was greater than 75%.

LIMITATION

When examining PrEP in high-risk MSM, the investigators did not model a mix of low- and high-risk MSM because of lack of data on mixing patterns.

CONCLUSION

PrEP in the general MSM population could prevent a substantial number of HIV infections, but it is expensive. Use in high-risk MSM compares favorably with other interventions that are considered cost-effective but could result in annual PrEP expenditures of more than $4 billion.

PRIMARY FUNDING SOURCE

National Institute on Drug Abuse, Department of Veterans Affairs, and National Institute of Allergy and Infectious Diseases.

Relationship factors associated with HIV risk among a sample of gay male couples

More HIV prevention research is needed to better understand how relationship factors may affect sexual risk behaviors among gay male couples. Our cross-sectional study collected dyadic data from 144 gay male couples to examine which relationship factors and characteristics were associated with men having UAI with a secondary sex partner. We targeted male couples by using a variety of recruitment strategies. Multilevel random-effects logistic regression modeling was used to examine which factors were predictive of men in gay couples who had UAI with a secondary sex partner. Analyses revealed that men were less likely to have had UAI with a secondary sex partner if they reported being in a strictly monogamous relationship, receiving an HIV test within the previous 3 months, and being committed to their sexual agreement. Future HIV prevention interventions must consider how relationship factors may influence sexual risk behaviors among gay male couples.

Partner-provided social support influences choice of risk reduction strategies in gay male couples

We investigated the influence of partner-provided HIV-specific and general social support on the sexual risk behavior of gay male couples with concordant, discordant, or serostatus-unknown outside partners. Participants were 566 gay male couples from the San Francisco Bay Area. HIV-specific social support was a consistent predictor for reduced unprotected anal intercourse (UAI) with both concordant outside partners (all couple types) and outside partners of discordant or unknown serostatus (concordant negative and discordant couples). General social support was associated with increased UAI with concordant outside partners for concordant negative and concordant positive couples (i.e., serosorting). Our findings suggest that prevention efforts should target couples and identify the level of HIV-specific support that partners provide. Partner-provided support for HIV-related behaviors could be an additional construct to consider in gay male relationships, akin to relationship satisfaction and commitment, as well as an important component of future HIV prevention interventions.

HIV treatment as prevention: systematic comparison of mathematical models of the potential impact of antiretroviral therapy on HIV incidence in South Africa

BACKGROUND

Many mathematical models have investigated the impact of expanding access to antiretroviral therapy (ART) on new HIV infections. Comparing results and conclusions across models is challenging because models have addressed slightly different questions and have reported different outcome metrics. This study compares the predictions of several mathematical models simulating the same ART intervention programmes to determine the extent to which models agree about the epidemiological impact of expanded ART.

METHODS AND FINDINGS

Twelve independent mathematical models evaluated a set of standardised ART intervention scenarios in South Africa and reported a common set of outputs. Intervention scenarios systematically varied the CD4 count threshold for treatment eligibility, access to treatment, and programme retention. For a scenario in which 80% of HIV-infected individuals start treatment on average 1 y after their CD4 count drops below 350 cells/µl and 85% remain on treatment after 3 y, the models projected that HIV incidence would be 35% to 54% lower 8 y after the introduction of ART, compared to a counterfactual scenario in which there is no ART. More variation existed in the estimated long-term (38 y) reductions in incidence. The impact of optimistic interventions including immediate ART initiation varied widely across models, maintaining substantial uncertainty about the theoretical prospect for elimination of HIV from the population using ART alone over the next four decades. The number of person-years of ART per infection averted over 8 y ranged between 5.8 and 18.7. Considering the actual scale-up of ART in South Africa, seven models estimated that current HIV incidence is 17% to 32% lower than it would have been in the absence of ART. Differences between model assumptions about CD4 decline and HIV transmissibility over the course of infection explained only a modest amount of the variation in model results.

CONCLUSIONS

Mathematical models evaluating the impact of ART vary substantially in structure, complexity, and parameter choices, but all suggest that ART, at high levels of access and with high adherence, has the potential to substantially reduce new HIV infections. There was broad agreement regarding the short-term epidemiologic impact of ambitious treatment scale-up, but more variation in longer term projections and in the efficiency with which treatment can reduce new infections. Differences between model predictions could not be explained by differences in model structure or parameterization that were hypothesized to affect intervention impact.

Mathematical models for the study of HIV spread and control amongst men who have sex with men

For a quarter of century, mathematical models have been used to study the spread and control of HIV amongst men who have sex with men (MSM). We searched MEDLINE and EMBASE databases up to the end of 2010 and reviewed this literature to summarise the methodologies used, key model developments, and the recommended strategies for HIV control amongst MSM. Of 742 studies identified, 127 studies met the inclusion criteria. Most studies employed deterministic modelling methods (80%). Over time we saw an increase in model complexity regarding antiretroviral therapy (ART), and a corresponding decrease in complexity regarding sexual behaviours. Formal estimation of model parameters was carried out in only a small proportion of the studies (22%) while model validation was considered by an even smaller proportion (17%), somewhat reducing confidence in the findings from the studies. Nonetheless, a number of common conclusions emerged, including (1) identification of the importance of assumptions regarding changes in infectivity and sexual contact rates on the impact of ART on HIV incidence, that subsequently led to empirical studies to gather these data, and (2) recommendation that multiple strategies would be required for effective HIV control amongst MSM. The role of mathematical models in studying epidemics is clear, and the lack of formal inference and validation highlights the need for further developments in this area. Improved methodologies for parameter estimation and systematic sensitivity analysis will help generate predictions that more fully express uncertainty, allowing better informed decision making in public health.

The cost-effectiveness of symptom-based testing and routine screening for acute HIV infection in men who have sex with men in the USA

OBJECTIVE

Acute HIV infection often causes influenza-like illness (ILI) and is associated with high infectivity. We estimated the effectiveness and cost-effectiveness of strategies to identify and treat acute HIV infection in men who have sex with men (MSM) in the USA.

DESIGN

Dynamic model of HIV transmission and progression.

INTERVENTIONS

We evaluated three testing approaches: viral load testing for individuals with ILI, expanded screening with antibody testing, and expanded screening with antibody and viral load testing. We included treatment with antiretroviral therapy for individuals identified as acutely infected.

MAIN OUTCOME MEASURES

New HIV infections, discounted quality-adjusted life years (QALYs) and costs, and incremental cost-effectiveness ratios.

RESULTS

At the present rate of HIV-antibody testing, we estimated that 538,000 new infections will occur among MSM over the next 20 years. Expanding antibody screening coverage to 90% of MSM annually reduces new infections by 2.8% and costs US$ 12,582 per QALY gained. Symptom-based viral load testing with ILI is more expensive than expanded antibody screening, but is more effective and costs US$ 22,786 per QALY gained. Combining expanded antibody screening with symptom-based viral load testing prevents twice as many infections compared to expanded antibody screening alone, and costs US$ 29,923 per QALY gained. Adding viral load testing to all annual HIV tests costs more than US$ 100,000 per QALY gained.

CONCLUSION

Use of HIV viral load testing in MSM with ILI prevents more infections than does expanded annual antibody screening alone and is inexpensive relative to other screening interventions. Clinicians should consider symptom-based viral load testing in MSM, in addition to encouraging annual antibody screening.

Serosorting and sexual risk behaviour according to different casual partnership types among MSM: the study of one-night stands and sex buddies

Among HIV-negative men who have sex with men (MSM), any incident of unprotected anal intercourse (UAI) between casual partners is usually regarded as risky for HIV transmission. However, men are increasingly using knowledge of their casual partner's HIV-status to reduce HIV risk during UAI (i.e., serosorting). Since familiarity between casual partners may lead to higher levels of UAI and serosorting, we examined how often men have UAI and practice serosorting with three types of casual partnerships that differ in their degree of familiarity. We included 240 HIV-negative men of the Amsterdam Cohort Study among MSM. We distinguished three types of casual partnerships: one-night stand ("met by chance and had sex only once"); multiple-time casual partner ("met and had sex with several times") and the "regular" casual partner ("sex buddy"). Serosorting was defined as UAI with an HIV-concordant partner. Generalised estimating equations analyses were used to examine the association between type of casual partnership and sexual risk behaviour. Analyses revealed that men with a sex buddy were more likely to have UAI than men with a one-night stand (OR [95%CI] 2.39 [1.39-4.09]). However, men with a sex buddy were also more likely to practice serosorting than men with a one-night stand (OR [95%CI] 5.20 [1.20-22.52]). Men with a sex buddy had more UAI but also reported more serosorting than men with a one-night stand. As a result, the proportion of UAI without serosorting is lower for men with a sex buddy, and therefore men might have less UAI at risk for HIV with this partner type. However, the protective value of serosorting with a sex buddy against HIV transmission needs to be further established. At this time, we suggest that a distinction between the one-night stand and the sex buddy should be incorporated in future studies as men behave significantly different with the two partner types.

'Gay boy talk' meets 'girl talk': HIV risk assessment assumptions in young gay men's sexual health communication with best friends

Young adults, particularly young gay men (YGM), are vulnerable to human immunodeficiency virus (HIV). Yet, little is known about how YGM discuss sexual health issues with their friends ('gay boy talk'). We conducted semi-structured interviews with YGM and their best friends (11 YGM/YGM dyads and 13 YGM/heterosexual female dyads). In this paper, we examine risk assessment assumptions conveyed within YGM's communication about sexual health with their friends and how, if at all, the sexual scripts guiding these assumptions may differ between YGM and young women. Findings demonstrated that, while these young adults clearly intended to support their friends and promote safer sex, they also conveyed assumptions about HIV risk assessment, especially regarding sexual partner selection, that may actually increase their friends' risk for HIV infection. Since inaccurate HIV risk assessment assumptions were transmitted via sexual health communication between peers, it is suggested that such assumptions may need to be addressed in HIV prevention programs working with YGM and their friends. Further, gender differences were identified within the sexual scripts shared between YGM and their friends, suggesting that such interventions should be tailored to the specific needs of different friendship networks.

Increasing risk behaviour can outweigh the benefits of antiretroviral drug treatment on the HIV incidence among men-having-sex-with-men in Amsterdam

Background

The transmission through contacts among MSM (men who have sex with men) is one of the dominating contributors to HIV prevalence in industrialized countries. In Amsterdam, the capital of the Netherlands, the MSM risk group has been traced for decades. This has motivated studies which provide detailed information about MSM's risk behavior statistically, psychologically and sociologically. Despite the era of potent antiretroviral therapy, the incidence of HIV among MSM increases. In the long term the contradictory effects of risk behavior and effective therapy are still poorly understood.

Methods

Using a previously presented Complex Agent Network model, we describe steady and casual partnerships to predict the HIV spreading among MSM. Behavior-related parameters and values, inferred from studies on Amsterdam MSM, are fed into the model; we validate the model using historical yearly incidence data. Subsequently, we study scenarios to assess the contradictory effects of risk behavior and effective therapy, by varying corresponding values of parameters. Finally, we conduct quantitative analysis based on the resulting incidence data.

Results

The simulated incidence reproduces the ACS historical incidence well and helps to predict the HIV epidemic among MSM in Amsterdam. Our results show that in the long run the positive influence of effective therapy can be outweighed by an increase in risk behavior of at least 30% for MSM.

Conclusion

We recommend, based on the model predictions, that lowering risk behavior is the prominent control mechanism of HIV incidence even in the presence of effective therapy.

A decade of modelling research yields considerable evidence for the importance of concurrency: a response to Sawers and Stillwaggon

In their recent article, Sawers and Stillwaggon critique the "concurrency hypothesis" on a number of grounds. In this commentary, I focus on one thread of their argument, pertaining to the evidence derived from modelling work. Their analysis focused on the foundational papers of Morris and Kretzschmar; here, I explore the research that has been conducted since then, which Sawers and Stillwaggon leave out of their review. I explain the methodological limitations that kept progress on the topic slow at first, and the various forms of methodological development that were pursued to overcome these. I then highlight recent modelling work that addresses the various limitations Sawers and Stillwaggon outline in their article. Collectively, this line of research provides considerable support for the modelling aspects of the concurrency hypothesis, and renders their critique of the literature incomplete and obsolete. It also makes clear that their call for "an end (or at least a moratorium) to research on sexual behaviour in Africa" that pertains to concurrency is unjustified.

Ongoing HIV-1 transmission among men who have sex with men in Amsterdam: a 25-year prospective cohort study

BACKGROUND

To examine the suggested resurgence of the HIV epidemic among men who have sex with men (MSM), we studied trends in HIV-1 incidence rates, sexual risk behaviour, risk factors for HIV-1 seroconversion, and source of HIV-1 infection among MSM in the Amsterdam Cohort Studies from 1984 to 2009.

METHODS

Trends in HIV-1 incidence and risk factors for HIV-1 infection were studied using Poisson regression. Trends in sexual risk behaviour were evaluated using logistic regression, correcting for intra-individual correlation via generalized estimating equations. Trends in the source of HIV-1 infection were modelled via logistic regression.

RESULTS

Of 1642 HIV-1-negative individuals, 217 seroconverted during follow-up. HIV-1 incidence rates strongly decreased from 8.6/100 person-years in 1985 to 1.3/100 person-years in 1992; remained relatively stable around 1.0/100 person-years between 1992 and 1996, and slowly increased to 2.0/100 person-years in 2009 (P = 0.14; linear trend 1996-2009). Reports of unprotected anal intercourse (UAI) increased significantly from 1996 onwards. HIV-1 seroconversion was associated with receptive UAI with casual partners, more than five sexual partners, a history of gonorrhoea (all in the preceding 6 months), and a lower educational level. Currently, MSM are more likely to have contracted HIV-1 from casual partners than from steady partners, but trends of recent years suggest that steady partners became a growing source with increasing age.

CONCLUSIONS

Following increases in sexual risk behaviour from 1996 onwards, HIV-1 continues to spread among MSM. Targeted prevention messages should continue to focus on sexual behaviour with casual partners, but also on sexual behaviour within steady relationships.

Responses from the Lesbian & Gay Psychology Section to Crossley's Making sense of barebacking

The aim of the present study is to summarize key responses to Crossley's (2004) article 'Making sense of barebacking' from members of the British Psychological Society's Lesbian & Gay Psychology Section. These responses are assembled into four main themes: (1) terminology, including descriptions of sexual behaviour that are inaccurate and pejorative; (2) representations that endorse culturally dominant and stigmatizing stereotypes of gay men as hedonistic, promiscuous, morally irresponsible and interested in sex rather than relationships; (3) methodology, particularly the use of autobiographical and fictional accounts as reliable sources of data about HIV risk; (4) ethics, especially the infringement of the dignity and the worth of those researched or represented. We welcome attempts to address the continuing problems of HIV/AIDS but recommend that authors and editors enter into dialogue with colleagues who are members of sexual minority communities as part of the research process.

HIV transmission by stage of infection and pattern of sexual partnerships

BACKGROUND

Most model analyses examining the role of primary HIV infection in the HIV epidemic ignore the fact that HIV is often transmitted through long-term, concurrent sexual partnerships. We sought to understand how duration and concurrency of sexual partnerships affect the role of transmissions during primary HIV infection.

METHODS

We constructed a stochastic individual-based model of HIV transmission in a homogeneous population where partnerships form and dissolve. Using observed contagiousness by stage of HIV infection, the fraction of transmissions during primary HIV infection at equilibrium was examined across varying partnership durations and concurrencies.

RESULTS

The fraction of transmissions during primary HIV infection has a U-shaped relationship with partnership duration. The fraction drops with increasing partnership duration for partnerships with shorter average duration but rises for partnerships with longer average duration. Partnership concurrency modifies this relationship. The fraction of transmissions during primary HIV infection increases with increasing partnership concurrency for partnerships with shorter average duration, but decreases for partnerships with longer average duration.

CONCLUSIONS

Partnership patterns strongly influence the transmission of HIV and do so differentially by stage of infection. Dynamic partnerships need to be taken into account to make a robust inference on the role of different stages of HIV infection.

Can migrants from high-endemic countries cause new HIV outbreaks among heterosexuals in low-endemic countries?

OBJECTIVES

To investigate how the sexual behaviour of migrants originating from HIV-endemic countries affects the spread of HIV among heterosexuals in low-endemic countries.

METHODS

A mathematical model is developed describing the transmission of HIV in heterosexual partnerships between African migrants, Caribbean migrants, and local natives. The model accounts for infection of migrants before migration and during trips to their home country. The model is parameterized using data from the Netherlands.

RESULTS

Among new and newly imported, heterosexually acquired, infections in 2010 in the Netherlands, the individual acquiring HIV is an African in 53% of cases, a Caribbean in 26% of cases, and a Dutch native in 21% of cases. The percentage of new infections acquired outside the Netherlands is 40% among African migrants and 32% among Caribbean migrants; these are mostly acquired before migration to the Netherlands. The prevalence of HIV in the Netherlands is hardly affected by changes in risk behaviour of migrants during trips to their home country after migration. If migrants mix more with the Dutch in forming partnerships, then HIV prevalence among migrants will decrease. The more initiating antiviral therapy is delayed among migrants, the higher the resulting prevalence in their own ethnic group and among the Dutch.

CONCLUSION

The serostatus of individuals migrating to low-prevalence countries as well as their sexual behaviour in the country of residence affect considerably the spread of HIV. Preventive measures should focus on targeted interventions, promoting safe sex practices, HIV testing, and entry to specialized HIV care among migrants.

Modelling sexual transmission of HIV: testing the assumptions, validating the predictions

PURPOSE OF REVIEW

To discuss the role of mathematical models of sexual transmission of HIV: the methods used and their impact.

RECENT FINDINGS

We use mathematical modelling of 'universal test and treat' as a case study to illustrate wider issues relevant to all modelling of sexual HIV transmission.

SUMMARY

Mathematical models are used extensively in HIV epidemiology to deduce the logical conclusions arising from one or more sets of assumptions. Simple models lead to broad qualitative understanding, whereas complex models can encode more realistic assumptions and, thus, be used for predictive or operational purposes. An overreliance on model analysis in which assumptions are untested and input parameters cannot be estimated should be avoided. Simple models providing bold assertions have provided compelling arguments in recent public health policy, but may not adequately reflect the uncertainty inherent in the analysis.

27 years of the HIV epidemic amongst men having sex with men in the Netherlands: an in depth mathematical model-based analysis

BACKGROUND

There has been increasing concern about a resurgent epidemic of HIV-1 amongst men having sex with men in the Netherlands, which has parallels with similar epidemics now occurring in many other countries.

METHODS

A transmission model applicable to HIV-1 epidemics, including the use of antiretroviral therapy, is presented in a set of ordinary differential equations. The model is fitted by maximum likelihood to national HIV-1 and AIDS diagnosis data from 1980 to 2006, estimating parameters on average changes in unsafe sex and time to diagnosis. Robustness is studied with a detailed univariate sensitivity analysis, and a range of hypothetical scenarios are explored for the past and next decade.

RESULTS

With a reproduction number around the epidemic threshold one, the HIV-1 epidemic amongst men having sex with men in the Netherlands is still not under control. Scenario analysis showed that in the absence of antiretroviral therapy limiting infectiousness in treated patients, the epidemic could have been more than double its current size. Ninety percent of new HIV transmissions are estimated to take place before diagnosis of the index case. Decreasing time from infection to diagnosis, which was 2.5 years on average in 2006, can prevent many future infections.

CONCLUSIONS

Sexual risk behaviour amongst men having sex with men who are not aware of their infection is the most likely factor driving this epidemic.

Development and validation of the sexual agreement investment scale

Sexual agreements are ubiquitous among gay men. Lower levels of investment in these agreements may be associated with breaking them or engaging in risky sexual behavior. A scale was developed to measure agreement investment levels among gay men. Qualitative data from 78 gay men in committed relationships were analyzed to inform item development, followed by quantitative analyses of two larger samples (n = 380, n = 1,001) to assess construct, convergent, and discriminant validity. The Sexual Agreement Investment Scale (SAIS) is a psychometrically sound measure of the level of investment in sexual agreements among gay men in relationships. Men with higher agreement investment were less likely to break agreements and less likely to engage in unprotected anal intercourse with outside partners. The SAIS can be used to measure investment in sexual agreements and its impact on sexual behavior in a wide variety of settings, including research on relationships, sexuality, couples therapy and HIV prevention.

Effects of a short individually tailored counselling session for HIV prevention in gay and bisexual men receiving Hepatitis B vaccination

Background

There is currently a trend towards unsafe unprotected anal intercourse (UAI) among men who have sex with men. We evaluated a short individual counselling session on reducing UAI among gay and bisexual men.

Methods

A quasi-experimental design was used to evaluate the counselling session. This session was conducted during consulting hours at four municipal health clinics during a Hepatitis B vaccination campaign. These clinics offered free vaccination to high-risk groups, such as gay and bisexual men.

All gay and bisexual men attending health clinics in four cities in the Netherlands were asked to participate. Each participant in the intervention group received a fifteen-minute individual counselling based on the Theory of Planned Behaviour and Motivational Interviewing. Changes in UAI were measured over a 5-months period, using self-administered questionnaires. UAI was measured separately for receptive and insertive intercourse in steady and casual partners. These measures were combined in an index-score (range 0–8).

Results

While UAI in the counselling group remained stable, it increased in the controls by 66% from 0.41 to 0.68. The results show that the intervention had a protective effect on sexual behaviour with steady partners. Intervention effects were strongest within steady relationships, especially for men whose steady-relationship status changed during the study. The intervention was well accepted among the target group.

Conclusion

The fifteen-minute individually tailored counselling session was not only well accepted but also had a protective effect on risk behaviour after a follow-up of six months.

Serostatus differences and agreements about sex with outside partners among gay male couples

This article describes agreements gay male couples make about sex outside the relationship and how the process of making those agreements, and their perceived quality, varies depending on couple serostatus. Data include 191 couples recruited in the San Francisco Bay Area from June to December 2004. Monogamous agreements were reported by 56% of participants in concordant-negative, 47% in concordant-positive, and 36% in discordant relationships. The remaining participants reported agreements allowing sex with outside partners in some form. Agreement quality was lowest among men in discordant relationships. Overall, few (30%) reported breaking their agreements; only half of whom reported disclosing those breaks to their partners. Although differences in agreement type, quality, and satisfaction were found among the three couple serostatus groups, rates of breaks and their disclosure did not vary significantly by group. Future HIV prevention efforts aimed at couples must integrate both couple serostatus and relationship-based issues.

A resurgent HIV-1 epidemic among men who have sex with men in the era of potent antiretroviral therapy

OBJECTIVE

Reducing viral load, highly active antiretroviral therapy has the potential to limit onwards transmission of HIV-1 and thus help contain epidemic spread. However, increases in risk behaviour and resurgent epidemics have been widely reported post-highly active antiretroviral therapy. The aim of this study was to quantify the impact that highly active antiretroviral therapy had on the epidemic.

DESIGN

We focus on the HIV-1 epidemic among men who have sex with men in the Netherlands, which has been well documented over the past 20 years within several long-standing national surveillance programs.

METHODS

We used a mathematical model including highly active antiretroviral therapy use and estimated the changes in risk behaviour and diagnosis rate needed to explain annual data on HIV and AIDS diagnoses.

RESULTS

We show that the reproduction number R(t), a measure of the state of the epidemic, declined early on from initial values above two and was maintained below one from 1985 to 2000. Since 1996, when highly active antiretroviral therapy became widely used, the risk behaviour rate has increased 66%, resulting in an increase of R(t) to 1.04 in the latest period 2000-2004 (95% confidence interval 0.98-1.09) near or just above the threshold for a self-sustaining epidemic. Hypothetical scenario analysis shows that the epidemiological benefits of highly active antiretroviral therapy and earlier diagnosis on incidence have been entirely offset by increases in the risk behaviour rate.

CONCLUSION

We provide the first detailed quantitative analysis of the HIV epidemic in a well defined population and find a resurgent epidemic in the era of highly active antiretroviral therapy, most likely predominantly caused by increasing sexual risk behaviour.

Prospective study of attitudinal and relationship predictors of sexual risk in the multicenter AIDS cohort study

We examined the influence of attitudes concerning HIV transmission, safe sex, and sexual sensation seeking, as well as negotiated risk reduction with primary partners, on the proportion of unprotected sexual partners (%UASP) among men who have sex with men (MSM). Participants were 263 HIV-seropositive and 238 HIV-seronegative MSM in the Multicenter AIDS Cohort Study between 1999 and 2003 who completed a 20-item attitude survey twice. Behavioral data were collected concurrently and 6-12 months after each survey. Among seropositives, decreased HIV concern and increased safer sex fatigue were associated with higher %UASP at 6 and 12 months. Among seronegatives, increased %UASP at 12 months was associated with safer sex fatigue. At 6 months and 12 months, risk reduction agreements were associated with increased %UASP among seronegatives in seroconcordant monogamous relationships, reflecting their abandonment of condoms in such partnerships. We conclude that HIV prevention efforts should target modifiable attitudes (reduced concern about HIV and safer sex fatigue) and increases in sexual risk-taking of MSM, particularly among HIV+ men having sex with serodiscordant partners.

Study of partner-related and situational risk factors for symptomatic male urethritis

During the last decade, the incidence of male urethritis stopped declining in France. Risk factors associated with unprotected intercourse have been extensively studied in men who have sex with men, but not in men in general. The purpose of the study was to determine major risk factors for urethritis among men and to describe the sociodemographic and medical characteristics of this population in 2005. We conducted a prospective case-crossover study of sexual behaviors among men with acute urethritis attending at general practitioners or sexually transmitted infection (STI) clinics in France. Each patient filled out a selfcompleted questionnaire focusing on sociodemographic characteristics, and on sexual behaviors for the month before urethritis onset and for the preceding 3 months. The doctor reported medical information on a separate questionnaire. Between January and September 2005, 121 cases of male urethritis, defined as recent-onset pain on micturition and/or purulent or mucoid discharge, were included. Median age was 33 years, 22.3% were MSM, 55.1% were single, and 72.0% had at least high school education. Conditional logistic regression analysis showed that intercourse with only casual partners or with both casual and steady partners (OR = 2.6, CI 95%: 0.8-8.7, and OR = 8.7, CI 95%: 2.7-28.0), as well as inconsistent condom use (OR = 5.8, CI 95%: 1.7-19.2) significantly increased the risk of male urethritis. STI prevention campaigns should continue to focus on consistent condom use and should not neglect men over 30 years of age.

The impact of HIV treatment on risk behaviour in developing countries: A systematic review

In developing countries, access to antiretroviral therapy (ART) is improving as HIV treatment becomes a greater priority in the global fight against AIDS. While ART has clearly beneficial clinical effects, increased access to treatment may also affect sexual behaviour. To examine the strength of evidence for the impact of medical treatment for HIV-positive individuals on behavioural outcomes in developing countries, we conducted a comprehensive search of the peer-reviewed literature. Studies were included if they provided clinical treatment to HIV-positive individuals in a developing country, compared behavioural, psychological, social, care, or biological outcomes related to HIV-prevention using a pre/post or multi-arm study design, and were published between January 1990 and January 2006. Only three studies were identified that met the inclusion criteria. All were conducted in Africa, utilized before/after or multi-arm study designs, and relied on self-reported behaviour. In all three studies, a majority of HIV-infected individuals reported being sexually abstinent, and access to ART was not associated with an increase in HIV-related risky sexual behaviours. However, one cross-sectional study found that ART patients were more likely to report STD treatment. The available evidence indicates a significant reduction in risk behaviour associated with ART in developing countries. However, there are few existing studies and the rigor of these studies is weak. More studies are needed to build an evidence base on which to make programmatic and policy decisions.

[Prevention strategies to control the HIV epidemic. Successes, problems, and perspectives]

After the recognition of AIDS as a new disease entity and isolation of HIV as the causative agent in the early 1980s, a controversial debate about adequate measures to reduce the spread of HIV developed. In all developed countries a strategy based on education about modes of HIV transmission, condom promotion and harm reduction approaches to intravenous drug use finally succeeded and became instrumental for the relative containment of HIV within most at risk populations. However, in most developing countries as well as in the countries of Eastern Europe, the success of HIV pre vention remained limited for various reasons. In addition, for several years now, in the western developed countries sexual risk behaviour, newly diagnosed HIV infections, and the incidence of various other sexually transmitted infections seem to re-increase, especially among men who have sex with men (MSM). These increases coincide with a dramatic improvement of therapeutic options for the treatment of HIV infection. The relationship between improvement of therapy and changes in sexual risk behaviour has to be further defined. It is hypothesized that HIV serosorting as a predominant risk management strategy among MSM results in an accumulation of health risks in the HIV-positive MSM population. In addition to and beyond established condom promotion, additional biomedical approaches to reduce transmissibility and susceptibility for HIV will be necessary to regain control of the spread of HIV in highly sexually active populations.

HIV incidence and HIV testing behavior in men who have sex with men: using three incidence sources, The Netherlands, 1984-2005

BACKGROUND

In The Netherlands, the western part, including Rotterdam and Amsterdam harbors the majority of the known HIV-infected population, of whom men who have sex with men (MSM) comprise the largest transmission category. Given a general rise in sexually transmitted infections (STI) and risky sexual behavior, we examine the HIV incidence among MSM in the Netherlands with data from three different sources.

METHODS

To describe the HIV epidemic among MSM we use: a prospective cohort study in Rotterdam (ROHOCO: 1998-2003, n = 265) and another in Amsterdam (ACS: 1984-2005, n = 1498]) plus an anonymous HIV surveillance study (Amsterdam STI clinic: 1991-2004, n = 3733) in which HIV-positive MSM were tested with a less-sensitive HIV assay. We evaluated calendar trends in HIV incidence, also focusing on age effects.

RESULTS

Since the start of the HIV epidemic in the early 1980s, incidence has declined strongly in the ACS. In recent years, an increase was noted among older MSM attending the Amsterdam STI clinic (P = 0.0334). In both cohort studies, HIV incidence was lower and recent time-trends were not statistically significant. Among recently infected men at the STI clinic, only 40% accepted named HIV testing at their STI consultation.

CONCLUSIONS

Data suggest that among MSM in the Netherlands, the HIV incidence is between one and four infections per 100 person-years. The epidemic expands among older STI clinic attendees. Prevention should be developed specifically for older men, along with a more efficient HIV testing approach such as routine HIV testing of MSM when they are screened for STI.

Increases in sexually transmitted infections and sexual risk behaviour without a concurrent increase in HIV incidence among men who have sex with men in San Francisco: a suggestion of HIV serosorting?

BACKGROUND

Sexually transmitted infections (STI) and unprotected anal intercourse (UAI) have been increasing among men who have sex with men (MSM) in San Francisco. However, HIV incidence has stabilised.

OBJECTIVES

To describe recent trends in sexual risk behaviour, STI, and HIV incidence among MSM in San Francisco and to assess whether increases in HIV serosorting (that is, selective unprotected sex with partners of the same HIV status) may contribute to preventing further expansion of the epidemic.

METHODS

The study applies an ecological approach and follows the principles of second generation HIV surveillance. Temporal trends in biological and behavioural measures among MSM were assessed using multiple pre-existing

DATA SOURCES

STI case reporting, prevention outreach programmatic data, and voluntary HIV counselling and testing data.

RESULTS

Reported STI cases among MSM rose from 1998 through 2004, although the rate of increase slowed between 2002 and 2004. Rectal gonorrhoea cases increased from 157 to 389 while early syphilis increased from nine to 492. UAI increased overall from 1998 to 2004 (p<0.001) in community based surveys; however, UAI with partners of unknown HIV serostatus decreased overall (p<0.001) among HIV negative MSM, and among HIV positive MSM it declined from 30.7% in 2001 to a low of 21.0% in 2004 (p<0.001). Any UAI, receptive UAI, and insertive UAI with a known HIV positive partner decreased overall from 1998 to 2004 (p<0.001) among MSM seeking anonymous HIV testing and at the STI clinic testing programme. HIV incidence using the serological testing algorithm for recent HIV seroconversion (STARHS) peaked in 1999 at 4.1% at the anonymous testing sites and 4.8% at the STI clinic voluntary testing programme, with rates levelling off through 2004.

CONCLUSIONS

HIV incidence among MSM appears to have stabilised at a plateau following several years of resurgence. Increases in the selection of sexual partners of concordant HIV serostatus may be contributing to the stabilisation of the epidemic. However, current incidence rates of STI and HIV remain high. Moreover, a strategy of risk reduction by HIV serosorting can be severely limited by imperfect knowledge of one's own and one's partners' serostatus.

Number of risk acts by relationship status and partner serostatus: Findings from the HIM cohort of homosexually active men in Sydney, Australia

In recent years, increases in both risk behavior and in seroconversion among homosexually active men have been noted in a number of parts of the world. Data were available from 903 HIV negative homosexual men regarding number of acts of unprotected anal intercourse (UAI), separated into receptive and insertive UAI, with and without ejaculation, with steady and with casual partners. Partners were classified according to serostatus as reported by respondents. Men (N = 325) reported 13,692 UAI acts, most of which were with steady partners, of whom most were reported to be HIV-negative. With HIV-positive partners, both steady and casual, and with casual partners of unknown serostatus, receptive UAI with ejaculation was relatively rare. Insertive UAI without ejaculation was relatively common with casual partners of unknown serostatus. Patterns of UAI suggest that risk of transmission may be greater with steady partners. Men appear to modify practice according to both the nature of the relationship (steady or casual) and (assumed) serostatus of partner.

Vulnerability to HIV among regular male partners and the social coding of intimacy in modern societies

Recent studies suggest that regular male partners who engage in unprotected anal intercourse (UAI) are currently at risk of HIV infection. In this paper, we try to offer a single and coherent theoretical framework to understand this profile of HIV infection. Data on transformations of intimacy in recent times, UAI and condom use between regular male partners and their correlates are reviewed. Firstly, we describe the social and historical context of modern intimate relationships. The differentiation of intimate relationships and their newfound autonomy from traditional social forces are explored to determine how they may contribute to the destabilization of intimate trajectories. Secondly, we theorize UAI as an attempt to stabilize intimate relationships and a strategy to reaffirm mutual trust, desire and intimacy between regular male partners. The impacts of such an approach to social vulnerability and HIV prevention are also discussed.

The epidemiological impact of antiretroviral use predicted by mathematical models: a review

This review summarises theoretical studies attempting to assess the population impact of antiretroviral therapy (ART) use on mortality and HIV incidence. We describe the key parameters that determine the impact of therapy, and argue that mathematical models of disease transmission are the natural framework within which to explore the interaction between antiviral use and the dynamics of an HIV epidemic. Our review focuses on the potential effects of ART in resource-poor settings. We discuss choice of model type and structure, the potential for risk behaviour change following widespread introduction of ART, the importance of the stage of HIV infection at which treatment is initiated, and the potential for spread of drug resistance. These issues are illustrated with results from models of HIV transmission. We demonstrate that HIV transmission models predicting the impact of ART use should incorporate a realistic progression through stages of HIV infection in order to capture the effect of the timing of treatment initiation on disease spread. The realism of existing models falls short of properly reproducing patterns of diagnosis timing, incorporating heterogeneity in sexual behaviour, and describing the evolution and transmission of drug resistance. The uncertainty surrounding certain effects of ART, such as changes in sexual behaviour and transmission of ART-resistant HIV strains, demands exploration of best and worst case scenarios in modelling, but this must be complemented by surveillance and behavioural surveys to quantify such effects in settings where ART is implemented.

Effectiveness of Highly Active Antiretroviral Therapy in Reducing Heterosexual Transmission of HIV

Highly active antiretroviral therapy (HAART) has been shown to be highly effective in reducing plasma levels of HIV RNA; therefore, these treatments could diminish the risk of transmission. We analyzed 393 steady heterosexual couples, of which one partner had been previously diagnosed with HIV infection (index case) and where the nonindex partner reported his or her sexual relationship with the index case as the unique risk exposure. These couples were consecutively enrolled in the period 1991 through 2003 when the nonindex partners took their first HIV test. HIV prevalence among partners of index cases who had not received antiretroviral therapy was 8.6%, whereas no partner was infected in couples in which the index case had been treated with HAART (P = 0.0123). HIV prevalence among nonindex partners declined from 10.3% during the pre-HAART period (1991-1995) to 1.9% during the late HAART period (1999-2003; P = 0.0061). In the multivariate analysis, this decline held (odds ratio = 0.14, 95% confidence interval: 0.03-0.66) after adjusting for length of partnership, unprotected coitus, and pregnancies as well as gender, CD4 lymphocyte count, AIDS-defining diseases, and sexually transmitted infections in the index case. When HAART became widely available, a reduction of approximately 80% in heterosexual transmission of HIV was observed, irrespective of changes in other factors that affect transmission.

Unrecognized HIV infection, risk behaviors, and perceptions of risk among young men who have sex with men: opportunities for advancing HIV prevention in the third decade of HIV/AIDS

This study evaluated the magnitude and distribution of unrecognized HIV infection among young men who have sex with men (MSM) and of those with unrecognized infection, the prevalence and correlates of unprotected anal intercourse (UAI), perceived low risk for infection, and delayed HIV testing. MSM aged 15-29 years were approached, interviewed, counseled, and tested for HIV at 263 randomly sampled venues in 6 US cities from 1994-2000. Of 5649 MSM participants, 573 (10%) tested positive for HIV. Of these, 91% of black, 69% of Hispanic, and 60% of white MSM (77% overall) were unaware of their infection. The 439 MSM with unrecognized infection reported a total of 2253 male sex partners in the previous 6 months; 51% had UAI; 59% perceived that they were at low risk for being infected; and 55% had not tested in the previous year. The HIV epidemic among MSM in the United States continues unabated, in part, because many young HIV-infected MSM are unaware of their infection and unknowingly expose their partners to HIV. To advance HIV prevention in the third decade of HIV/AIDS, prevention programs must reduce unrecognized infection among young MSM by increasing the demand for and availability of HIV testing services.

Similar Sexual Behaviors With Casual Partners Among Gay Men With and Without a Regular Partner

OBJECTIVES

We hypothesized that men in a relationship would engage in less risky sexual activity with their casual partners compared to men with only casual partners.

STUDY

Cross-sectional data were collected between 1999 and 2003 in a study of young human immunodeficiency virus-seronegative gay men in Vancouver, British Columbia. We assessed the substance use and sexual behaviors of 156 men and compared those having casual partners in addition to 1 regular partner for the duration of the previous year (n = 43) to those reporting only casual partners during the previous year (n = 113).

RESULTS

Men with just 1 regular partner were not significantly different from men without a regular partner with regard to sexual behaviors reportedly engaged in with casual partners, self-reported sexually transmitted infections diagnosed within the previous year, or demographic characteristics. Only previous use of poppers and higher frequencies of marijuana use were found among men without a regular partner.

CONCLUSIONS

In our cohort, young gay men in a relationship do not appear less likely to engage in risky sex with casual partners than men with casual partners only.

Transmission of HIV-1 during primary infection: relationship to sexual risk and sexually transmitted infections

OBJECTIVE

To study primary HIV-1 infections (PHI) using molecular and epidemiological approaches in order to assess correlates of transmission in this population.

METHODS

Individuals with PHI were recruited prospectively from a discrete cohort of 1235 individuals under follow-up in a well-defined geographical area between 1999 and 2003. PHI was diagnosed by one of the following: negative HIV antibody test within 18 months, evolving antibody response, or application of the serological testing algorithm for recent HIV seroconversion. The pol gene was sequenced to identify genotypic resistance and facilitate molecular epidemiological analysis. Clinical data were collected and linked in an irretrievable fashion when informed consent was obtained.

RESULTS

A total of 103 individuals with PHI diagnosed between 1999 and 2003 were included in the study; 99 (96%) were male and 90 (91%) were men who have sex with men. Viruses from 35 out of 103 (34%) appeared within 15 phylogenetically related clusters. Significant associations with clustering were: young age, high CD4 cell count, number of sexual contacts, and unprotected anal intercourse (UAI) in the 3 months before diagnosis (P < 0.05 for all). High rates of acute sexually transmitted infections (STI) were observed in both groups with a trend towards higher rates in those individuals with viruses within a cluster (42.9 versus 27.9%; P = 0.13).

CONCLUSION

High rates of partner change, UAI and STI are factors that facilitate onward transmission during PHI. More active identification of individuals during PHI, the management of STI and highly active antiretroviral therapy may all be useful methods to break transmission networks.

Negotiated Safety Relationships and Sexual Behavior Among a Diverse Sample of HIV-Negative Men Who Have Sex With Men

OBJECTIVE

To examine the prevalence of negotiated safety (NS) in a diverse sample of HIV-negative men who have sex with men (MSM), characteristics of MSM practicing NS, and adherence to NS.

METHODS

This was a cross-sectional survey of San Francisco MSM recruited from venues and community organizations. NS relationships were defined as those in which HIV-negative men were in seroconcordant primary relationships for >/=6 months, had unprotected anal intercourse (UA) together, and had rules prohibiting UA with others. Adherence to NS was determined from self-reported sexual behavior in the prior 3 months. Presence of an agreement with NS partners to disclose rule breaking was also determined.

RESULTS

Of 340 HIV-negative participants, 76 (22%) reported a current seroconcordant primary relationship for >/=6 months. Of these 76 men, 38 (50%) had NS relationships, 30 (39%) had no UA with primary partners, and 8 (11%) had UA with primary partners without rules prohibiting UA with others. In multivariate analysis, NS was more common than no UA with primary partners in younger men. Among 38 NS men, 29% violated their NS-defining rule in the prior 3 months, including 18% who reported UA with others, and 18% reported a sexually transmitted infection (STI) in the prior year. Only 61% of NS men adhered fully to rules and agreed to disclose rule breaking.

CONCLUSIONS

Although NS was commonly practiced among HIV-negative men in seroconcordant relationships, some men violated NS-defining rules, placing themselves and potentially their primary partners at risk for HIV infection. Prevention efforts regarding NS should emphasize the importance of agreement adherence, disclosure of rule breaking, and routine STI testing.

Primary HIV infection as source of HIV transmission within steady and casual partnerships among homosexual men

OBJECTIVE

To assess the contribution of primary or acute HIV infection to the transmission of HIV among homosexual men in Amsterdam and to investigate how the initiation of treatment during primary HIV infection (PHI) can affect the incidence of HIV infection.

METHODS

A mathematical model describing HIV transmission among homosexual men was developed. In the model, men are involved in both steady and casual partnerships. Infectivity is higher during PHI than during chronic HIV infection. Highly active antiretroviral therapy reduces infectivity and increases the time to the development of AIDS. Its effect is enhanced if treatment is initiated during PHI. HIV incidence and the fraction of transmission attributed to PHI were calculated for different levels of treatment efficacy.

RESULTS

Primary infections account for 35% of HIV transmissions from casual partners and 6% of transmissions from steady partners. Among all new infections, only 11% occurs during PHI. Therefore, the effect of treatment during PHI on the incidence of HIV is limited. However, in a community with higher risky behaviour among casual partners, the fraction of transmission attributed to PHI increases to 25%.

CONCLUSION

Primary infections play a more important role in transmission from casual partners than in transmission from steady partners. Therefore, in communities in which steady partners account for the majority of new infections and the epidemic is at an advanced phase, the contribution of PHI to the transmission of HIV is rather small and the effect of early treatment on the incidence of HIV is limited.