Probability Of HIV Transmission Research Articles

To See the Efficacy of Behavioral Interventions on Consistent Condom Use Among Female Sex Workers in Lahore, Pakistan

BackgroundSince the beginning of the AIDS epidemic, female sex workers in developing countries have been one of the groups most vulnerable to HIV infection due to their large numbers and rapid change of sexual partners. High rates of other sexually transmitted infections (STIs) and sexual practices such as dry sex or sex during menses further increase the probability of HIV transmission in FSWs. They also are often in a poor position to negotiate safe sex because of social, economic, cultural and legal factors.Method A sample of 300 FSWs from 18 to 45 years of age was recruited using a structured questionnaire. FSWs were randomly assigned to the intervention group and control group. Intervention group attended counseling sessions over a period of one month. Behavioral Interventions were used during counseling sessions including psycho-educational sessions to improve knowledge about HIV prevention and role rehearsal for safe sex behavior, negotiation, communication skills, proper condom use skills, and to help sex partners develop supportive behavior for consistent condom use. Control group didn’t receive any intervention but free condom provision was made possible for both the intervention and control group. Regular follow up sessions were given for 6 months to the participants of intervention and control group.ResultsDescriptive analysis of the baseline interview indicated knowledge about HIV prevention 5.5%, safe sex behavior 10%, negotiation skills 6%, communication skills 6%, and supportive behavior of sex partners 2% that remained the same among females in control group. Females in the intervention group improved knowledge about HIV prevention by 79%, safe sex behavior 70%, negotiation skills 76%, communication skills 75%, and supportive behavior of sex partners 72% at follow up sessions. A multivariate analysis of variance demonstrated increased knowledge about HIV prevention, improved safe sex behavior, improved negotiation skills, improved communication skills and supportive behavior of sex partners (p ConclusionStatistically significant results indicate that behavioral interventions that are gender sensitive, involve sex partners and carried out consistently can effectively enhance consistent condom use among female sex workers.

4.0 When to start

4.0 When to start

Effect of Early Antiretroviral Therapy on Sexual Behaviors and HIV-1 Transmission Risk Among Adults With Diverse Heterosexual Partnership Statuses in Côte d'Ivoire

Background. The effect of early initiation of antiretroviral therapy (ART; ie, at CD4+ T-cell counts >350 cells/mm3) on sexual behaviors and human immunodeficiency virus type 1 (HIV) transmission risk has not been documented in populations other than HIV-serodiscordant couples in stable relationships.Methods. On the basis of data from a behavioral study nested in a randomized, controlled trial (Temprano-ANRS12136) of early ART, we compared proportions of risky sex (ie, unprotected sex with a partner of negative/unknown HIV status) reported 12 months after inclusion between participants randomly assigned to initiate ART immediately (hereafter, “early ART”) or according to ongoing World Health Organization criteria. Group-specific HIV transmission rates were estimated on the basis of sexual behaviors and viral load–specific per-act HIV transmission probabilities. The ratio of transmission rates was computed to estimate the protective effect of early ART.Results. Among 957 participants (baseline median CD4+ T-cell count, 478 cells/mm3), 46.0% reported sexual activity in the past month; of these 46.0%, sexual activity for 41.5% involved noncohabiting partners. The proportion of subjects who engaged in risky sex was 10.0% in the early ART group, compared with 12.8% in the standard ART group (P = .17). After accounting for sexual behaviors and viral load, we estimated that the protective effect of early ART was 90% (95% confidence interval, 81%–95%).Conclusion. Twelve months after inclusion, patients in the early and standard ART groups reported similar sexual behaviors. Early ART decreased the estimated risk of HIV transmission by 90%, suggesting a major prevention benefit among seronegative sex partners in stable or casual relationships with seropositive individuals.

Current status and prospects for development of a vaccine against Trichomonas vaginalis infections

Trichomonas vaginalis is a sexually transmitted pathogen with an annual worldwide incidence of over 276 million infections, the highest of all curable and non-viral STI. A large proportion of cases are asymptomatic and under-diagnosed with conventional diagnostic tools. Infection has important maternal and fetal health consequences and can lead to a higher probability of HIV transmission and susceptibility. Lack of affordable accurate diagnostic tests globally and metronidazole resistance hinder T. vaginalis control efforts. Based on data from current vaccination studies in animal models, a human vaccine is achievable to intervene on the substantial incidence of infection.

P4.058 Incorrect Inferences About Male Circumcision and Female HIV Infection Risk: Evidence from a Randomised Trial in Malawi

<h3>Background</h3> Incorrect inferences drawn from large-scale HIV prevention messages may hinder their efficacy. We examined whether individuals who are told that medical male circumcision (MMC) partially reduces female-to-male HIV transmission also erroneously infer a reduction in male-to-female HIV transmission risk. <h3>Methods</h3> In 2008, nearly 1,000 men in rural Malawi were randomised to receive the information that MMC reduces female-to-male HIV transmission risk, with follow-up in 2009. Data was collected on perceived HIV transmission risks faced by men and women by the circumcision status of the male partner. Descriptive and multivariate variable regression analyses were used to assess whether beliefs about male circumcision and female HIV risk varied by receipt of information about MMC and by whether individuals believed that that MMC partially protects men from HIV infection. <h3>Results</h3> Men randomised to information about MMC were more likely to believe that circumcised men faced reduced HIV transmission risks relative to uncircumcised men (50% versus 65%, p &lt; 0.01). They were also more likely to believe that sex with a circumcised male would confer lower transmission risks for women vis-à-vis sex with an uncircumcised male (38% versus 50%, p &lt; 0.01). Multivariate Ordinary Least Squares (OLS) regression analyses supported these findings and demonstrated that those who internalised beliefs about the risk reduction benefits accruing to men believed, on average, that circumcision conferred a 22 percentage point greater risk reduction (p &lt; 0.01) in the probability of HIV transmission from men to women than men who did not believe that MMC protects men. Moreover, instrumental variable analyses indicated that the result from the OLS regression were an underestimation of the true effect. <h3>Conclusions</h3> Our results point toward the need for male circumcision campaigns to make explicit that male circumcision does not directly protect women from HIV infection.

The importance of extended high viremics in models of HIV spread in South Africa

Recent studies found a substantial fraction of 'extended high viremics' among HIV-1 subtype C, the most common subtype in southern Africa. Extended high viremics are HIV infected individuals who maintain a high viral load for a longer time period than usual after the initial infection. They are more infectious during this period, and their infection progresses to full-blown AIDS and death much faster than usual. This study investigates the impact of extended high viremics on the spread of the HIV epidemic in South Africa. We develop a simple deterministic compartmental model for HIV infection that includes extended high viremics. As the available data on extended high viremics are limited, we parameterize this model using only the fraction of extended high viremics among new infections and the reduced life-span of extended high viremics. We find that without extended high viremics, the HIV prevalence in South Africa would have remained close to its 1990 level, instead of increasing to the current epidemic levels. We also find that the greater the fraction of extended high viremics among susceptibles, the greater the steady-state HIV prevalence and the more sensitive the steady-state prevalence is to the HIV transmission probability. These results suggest that extended high viremics have an impact on the HIV epidemic in South Africa; justify the need for comprehensive epidemiological studies since the current data is limited; and suggest that future models of HIV for southern Africa should explicitly model extended high viremics.

Statistical models to estimate male-to-female HIV transmission probabilities

Statistical models to estimate male-to-female HIV transmission probabilities

High coverage of ART associated with decline in risk of HIV acquisition in rural KwaZulu-Natal, South Africa.

The landmark HIV Prevention Trials Network (HPTN) 052 trial in HIV-discordant couples demonstrated unequivocally that treatment with antiretroviral therapy (ART) substantially lowers the probability of HIV transmission to the HIV-uninfected partner. However, it has been vigorously debated whether substantial population-level reductions in the rate of new HIV infections could be achieved in "real-world" sub-Saharan African settings where stable, cohabiting couples are often not the norm and where considerable operational challenges exist to the successful and sustainable delivery of treatment and care to large numbers of patients. We used data from one of Africa's largest population-based prospective cohort studies (in rural KwaZulu-Natal, South Africa) to follow up a total of 16,667 individuals who were HIV-uninfected at baseline, observing individual HIV seroconversions over the period 2004 to 2011. Holding other key HIV risk factors constant, individual HIV acquisition risk declined significantly with increasing ART coverage in the surrounding local community. For example, an HIV-uninfected individual living in a community with high ART coverage (30 to 40% of all HIV-infected individuals on ART) was 38% less likely to acquire HIV than someone living in a community where ART coverage was low (<10% of all HIV-infected individuals on ART).

HIV-Negative Gay Men’s Perceived HIV Risk Hierarchy: Imaginary or Real?

HIV-related risk perceptions and risk practices among gay men have changed over time. We revisited perceived HIV risk and engagement in anal intercourse with casual partners among HIV-negative gay men who participated in one of the Sydney Gay Community Periodic Surveys (GCPS). Perceived HIV risk was assessed by a range of anal intercourse practices combined with pre-specified casual partners' HIV status and viral load levels. Perceived HIV risk forms a potential hierarchy, broadly reflecting differences in the probability of HIV transmission through various anal intercourse practices. To a lesser extent, it also varies by casual partners' HIV status and viral load. Men who had unprotected anal intercourse with casual partners (UAIC) perceived lower HIV risk than those who used condoms consistently in the 6 months prior to survey. Recognising the complex associations between risk perceptions and risk practices helps to better address challenges arising from the 'Treatment as Prevention' (TasP).

Variable effect of co-infection on the HIV infectivity: Within-host dynamics and epidemiological significance

BackgroundRecent studies have implicated viral characteristics in accounting for the variation in the HIV set-point viral load (spVL) observed among individuals. These studies have suggested that the spVL might be a heritable factor. The spVL, however, is not in an absolute equilibrium state; it is frequently perturbed by immune activations generated by co-infections, resulting in a significant amplification of the HIV viral load (VL). Here, we postulated that if the HIV replication capacity were an important determinant of the spVL, it would also determine the effect of co-infection on the VL. Then, we hypothesized that viral factors contribute to the variation of the effect of co-infection and introduce variation among individuals.MethodsWe developed a within-host deterministic differential equation model to describe the dynamics of HIV and malaria infections, and evaluated the effect of variations in the viral replicative capacity on the VL burden generated by co-infection. These variations were then evaluated at population level by implementing a between-host model in which the relationship between VL and the probability of HIV transmission per sexual contact was used as the within-host and between-host interface.ResultsOur within-host results indicated that the combination of parameters generating low spVL were unable to produce a substantial increase in the VL in response to co-infection. Conversely, larger spVL were associated with substantially larger increments in the VL. In accordance, the between-host model indicated that co-infection had a negligible impact in populations where the virus had low replicative capacity, reflected in low spVL. Similarly, the impact of co-infection increased as the spVL of the population increased.ConclusionOur results indicated that variations in the viral replicative capacity would influence the effect of co-infection on the VL. Therefore, viral factors could play an important role driving several virus-related processes such as the increment of the VL induced by co-infections. These results raise the possibility that biological differences could alter the effect of co-infection and underscore the importance of identifying these factors for the implementation of control interventions focused on co-infection.

Is Sex Like Driving? Risk Compensation Associated with Male Circumcision in Kisumu, Kenya

Mass adult male circumcision campaigns for HIV prevention are underway across much of Sub-Saharan Africa. However, concern remains about risk compensation associated with the reduction in the probability of HIV transmission per risky act. This paper examines the behavioral response to male circumcision in Kisumu, Kenya. Contrary to the presumption of risk compensation, we nd that the response due to the perceived reduction in HIV transmission appears to have been a reduction in risky sexual behavior. We suggest a mechanism for this nding:

Modulating HIV Infection by Controlling the Kinetics of SEVI Fibrillization

Despite the rapid progress of the AIDS pandemic, HIV is a surprisingly weak pathogen in vitro. The large difference between in vitro and in vivo infection rates suggests that cofactors absent in vitro but essential for the natural transmission of the virus may be responsible for this discrepancy. A recently identified peptide in human semen, PAP248-286, has emerged as a candidate for the missing cofactor as it dramatically enhances the infectivity of HIV by up to five orders of magnitude. The PAP248-286 peptide fragment has been shown to only induce its synergistic effect with HIV infection when in the form of amyloid fibers. Amyloid formation by PAP248-286 into the active SEVI form is a slow process during which it is susceptible to being degraded and inactivated. Therefore, initiators of this fibrillization process would be an indirect cause of the increase in viral infectivity. For this reason, we have searched for possible inhibitors and enhancers of SEVI amyloid formation including metals, lipids, other amyloids, and polyphenolic inhibitors. The effects of the metals are metal specific, with some enhancing kinetics, while others either inhibit or have little effect. High resolution structures of PAP248-286 and the green tea extract compound epigallocatechin gallate (EGCG) show binding to the monomer subunit through the lysine side-chains and inhibiting fiber growth, which could prove as an effective preventative measure for HIV infection. In contrast, amyloidogenic fibers produced by E. coli are seen to strongly enhance the kinetics of SEVI formation and HIV infectivity, indicating that bacterial infection could enhance the probability of HIV transmission. This phenomenon appears to be quite general and could be an important seeding mechanism for other amyloid proteins.1) Biophysical. Journal (2009), 97(9), 2474-2483.2) Biochemica et Biophysica Acta, Biomembranes (2011), 1808(4), 1161-1169.

The role of sexually transmitted infections in the evolution of the South African HIV epidemic

To assess the extent to which sexually transmitted infections (STIs) have contributed to the spread of HIV in South Africa and to estimate the extent to which improvements in STI treatment have reduced HIV incidence. A mathematical model was used to simulate interactions between HIV and six other STIs (genital herpes, syphilis, chancroid, gonorrhoea, chlamydial infection and trichomoniasis) as well as bacterial vaginosis and vaginal candidiasis. The effects of STIs on HIV transmission probabilities were assumed to be consistent with meta-analytic reviews of observational studies, and the model was fitted to South African HIV prevalence data. The proportion of new HIV infections in adults that were attributable to curable STIs reduced from 39% (uncertainty range: 24-50%) in 1990 to 14% (8-18%) in 2010, while the proportion of new infections attributable to genital herpes increased. Syndromic management programmes are estimated to have reduced adult HIV incidence by 6.6% (3.3-10.3%) between 1994 and 2004, by which time syndromic management coverage was 52%. Had syndromic management been introduced in 1986, with immediate achievement of 100% coverage and a doubling of the rate of health seeking, HIV incidence would have reduced by 64% (36-82%) over the next decade, but had the same intervention been delayed until 2004, HIV incidence would have reduced by only 5.5% (2.8-9.0%). Sexually transmitted infections have contributed significantly to the spread of HIV in South Africa, but STI control efforts have had limited impact on HIV incidence because of their late introduction and suboptimal coverage.

P1-S4.25 Using mathematical models to understand the causes of the ecological association seen between HIV and HSV-2 in female sex workers in Southern India

BackgroundCross-sectional surveys of female sex workers (FSW) in multiple districts in Southern India show a strong ecological association between HIV and HSV-2 prevalence. Modelling was used to determine if this...

Time trends of syphilis and HSV-2 co-infection among men who have sex with men in the German HIV-1 seroconverter cohort from 1996–2007

ObjectivesNumbers of newly diagnosed HIV infections among men who have sex with men (MSM) in Germany increased after the year 2000. We sought to explore trends in STI co-infections around...

Role of acute and early HIV infection in the sexual transmission of HIV

Acute HIV infection (AHI), the earliest period after HIV acquisition, is only a few weeks in duration. In this brief period, the concentration of HIV in blood and genital secretions is extremely high, increasing the probability of HIV transmission. Although a substantial role of AHI in the sexual transmission of HIV is biologically plausible, the significance of AHI in the epidemiological spread of HIV remains uncertain. AHI is diagnosed by detecting viral RNA or antigen in the blood of persons who are HIV seronegative. Depending on the setting, persons with AHI represent between 1 and 10% of persons with newly diagnosed HIV infection. The high concentration of virus during AHI leads to increased infectiousness, possibly as much as 26 times greater than during chronic infection. In mathematical models, the estimated proportion of transmission attributed to AHI has varied considerably, depending on model structure, model parameters, and the population. Key determinants include the stage of the HIV epidemic and the sexual risk profile of the population. Despite its brief duration, AHI plays a disproportionate role in the sexual transmission of HIV infection. Detection of persons with AHI may provide an important opportunity for transmission prevention.

Commentary: Probability of HIV transmission through anal intercourse

Anal intercourse remains the most common means of HIV transmission in most of the developed world. Homosexual men represent the largest group of new HIV infections in Australia, Western Europe and North America. The rate of HIV diagnosis in homosexual men is on the increase in these regions, and this most likely reflects a true increase in HIV incidence. Recently, it has become clear that homosexual men are also disproportionately affected by HIV in much of the developing world. A detailed understanding of the transmissibility of HIV through anal intercourse, and co-factors which influence risk, is fundamental to the formulation of effective HIV prevention policy for homosexual men, and is also important for the substantial minority of heterosexual people who also practice anal intercourse. A recent review of the per-contact probability of HIV transmission by vaginal intercourse included data from dozens of prospective studies. In contrast, there is a dearth of data on per-contact probability of HIV transmission by anal intercourse. In the review published in this issue of the IJE, Baggaley et al. were able to include only four studies, including only a single prospective cohort. They were unable to find a single published estimate of the per-contact probability of HIV transmission by insertive anal intercourse. Baggaley et al. report a summary per-contact probability of transmission through receptive anal intercourse of 1.4%. This accords precisely with the probability calculated in an Australian study published in early 2010. In that study, the authors also concluded that the per-contact probability of transmission was lower (0.65%) when withdrawal prior to ejaculation occurred. The study also provided the first estimate of per-contact probability of HIV transmission via insertive anal sex, of 0.11% when the insertive partner was circumcised and 0.62% when the insertive partner was uncircumcised. The summary estimate of probability of transmission by receptive anal intercourse of 1.4% is 18-fold higher than the estimated probability of male to female vaginal intercourse, and goes some way to explain why homosexual men have been so disproportionately affected by HIV in developed countries. What co-factors might influence HIV transmission by anal intercourse? For vaginal intercourse, HIV viral load in the index case is of fundamental importance. In people with earlyand late-stage HIV disease, both of which are associated with high HIV viral load, per-contact probability of transmission through vaginal intercourse has been estimated to be 7–9-fold higher than in the asymptomatic phase of HIV infection. In untreated African heterosexuals, the HIV transmission rate increased dramatically with increasing viral load from 0 in couples where the HIV positive partner had a viral load of less than 1500, to 23 per 100 person-years in those couples where the viral load was more than 50 000. As effective antiretroviral therapy generally reduces HIV viral load to undetectable levels, it has been hypothesized that widespread treatment of HIV within a population could have a central role in HIV prevention. The question of whether anti-retroviral therapy will reduce sexual transmission of HIV is being addressed directly by a randomized controlled trial of early vs standard anti-retroviral therapy of HIV-infected individuals in serodiscordant couples. The primary outcome in this study is the rate of HIV infection in the initially HIV-negative partner. Trial results, anticipated to be available in 2013, will shape our understanding of the role of HIV treatment in HIV prevention in heterosexuals. What do we know about the effect of viral load and anti-retroviral therapy on rates of HIV transmission by anal intercourse? The fact that the per-contact probability of transmission is close to 20-fold higher than for vaginal intercourse is a strong argument for not simply extrapolating data from heterosexual populations. The modelling data presented by Baggaley and others demonstrate that the Published by Oxford University Press on behalf of the International Epidemiological Association

Short communication: Prioritizing communities for HIV prevention in sub-Saharan Africa.

HIV prevalence is the most commonly used measure to prioritize communities for HIV prevention. We show that data on two HIV infection stages (early vs. nonearly and late vs. nonlate) allow estimation of two better measures of prevention need: HIV incidence (for prevention of HIV acquisition) and expected probability of HIV transmission in unprotected sex acts between HIV-infected community members and susceptible individuals (for prevention of HIV transmission). The three ranking schemes--by prevalence, incidence, and transmission probability--lead to significantly different community rank orders. Disease stage information should be collected in HIV surveys.

Available evidence does not support serosorting as an HIV risk reduction strategy: author's reply.

Heymer and Wilson offer four critiques of our study [1], concluding that serosorting may be more harmful than helpful in many settings. We address each critique below. First, they were concerned that our model does not consider how strategic positioning and negotiated safety ‘change with or without disclosure of HIV status’. We did not explicitly model HIV status disclosure. However, we randomly assigned nondisclosers serostatuses based on local prevalence and partitioned strategic positioning and negotiated safety behaviors accordingly. Thus, our model considered both behaviors, parameterized using our 2003 RDD study data of men who have sex with men (MSM) [2] and with weighted averages across nondisclosing and disclosing serodiscordant couples, hence avoiding bias. Second, they express concern that, given 16% HIV prevalence, our ‘model would require partner acquisition rates among HIV-positive men to be much greater than among HIV-negative men’. We did not assume a priori that HIV-positive men have higher partner acquisition rates; rather, men with higher partner acquisition rates were more likely to become HIV-positive. Third, our colleagues ‘question the relevance of using equilibrium prevalence to gain insight into the effect of a recent phenomenon on an epidemic where there is increasing incidence’. We interpret this as favoring a focus on the immediate effects of serosorting to an individual, as in Heymer and Wilson’s recent study [3]. We find value in understanding both short-term individual and long-term population effects, and explored both in our study. For the latter, equilibrium prevalence is a commonly used metric with cross-study comparability; its use does not imply that researchers expect behavior to remain constant indefinitely. Both calculations supported our conclusion that in Seattle MSM, serosorting is more protective than not serosorting, all else equal. Additionally, overall HIV incidence among Seattle MSM appears to be stable [4]. Perhaps most importantly, Heymer and Wilson question our decision to parameterize our model assuming that MSM test one to two times per year, an estimate they believe exceeds common rates. We derived our testing estimates directly from the Seattle RDD study [2], where men reporting one anal sex partner and more than oneanal sex partner in the previous year reported testing a median of 12 and 6 months prior to interview, respectively. Mean intertest interval was 18 and 10 months. Among MSM tested in five US STD clinics, the median intertest interval was 243 days [5]. Few US data are available on the proportion of HIV cases that are undiagnosed, and the representativeness of what data we have is mostly unknown. However, the 2008 National HIV Behavioral Surveillance System (NHBS) survey conducted in King County, Washington, found that 13% of MSM testing HIV-positive were unaware of their infection (H. Theide, personal communication). Data from the same survey in San Francisco, California, found that 14.5% of HIV-positive MSM were undiagnosed (W. McFarland, personal communication). A 2001 population-based survey of California MSM found nearly 10% of HIV-infected MSM were undiagnosed [6]. These numbers are at odds with national data from the 2005 NHBS survey and a CDC estimate that 24.5% of HIV-positive US MSM are undiagnosed [7,8]. We believe there is evidence to support our estimate for Seattle. However, the USA is a large, heterogeneous nation and there are almost certainly regions where testing is less frequent and a greater proportion of infected MSM are undiagnosed. To assess that issue further, we explored the effect of serosorting under a wider array of testing frequencies (Fig. 1). In our model, serosorting has no impact on equilibrium HIV prevalence or the risk of HIV acquisition when high-risk and low-risk MSM test every 1.5 and 3 years, respectively. Serosorting has a small deleterious effect at lower testing frequencies. Fig. 1 Equilibrium HIV prevalence (a) and transmission probabilities during unprotected receptive anal intercourse with a randomly chosen apparent negative (b) by testing frequency We agree with Heymer and Wilson that the benefit of serosorting is highly context-specific and depends on testing frequency, accurate disclosure and behavior. Our findings suggest that, in Seattle, serosorting, as practiced, probably somewhat protects MSM both at the individual and population levels, a finding supported by empirical observation [9,10]. However, that may not be true throughout the USA and should not be construed to suggest that we are promoting serosorting. We believe that our modeling study and Heymer and Wilson‘s [3] have some similar conclusions. Serosorting would be protective if the proportion of HIV-infected MSM that are undiagnosed is less than 20% (as shown in Fig. 1 [3]), and serosorting can potentially increase risk when the population does not test frequently. These concordant findings should reinforce the importance of increasing the proportion and frequency of MSM testing for HIV.

Per-contact probability of HIV transmission in homosexual men in Sydney in the era of HAART

The objective of this study is to estimate per-contact probability of HIV transmission in homosexual men due to unprotected anal intercourse (UAI) in the era of HAART. Data were collected from a longitudinal cohort study of community-based HIV-negative homosexual men in Sydney, Australia. A total of 1427 participants were recruited from June 2001 to December 2004. They were followed up with 6-monthly detailed behavioral interviews and annual testing for HIV till June 2007. Data were used in a bootstrapping method, coupled with a statistical analysis that optimized a likelihood function for estimating the per-exposure risks of HIV transmission due to various forms of UAI. During the study, 53 HIV seroconversion cases were identified. The estimated per-contact probability of HIV transmission for receptive UAI was 1.43% [95% confidence interval (CI) 0.48-2.85] if ejaculation occurred inside the rectum, and it was 0.65% (95% CI 0.15-1.53) if withdrawal prior to ejaculation was involved. The estimated transmission rate for insertive UAI in participants who were circumcised was 0.11% (95% CI 0.02-0.24), and it was 0.62% (95% CI 0.07-1.68) in uncircumcised men. Thus, receptive UAI with ejaculation was found to be approximately twice as risky as receptive UAI with withdrawal or insertive UAI for uncircumcised men and over 10 times as risky as insertive UAI for circumcised men. Despite the fact that a high proportion of HIV-infected men are on antiretroviral treatment and have undetectable viral load, the per-contact probability of HIV transmission due to UAI is similar to estimates reported from developed country settings in the pre-HAART era.