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

Optimizing HIV care: Insights from CD4 count, viral load, and demographic factors in a tertiary care centre of South Gujarat

PURPOSE

Despite advancements in prevention, diagnosis, and treatment, HIV/AIDS remains a critical health concern, particularly in India. This study contributes valuable insights into HIV management strategies. This prospective and retrospective longitudinal observational study aimed to analyze the trends in CD4 cell count and viral load suppression among adult People Living with HIV (PLHIV) undergoing antiretroviral therapy (ART) and evaluate the influence of demographic factors and ART adherence on these parameters at the ART Centre of New Civil Hospital, Surat, India.

MATERIALS & METHODS

Adult PLHIV registered and initiated on ART between June 2017 and May 2018 at ART-NCH, Surat with Continuous follow-up until 2023 were included in the study. Data was collected and Statistical analysis was performed using Microsoft Excel and SPSS software. Other factors were evaluated for their influence on treatment outcomes.

RESULTS

A longitudinally analyzed data from 365 adult PLHIV receiving ART with continuous follow-up until 2023 revealed significant trends, with CD4 counts increasing from 425 (1st month) to 612.67 (24th month), indicating improving immune function. Individuals on first-line ART regimens had significantly higher odds (OR: 3.5, 95 % CI: 1.1-11.3) of achieving CD4 counts ≥350 compared to those on second-line regimens. Adherence to treatment (OR: 1.98, 95 % CI: 1.1-3.4) also increased the odds of attaining CD4 counts ≥350. Viral load suppression was achieved in 353 out of 365 participants.

CONCLUSION

This study highlights the need for tailored interventions to optimize immune recovery and viral load suppression among PLHIV. Recommendations include targeted intervention to improve long-term health outcomes.

Estimating contact network properties by integrating multiple data sources associated with infectious diseases

To effectively mitigate the spread of communicable diseases, it is necessary to understand the interactions that enable disease transmission among individuals in a population; we refer to the set of these interactions as a contact network. The structure of the contact network can have profound effects on both the spread of infectious diseases and the effectiveness of control programs. Therefore, understanding the contact network permits more efficient use of resources. Measuring the structure of the network, however, is a challenging problem. We present a Bayesian approach to integrate multiple data sources associated with the transmission of infectious diseases to more precisely and accurately estimate important properties of the contact network. An important aspect of the approach is the use of the congruence class models for networks. We conduct simulation studies modeling pathogens resembling SARS-CoV-2 and HIV to assess the method; subsequently, we apply our approach to HIV data from the University of California San Diego Primary Infection Resource Consortium. Based on simulation studies, we demonstrate that the integration of epidemiological and viral genetic data with risk behavior survey data can lead to large decreases in mean squared error (MSE) in contact network estimates compared to estimates based strictly on risk behavior information. This decrease in MSE is present even in settings where the risk behavior surveys contain measurement error. Through these simulations, we also highlight certain settings where the approach does not improve MSE.

How the Disruption in Sexually Transmitted Infection Care Due to the COVID-19 Pandemic Could Lead to Increased Sexually Transmitted Infection Transmission Among Men Who Have Sex With Men in The Netherlands: A Mathematical Modeling Study

BACKGROUND

During the COVID-19 pandemic, the disruption in care for sexually transmitted infections (STIs) and the social distancing measures have led to reductions in STI testing and sexual behavior. We assessed the impact of these COVID-19-related changes on transmission of Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (NG) among men who have sex with men (MSM) in The Netherlands.

METHODS

We developed a mathematical model for CT and NG transmission among MSM, accounting for COVID-19-related changes in sexual behavior and testing in 2020 to 2021. Changes in 2020 were estimated from data from the Dutch COVID-19, Sex, and Intimacy Survey among MSM and the National Database of STI Clinics. Because of the lack of data for 2021, we examined several scenarios covering a range of changes.

RESULTS

A reduction of 10% and 40% in STI testing of symptomatic and asymptomatic, respectively, individuals with a 10% to 20% reduction in numbers of casual partners (according to partner status and activity level) during the second lockdown, resulted in a 2.4% increase in CT prevalence, but a 2.8% decline in NG prevalence in 2021. A 5% and 30% reduction in STI testing of symptomatic and asymptomatic, respectively, individuals with the same reduction in casual partners resulted in a 0.6% increase in CT prevalence and a 4.9% decrease in NG prevalence in 2021.

CONCLUSIONS

The disruption in STI care due to COVID-19 might have resulted in a small increase in CT prevalence, but a decrease in NG prevalence. Scaling up STI care is imperative to prevent increases in STI transmission.

Changes in Sexual Behaviour Following PrEP Initiation Among Australian Gay and Bisexual Men in Relationships: Results from a Prospective Observational Study

Few studies have examined changes in sexual behaviour following HIV pre-exposure prophylaxis (PrEP) initiation among gay, bisexual and other men who have sex with men (GBM) in relationships. In a national, online, prospective study of GBM in Australia, we compared sexual behaviours prior to and after PrEP initiation among HIV-negative and unknown-HIV-status men (recruited 2014-2017) not taking PrEP at baseline and who completed at least one six-monthly follow-up by July 2018. Among men in relationships who did not initiate PrEP (n = 339), we compared their most recent survey to their prior one, while among men in relationships who initiated PrEP (n = 81), we compared follow-ups before and after PrEP initiation. Among the 81 PrEP-initiators who were in a relationship both before and after initiation, the proportion reporting their regular partner was on PrEP increased from 8.3 to 44.4% (p < 0.001) and the proportion reporting receptive CLAIC increased from 27.2 to 44.4% (p = 0.009). Overall, men who initiated PrEP were more likely to be in a relationship with a partner on PrEP, and it appeared they started PrEP around the same time. Receptive CLAIC also became more common.

The Rhythm of Risk: Sexual Behaviour, PrEP Use and HIV Risk Perception Between 1999 and 2018 Among Men Who Have Sex with Men in Amsterdam, The Netherlands

HIV risk perception plays a crucial role in the uptake of preventive strategies. We investigated how risk perception and its determinants changed between 1999 and 2018 in an open, prospective cohort of 1323 HIV-negative men who have sex with men (MSM). Risk perception, defined as the perceived likelihood of acquiring HIV in the past 6 months, changed over time: being relatively lower in 2008–2011, higher in 2012–2016, and again lower in 2017–2018. Irrespective of calendar year, condomless anal intercourse (AI) with casual partners and high numbers of partners were associated with higher risk perception. In 2017–2018, condomless receptive AI with a partner living with HIV was no longer associated with risk perception, while PrEP use and condomless AI with a steady partner were associated with lower risk perception. We showed that risk perception has fluctuated among MSM in the past 20 years. The Undetectable equals Untransmittable statement and PrEP coincided with lower perceived risk.

Uniformly accurate nonlinear transmission rate models arising from disease spread through pair contacts

We derive and asymptotically analyze mass-action models for disease spread that include transient pair formation and dissociation. Populations of unpaired susceptible individuals and infected individuals are distinguished from the population of three types of pairs of individuals: both susceptible, one susceptible and one infected, and both infected. Disease transmission can occur only within a pair consisting of one susceptible individual and one infected individual. We use perturbation expansion to formally derive uniformly valid approximations for the dynamics of the total infected and susceptible populations under different conditions including combinations of fast association, fast transmission, and fast dissociation limits. The effective equations are derived from the fundamental mass-action system without implicitly imposing transmission mechanisms, such as those used in frequency-dependent models. Our results represent submodels that show how effective nonlinear transmission can arise from pairing dynamics and are juxtaposed with density-based mass-action and frequency-based models.

Modelling the impact of tailored behavioural interventions on chlamydia transmission

Behavioural interventions tailored to psychological characteristics of an individual can effectively achieve risk-reducing behaviour. The impact of tailored interventions on population-level chlamydia prevalence is unknown. We aimed to assess the impact on overall chlamydia prevalence five years after the introduction of an intervention aimed at increasing self-efficacy, social norms, attitudes and intentions towards condom use (i.e., condom intervention), and an intervention aimed at increasing health goals and decreasing impulsiveness (i.e., impulsiveness intervention). A pair model, informed by longitudinal psychological and behavioural data of young heterosexuals visiting sexual health centers, with susceptible-infected-susceptible structure was developed. The intervention effect was defined as an increased proportion of each subgroup moving to the desired subgroup (i.e., lower risk subgroup). Interventions tailored to subgroup-specific characteristics, assuming differential intervention effects in each subgroup, more effectively reduced overall chlamydia prevalence compared to non-tailored interventions. The most effective intervention was the tailored condom intervention, which was assumed to result in a relative reduction in chlamydia prevalence of 18% versus 12% in the non-tailored scenario. Thus, it is important to assess multiple psychological and behavioural characteristics of individuals. Tailored interventions may be more successful in achieving risk-reducing behaviour, and consequently, reduce chlamydia prevalence more effectively.

Understanding the HIV Epidemic Among MSM in Baltimore: A Modeling Study Estimating the Impact of Past HIV Interventions and Who Acquired and Contributed to Infections

Supplemental Digital Content is Available in the Text.

Men who have sex with men (MSM) in the United States are disproportionately affected by HIV. We estimated the impact of past interventions and contribution of different population groups to incident MSM HIV infections.

Cost-effectiveness analysis of pre-exposure prophylaxis for the prevention of HIV in men who have sex with men in South Korea: a mathematical modelling study

In February 2018, the Ministry of Food and Drug Safety in Korea approved tenofovir disoproxil fumarate and emtricitabine (TDF/FTC) co-formulate for use in pre-exposure prophylaxis (PrEP) for the prevention of human immunodeficiency virus (HIV) infection. This study aimed to estimate the cost-effectiveness of PrEP in men who have sex with men (MSM), a major risk group emerging in Korea. A dynamic compartmental model was developed for HIV transmission and progression in MSM aged 15-64 years. With a combined model including economic analysis, we estimated averted HIV infections, changes in HIV prevalence, discounted costs, quality-adjusted life-years (QALYs), and incremental cost-effectiveness ratios (ICERs). PrEP was evaluated in both the general MSM and high-risk MSM populations and was assumed to reduce infection risk by 80%. Implementing PrEP in all MSM would avert 75.2% HIV infections and facilitate a gain of 37,372 QALYs at a cost of $274,822 per QALY gained over 20 years relative to the status quo. Initiating PrEP in high-risk MSM with an average of eight partners per year (around 20% of MSM) would improve the cost-effectiveness, averting 78.0% HIV infections and add 29,242 QALYs at a cost of $51,597 per QALY gained, which is within the willingness-to-pay threshold for Korea of $56,000/QALY gained. This result was highly sensitive to annual PrEP costs, quality-of-life for people who are on PrEP, and initial HIV prevalence. Initiating PrEP in a larger proportion of MSM in Korea would prevent more HIV infections, but at an increasing cost per QALY gained. Focusing PrEP on higher risk MSM and any reduction in PrEP cost would improve cost-effectiveness.

Different guidelines for pre-exposure prophylaxis (PrEP) eligibility estimate HIV risk differently: an incidence study in a cohort of HIV-negative men who have sex with men, Portugal, 2014-2018

IntroductionGuidelines for pre-exposure prophylaxis (PrEP) provide criteria to identify individuals at higher risk of HIV infection. We compared the ability to predict HIV seroconversion of four guidelines: the World Health Organization (WHO), the United States Public Health Service and Centers for Disease Control and Prevention (US CDC), the European AIDS Clinical Society (EACS) and the Portuguese National Health Service (PNHS).AimWe aimed to measure the association between guideline-specific eligibility and HIV seroconversion.MethodsWe studied 1,254 participants from the Lisbon Cohort of men who have sex with men with at least two evaluations between March 2014 and March 2018, corresponding to 1,724.54 person-years (PY) of follow-up. We calculated incidence rates (IR) according to each guideline eligibility definition and incident rate ratios (IRR) to test the association between eligibility at baseline and HIV seroconversion.ResultsWe found 28 incident cases (IR: 1.62/100 PY; 95% confidence interval (CI) 1.12-2.35). Guidelines' sensitivity varied from 60.7% (EACS) to 85.7% (PNHS) and specificity varied from 31.8% (US CDC) to 51.5% (EACS). IR was highest among those defined as eligible by the PNHS guideline (2.46/100 PY; IRR = 4.61; 95% CI: 1.60-13.27) and lowest for the WHO guideline (1.89/100 PY; IRR = 1.52; 95% CI: 0.69-3.35).ConclusionsBeing identified as eligible for PrEP was associated with a higher risk of infection. The magnitude of risk varied according to the guideline used. However, the number of HIV infections identified among ineligible participants highlights the potential for missing people who need PrEP.

Improving pairwise approximations for network models with susceptible-infected-susceptible dynamics

Network models of disease spread play an important role in elucidating the impact of long-lasting infectious contacts on the dynamics of epidemics. Moment-closure approximation is a common method of generating low-dimensional deterministic models of epidemics on networks, which has found particular success for diseases with susceptible-infected-recovered (SIR) dynamics. However, the effect of network structure is arguably more important for sexually transmitted infections, where epidemiologically relevant contacts are comparatively rare and longstanding, and which are in general modelled via the susceptible-infected-susceptible (SIS)-paradigm. In this paper, we introduce an improvement to the standard pairwise approximation for network models with SIS-dynamics for two different network structures: the isolated open triple (three connected individuals in a line) and the k-regular network. This improvement is achieved by tracking the rate of change of errors between triple values and their standard pairwise approximation. For the isolated open triple, this improved pairwise model is exact, while for k-regular networks a closure is made at the level of triples to obtain a closed set of equations. This improved pairwise approximation provides an insight into the errors introduced by the standard pairwise approximation, and more closely matches both higher-order moment-closure approximations and explicit stochastic simulations with only a modest increase in dimensionality to the standard pairwise approximation.

Strategies used by gay male HIV serodiscordant couples to reduce the risk of HIV transmission from anal intercourse in three countries

INTRODUCTION

There are few data about the range of strategies used to prevent sexual HIV transmission within gay male serodiscordant couples. We examined HIV prevention strategies used by such couples and compared differences between countries.

METHODS

Opposites Attract was a cohort study of male serodiscordant couples in Australia, Brazil and Thailand, from May 2014 (Australia) or May 2016 (Brazil/Thailand) to December 2016. At visits, HIV-positive partners had viral load (VL) tested; HIV-negative partners reported sexual behaviour and perceptions of their HIV-positive partner's VL results. Within-couple acts of condomless anal intercourse (CLAI) were categorized by strategy: condom-protected, biomedically protected (undetectable VL and/or pre-exposure prophylaxis [PrEP]), or not protected by either (HIV-negative partners engaging in insertive CLAI, receptive CLAI with withdrawal, or receptive CLAI with ejaculation).

RESULTS

A total of 343 couples were included in this analysis (153 in Australia, 93 in Brazil and 97 in Thailand). Three-quarters of HIV-positive partners were consistently virally suppressed (<200 copies/mL) during follow-up, and HIV-negative partners had correct perceptions of their partner's VL result for 76.5% of tests. One-third of HIV-negative partners used daily PrEP during follow-up. Over follow-up, 73.8% of couples had CLAI. HIV-negative partners reported 31,532 acts of anal intercourse with their HIV-positive partner. Of these, 46.7% were protected by condoms, 48.6% by a biomedical strategy and 4.7% of acts were not protected by these strategies. Australian couples had fewer condom-protected acts and a higher proportion of biomedically protected acts than Brazilian and Thai couples. Of the 1473 CLAI acts where the perceived VL was detectable/unknown and were not protected by PrEP (4.7% of all acts), two-thirds (n = 983) were when the HIV-negative partner was insertive (strategic positioning). Of the 490 acts when the HIV-negative partner was receptive, 261 involved withdrawal and 280 involved ejaculation. Thus, <1% of acts were in the highest risk category of receptive CLAI with ejaculation.

CONCLUSIONS

Couples used condoms, PrEP or perceived undetectable VL for prevention in the majority of anal intercourse acts. Only a very small proportion of events were not protected by these strategies. Variation between countries may reflect differences in access to HIV treatment, education, knowledge and attitudes.

A multilayer temporal network model for STD spreading accounting for permanent and casual partners

Sexually transmitted diseases (STD) modeling has used contact networks to study the spreading of pathogens. Recent findings have stressed the increasing role of casual partners, often enabled by online dating applications. We study the Susceptible-Infected-Susceptible (SIS) epidemic model –appropriate for STDs– over a two-layer network aimed to account for the effect of casual partners in the spreading of STDs. In this novel model, individuals have a set of steady partnerships (links in layer 1). At certain rates, every individual can switch between active and inactive states and, while active, it establishes casual partnerships with some probability with active neighbors in layer 2 (whose links can be thought as potential casual partnerships). Individuals that are not engaged in casual partnerships are classified as inactive, and the transitions between active and inactive states are independent of their infectious state. We use mean-field equations as well as stochastic simulations to derive the epidemic threshold, which decreases substantially with the addition of the second layer. Interestingly, for a given expected number of casual partnerships, which depends on the probabilities of being active, this threshold turns out to depend on the duration of casual partnerships: the longer they are, the lower the threshold.

Is increased screening and early antiretroviral treatment for HIV-1 worth the investment? An analysis of the public health and economic impact of improvement in the UK

OBJECTIVES

Early treatment of HIV-1 infection at all CD4 levels has demonstrated clinical and public health benefits. This analysis examined the costs, health outcomes, and cost-effectiveness of increased HIV-1 screening and early treatment initiation in the UK.

METHODS

A Markov model followed theoretical cohorts of men who have sex with men (MSM), heterosexuals, and people who inject drugs (PWID) with initially undiagnosed HIV-1 infection over their remaining lifetimes. The analysis examined increased HIV-1 screening (resulting in 10-50% improvements in diagnosis rates) versus current screening in sexual health services (SHS) and other settings, with all individuals initiating treatment within 3 months of diagnosis. Health status was modelled by viral load and CD4 cell count as individuals progressed to diagnosis and treatment. Individuals accrued quality-adjusted life-years (QALYs), incurred costs for screening and HIV-related clinical management, and were at risk of transmitting HIV-1 infection to their partners. Input parameter data were taken primarily from UK-specific published sources. All outcomes were discounted at 3.5% annually.

RESULTS

The model estimated that increased screening and early treatment resulted in fewer onward HIV transmissions, more QALYs, and higher total costs. For SHS, incremental cost-effectiveness ratios (ICERs) for heterosexuals (~£22 000/QALY gained) were within typical UK willingness-to-pay thresholds and were well below these thresholds for MSM (~£9500/QALY gained) and PWID (~£6500/QALY gained). Sensitivity analysis showed that model results were robust.

CONCLUSIONS

Increased HIV-1 screening and early treatment initiation may be a cost-effective strategy to reduce HIV transmission and improve health for MSM, heterosexuals, and PWID in the UK.

The Impact of Screening and Partner Notification on Chlamydia Prevalence and Numbers of Infections Averted in the United States, 2000-2015: Evaluation of Epidemiologic Trends Using a Pair-Formation Transmission Model

Population-level effects of control strategies on the dynamics of Chlamydia trachomatis transmission are difficult to quantify. In this study, we calibrated a novel sex- and age-stratified pair-formation transmission model of chlamydial infection to epidemiologic data in the United States for 2000–2015. We used sex- and age-specific prevalence estimates from the National Health and Nutrition Examination Surveys, case report data from national chlamydia surveillance, and survey data from the Youth Risk Behavior Survey on the proportion of the sexually active population aged 15–18 years. We were able to reconcile national prevalence estimates and case report data by allowing for changes over time in screening coverage and reporting completeness. In retrospective analysis, chlamydia prevalence was estimated to be almost twice the current levels in the absence of screening and partner notification. Although chlamydia screening and partner notification were both found to reduce chlamydia burden, the relative magnitude of their estimated impacts varied in our sensitivity analyses. The variation in the model predictions highlights the need for further data collection and research to improve our understanding of the natural history of chlamydia and the pathways through which prevention strategies affect transmission dynamics.

A dynamic network model to disentangle the roles of steady and casual partners for HIV transmission among MSM

HIV is a sexually transmitted infection (STI) whose transmission process is highly dependent on the sexual network structure of the population under consideration. Most sexual behaviour data is egocentric in nature. We develop a stochastic dynamic sexual network model that utilises this type of egocentric network data. The model incorporates both steady and casual sex partners, and can be seen as a stochastic form of a generalised pair-formation model. We model the spread of an infection where individuals are susceptible, infectious, or successfully treated (and unable to transmit) and derive analytical expressions for several epidemiological quantities. We use sexual behaviour and HIV prevalence data that was gathered among 403 MSM at an STI clinic in Stockholm. To accurately capture transmission dynamics for this population, we need to explicitly model both casual sex partners and steady partnerships. Our model yields an estimate for the mean time until diagnosis followed by successful treatment that is in line with literature. This study indicates that small reductions in the time to diagnosis, and thereby, beginning of treatment, may substantially reduce HIV prevalence. Moreover, we find that moderate increases in condom use with casual sex partners have greater impact on reducing prevalence than the same increases in condom use with steady sex partners. This result demonstrates the relative importance of casual contacts on the HIV transmission dynamics among MSM in Sweden. Our results highlight the importance of HIV testing and condom-use interventions, and the role that casual and steady partners play in this, in order to turn the epidemiological trend in Sweden towards decreased HIV incidence.

Sexual Agreements: A Scoping Review of Measurement, Prevalence and Links to Health Outcomes

A sexual agreement is a mutual understanding between two partners regarding sexual and relational behaviors both within and outside of their relationship. Sexual agreements have been central to research and programming efforts around HIV prevention, primarily for male couples. A comprehensive scoping review of the primary literature on sexual agreements, including negotiated safety, was performed to identify what is known about sexual agreements among couples (n = 66). Results indicate a wide range of prevalence of agreements and measurements used to characterize sexual agreements. Findings also report associations between sexual agreements and health and relational outcomes. Several knowledge gaps were identified; specifically, the need to expand sexual agreements research beyond MSM populations and the need to better understand agreement breaks, break disclosure, and how variation in agreement categorization may impact reported prevalence. This review demonstrates the importance of broadening the evidence-base of sexual agreements research and programmatic focus.

Young Men Who Have Sex with Men at High Risk for HIV, Bangkok MSM Cohort Study, Thailand 2006-2014

High HIV incidence has been reported in young men who have sex with men (YMSM) in North America and Western Europe, but there are limited data from Southeast Asia suggesting MSM may be the driver of the HIV epidemic in this region. We described HIV incidence and risk factors among 494 YMSM enrolled in a cohort study in Bangkok, Thailand. The HIV incidence was 7.4 per 100 person-years. In multivariable analysis, reporting use of an erectile dysfunction drug in combination with club drugs, having receptive or both insertive and receptive anal intercourse with men, having hepatitis A infection, having rectal Chlamydia trachomatis, having hepatitis B infection prior to HIV seroconversion, and reporting not always using condoms with male steady partners were significantly associated with HIV incidence in YMSM. Reduction in new HIV infections in YMSM are critical to reach targets set by Thailand and the region.

Concurrency of partnerships, consistency with data, and control of sexually transmitted infections

Sexually transmitted infections (STIs) are a globally increasing public health problem. Mathematical models, carefully matched to available epidemiological and behavioural data, have an important role to play in predicting the action of control measures. Here, we explore the effect of concurrent sexual partnerships on the control of a generic STI with susceptible-infected-susceptible dynamics. Concurrency refers to being in more than one sexual partnership at the same time, and is difficult to measure accurately. We assess the impact of concurrency through the development of three nested pair-formation models: one where infection can only be transmitted via stable sexual partnerships, one where infection can also be transmitted via casual partnerships between single individuals, and one where those individuals in stable partnerships can also acquire infection from casual partnerships. For each model, we include the action of vaccination before sexual debut to inform about the ability to control. As expected, for a fixed transmission rate, concurrency increases both the endemic prevalence of infection and critical level of vaccination required to eliminate the disease significantly. However, when the transmission rate is scaled to maintain a fixed endemic prevalence across models, concurrency has a far smaller impact upon the critical level of vaccination required. Further, when we also constrain the models to have a fixed number of new partnerships over time (both long-term and casual), then increasing concurrency can slightly decrease the critical level of vaccination. These results highlight that accurate measures and models of concurrency may not always be needed for reliable forecasts when models are closely matched to prevalence data. We find that, while increases in concurrency within a population are likely to generate public-health problems, the inclusion of concurrency may be unnecessary when constructing models to determine the efficacy of the control of STIs by vaccination.

In Australia, Most HIV Infections Among Gay and Bisexual Men are Attributable to Sex with 'New' Partners

It has been estimated that the majority of global HIV infections among gay and bisexual men (GBM) can be attributed to sex within a committed relationship. In Australia, however, negotiated safety, whereby HIV-negative regular partners agree to discard condoms with each other but commit to consistent condom use with other partners, has been promoted as a key component of the HIV prevention response. We asked GBM recently diagnosed with HIV to describe their relationship to the person they believed to be the source of their infection ('source person'). The majority (66.1%) ascribed their infection to a casual partner. A further 23.3% ascribed their infection to a non-committed and non-romantic partner (or 'fuckbuddy'). Only 10.6% believed they had acquired their HIV from a 'boyfriend' in the context of a committed romantic relationship, and 51.7% of these occurred within the first 3 months following their first sexual contact. Most men (61.5%) believed they had acquired their HIV infection on the first occasion they had sex with the source person. In the Australian context, negotiated safety appears to have minimised infections between regular partners. However, many HIV infections between regular partners may not be in the context of a romantic committed relationship, and yet this distinction between types of regular partners has been all but ignored. Furthermore, in this sample, most infections occurred on the occasion of first meeting, suggesting that the most useful indicators of risk may be the characteristics, contexts, and lengths of sexual partnerships and how sex is negotiated, rather than how GBM categorize their partner. Findings suggest more new HIV infections occur in new partnerships, than in established relationships.

Pair formation models for sexually transmitted infections: A primer

For modelling sexually transmitted infections, duration of partnerships can strongly influence the transmission dynamics of the infection. If partnerships are monogamous, pairs of susceptible individuals are protected from becoming infected, while pairs of infected individuals delay onward transmission of the infection as long as they persist. In addition, for curable infections re-infection from an infected partner may occur. Furthermore, interventions based on contact tracing rely on the possibility of identifying and treating partners of infected individuals. To reflect these features in a mathematical model, pair formation models were introduced to mathematical epidemiology in the 1980's. They have since been developed into a widely used tool in modelling sexually transmitted infections and the impact of interventions. Here we give a basic introduction to the concepts of pair formation models for a susceptible-infected-susceptible (SIS) epidemic. We review some results and applications of pair formation models mainly in the context of chlamydia infection.

Mining for pairs: shared clinic visit dates identify steady HIV-positive partnerships

OBJECTIVES

Here we examined the hypothesis that some stable HIV-infected partnerships can be found in cohort studies, as the patients frequently attend the clinic visits together.

METHODS

Using mathematical approximations and shuffling to derive the probabilities of sharing a given number of visits by chance, we identified and validated couples that may represent either transmission pairs or serosorting couples in a stable relationship.

RESULTS

We analysed 434 432 visits for 16 139 Swiss HIV Cohort Study patients from 1990 to 2014. For 89 pairs, the number of shared visits exceeded the number expected. Of these, 33 transmission pairs were confirmed on the basis of three criteria: an extensive phylogenetic tree, a self-reported steady HIV-positive partnership, and risk group affiliation. Notably, 12 of the validated transmission pairs (36%; 12 of 33) were of a mixed ethnicity with a large median age gap [17.5 years; interquartile range (IQR) 11.8-22 years] and these patients harboured HIV-1 of predominantly non-B subtypes, suggesting imported infections.

CONCLUSIONS

In the context of the surge in research interest in HIV transmission pairs, this simple method widens the horizons of research on within-pair quasi-species exchange, transmitted drug resistance and viral recombination at the biological level and targeted prevention at the public health level.

Optimizing HIV pre-exposure prophylaxis implementation among men who have sex with men in a large urban centre: a dynamic modelling study

INTRODUCTION

Once-daily tenofovir/emtricitabine-based pre-exposure prophylaxis (PrEP) can reduce HIV acquisition in men who have sex with men (MSM), by 44% in the iPrEx trial, and reaching up to 99% with high adherence. We examined the potential population-level impact and cost-effectiveness of different PrEP implementation strategies.

METHODS

We developed a dynamic, stochastic compartmental model of HIV transmission among the estimated 57,400 MSM in Toronto, Canada. Parameterization was performed using local epidemiologic data. Strategies examined included (1) uniform PrEP delivery versus targeting the highest risk decile of MSM (with varying coverage proportions); (2) increasing PrEP efficacy as a surrogate of adherence (44% to 99%); and (3) varying HIV test frequency (once monthly to once yearly). Outcomes included HIV infections averted and the incremental cost ($CAD) per incremental quality-adjusted-life-year (QALY) gained over 20 years.

RESULTS

Use of PrEP among all HIV-uninfected MSM at 25, 50, 75 and 100% coverage prevented 1970, 3427, 4317, and 4581 infections, respectively, with cost/QALY increasing from $500,000 to $800,000 CAD. Targeted PrEP for the highest risk MSM at 25, 50, 75 and 100% coverage prevented 1166, 2154, 2816, and 3012 infections, respectively, with cost/QALY ranging from $35,000 to $70,000 CAD. Maximizing PrEP efficacy, in a scenario of 25% coverage of high-risk MSM with PrEP, prevented 1540 infections with a cost/QALY of $15,000 CAD. HIV testing alone (Q3 months) averted 898 of infections with a cost savings of $4,000 CAD per QALY.

CONCLUSIONS

The optimal implementation strategy for PrEP over the next 20 years at this urban centre is to target high-risk MSM and to maximize efficacy by supporting PrEP adherence. A large health benefit of PrEP implementation could come from engaging undiagnosed HIV-infected individuals into care.

The Meaning of 'Regular Partner' in HIV Research Among Gay and Bisexual Men: Implications of an Australian Cross-Sectional Survey

Estimates of the proportion of HIV infections coming from within regular sexual relationships among gay and bisexual men (GBM) vary widely. Research surveys use various partner type categories, but there is little understanding of how men classify their partners. We conducted an online cross-sectional survey of Australian GBM exploring sexual relationships, including 2057 men reporting on 2566 regular partnerships. Just over half of the partnerships were considered 'relationships', while the remainder were non-romantic 'fuckbuddy'-style arrangements. In multivariable analysis, factors associated with considering the partnership a 'relationship' were: using a 'romantic' descriptor, partnership length, monogamous agreements, any condomless anal sex with each other, love, and commitment. The category of 'regular partner' can mask diverse partnership types, which have different meanings to GBM, associated behaviours, and HIV risks. Certain HIV prevention techniques may be more suited to particular types of partnerships. 'Fuckbuddy' arrangements need to be more explicitly acknowledged in HIV prevention.

Factors associated with unprotected anal sex with multiple non-steady partners in the past 12 months: results from the European Men-Who-Have-Sex-With-Men Internet Survey (EMIS 2010)

BACKGROUND

Practising unprotected anal intercourse (UAI) with high numbers of partners is associated with increased risk for acquiring and transmitting HIV and other sexually transmitted infections. Our aim was to describe factors associated with UAI with multiple partners in a large sample of MSM from 38 European countries recruited for an online survey in 2010.

METHODS

Data are from the European Men-Who-Have-Sex-With-Men Internet Survey (EMIS). The analysis was restricted to men who reported any anal sex with a non-steady partner in the past 12 months, and who were either never diagnosed with HIV, or who had been diagnosed with HIV more than 12 months ago, reported a detectable viral load and did not exclusively serosort (n = 91,477). Multivariable logistic regression was used to compare men reporting UAI with four or more (4+) non-steady partners to two comparison groups: a) no UAI with non-steady partners, and b) UAI with 1-3 non-steady partners.

RESULTS

Overall, 9.6% of the study population reported UAI with 4+ partners in the past 12 months. In both models, factors consistently associated with this behaviour were: having been diagnosed with HIV, lower educational levels, use of nitrite inhalants, drugs associated with sex and parties, or erectile dysfunction drugs in the past 4 weeks, using sex-on-site venues in the past 4 weeks, buying or selling sex in the past 12 months, having experienced physical violence due to sexual attraction to men in the past 12 months, reporting sexual happiness, being out to all or almost all of one's acquaintances, and knowing that ART reduces HIV transmissibility.

CONCLUSIONS

Effective antiretroviral treatment drastically reduces HIV transmission for men diagnosed with HIV, irrespective of partner numbers. Apart from reducing partner numbers or increasing condom use no other recommendations are currently in place to reduce the risk of HIV acquisition and onward transmission for HIV-negative men practicing UAI with multiple partners. A range of factors were identified as associated with UAI with four or more partners which allow the strengthening and targeting of prevention strategies to reduce HIV transmission risks resulting from condomless anal intercourse with multiple partners.

Factors Associated to a Reactive Result of Rapid-HIV Test in Socio-culturally Adapted Services in Primary Care in Spain

The Services of Prevention and Early diagnosis of HIV in Madrid (Spain) are set in selected primary care centers. Cultural mediators targeted to vulnerable groups (economic immigrants, MSM, sex workers…) perform risk assessment and counselling. Between 2010 and 2014 they performed 6 039 rapid-HIV test, 27.8 % in MSM, 41.2 % in men who have sex exclusively with women (MSW) and 31.0 % in women; 35.7 % in immigrants, mainly from Latin America. A reactive result was more common among MSM (6.0 %) compared to women (0.6 %) and MSW (0.5 %). In MSM it was associated to being immigrant and to antecedents of sexually transmitted infections (STI). Among MSW the factors associated to a reactive result were: seropositivity of sexual partner and heroine consumption, and in women: infrequent use of condoms, seropositivity of sexual partner and antecedents of STI. Preventive interventions to reduce risk of HIV transmission and for early detection should be adapted and targeted to high risk population.

Concurrency can drive an HIV epidemic by moving R0 across the epidemic threshold

OBJECTIVE

The objective of this study is to investigate whether concurrency can drive an HIV epidemic by moving R0 across the epidemic threshold.

DESIGN AND METHODS

We use a mathematical framework for a dynamic partnership network and the spread of a one-stage infection to study how concurrency is related to the basic reproduction number R0. Two concurrency indices were used to measure the level of concurrency. The model allows varying the level of concurrency in the population, while other key network properties such as partnership duration and lifetime number of partners are kept fixed. In this way, the effect of concurrency on R0 is investigated as an isolated phenomenon.

RESULTS

We find that an increase in concurrency is associated with an increase of R0. For plausible parameter sets for MSM populations, R0 is always above the epidemic threshold of 1. For scenarios that are plausible for sub-Saharan African populations, we show that increasing the level of concurrency can lead to R0 crossing the epidemic threshold. This occurs already at low levels of concurrency. Only a slight shift of the network structure from a purely monogamous population to one wherein individuals are allowed to have at most two partners is enough for this to happen.

CONCLUSION

Concurrency can be a driver of an HIV epidemic in sub-Saharan Africa for low levels of concurrency, although it is not decisive in MSM populations. A small increase in the level of concurrency can lead to R0 crossing the epidemic threshold in a sub-Saharan African setting.

Incident risk factors as predictors of HIV seroconversion in the Lisbon cohort of men who have sex with men: first results, 2011-2014

HIV incidence in men who have sex with men (MSM) is increasing in western countries, including Portugal. We aimed to estimate HIV incidence and to assess how individual short-term changes in exposures over time predict seroconversion. We evaluated participants of an open cohort of HIV-negative MSM enrolled after testing at a community-based voluntary HIV counselling and testing centre in Lisbon. At each evaluation a structured questionnaire was completed and HIV status was ascertained using rapid followed by confirmatory testing. Between April 2011 and February 2014, 804 MSM were followed for a total of 893 person-years. Predictors of HIV seroconversion were identified using Poisson generalised linear regression. The overall seroincidence was 2.80/100 person-years (95% confidence interval: 1.89-4.14). Men who seroconverted had a higher mean number of tests per year. Seroconversions were significantly associated with partner disclosure of HIV status during follow-up, newly-adopted unprotected anal intercourse (UAI) with a steady partner and being newly-diagnosed with syphilis during follow-up. Likewise, sexual intercourse with HIV-positive men, having an HIV-positive steady partner at least once during follow-up and persistent UAI with occasional partners were predictors of seroconversion. High HIV incidence in this cohort is likely driven by short-term contextual and behavioural changes during follow-up.

Modelling the HIV epidemic among MSM in the United Kingdom: quantifying the contributions to HIV transmission to better inform prevention initiatives

OBJECTIVES

HIV is a major public health problem among MSM in the United Kingdom with around 2400 new infections annually. We quantified the contribution of biological and behavioural factors.

DESIGN

Modelling study.

METHODS

A partnership-based model of HIV transmission among UK MSM aged 15-64 years was developed and calibrated to time series HIV prevalence. The calibration was validated using multiple surveillance datasets. Population-attributable fractions were used to estimate the contribution of behavioural and biological factors to HIV transmission over the period 2001-2002, 2014-2015, and 2019-2020.

RESULTS

The contribution of most biological and behavioural factors was relatively constant over time, with the key group sustaining HIV transmission being higher-sexual activity MSM aged below 35 years living with undiagnosed HIV. The effect of primary HIV infection was relatively small with 2014-2015 population-attributable fraction of 10% (3-28%) in comparison with other subsequent asymptomatic stages. Diagnosed men who were not on antiretroviral therapy (ART) currently contributed 26% (14-39%) of net infections, whereas ART-treated MSM accounted for 17% (10-24%). A considerable number of new infections are also likely to occur within long-term relationships.

CONCLUSION

The majority of the new HIV infections among MSM in the United Kingdom during 2001-2020 is expected to be accounted for by a small group of younger and highly sexually active individuals, living with undiagnosed HIV in the asymptomatic stage. Bringing this group into HIV/AIDS care by improving testing uptake is a vital step for preventing onward transmission and will determine the success of using ART as prevention.

"Same-sex relationship in a straight world": individual and societal influences on power and control in young men's relationships

Young men who have sex with men (YMSM) continue to experience higher rates of HIV infection than other populations. Recently, there have been recommendations to consider HIV prevention at the dyadic or couple level. Using a dyadic approach to HIV prevention would also address an unaddressed concern related to intimate partner violence (IPV) among YMSM. Although research on IPV among YMSM is still in its infancy, great strides have been made in the past 10 years to describe the prevalence and related correlates of IPV within older adult same-sex relationships. These studies have found rates of IPV among MSM to be similar to rates among heterosexual women, and to be on the rise. The present study is designed to provide insight into how power is conceptualized within YMSM relationships and the role it may play in relationship challenges. This study draws from qualitative data collected from 11 focus groups with 86 YMSM and 26 individual semi-structured interviews to understand relationship challenges and the experiences of YMSM involved in partner violence. YMSM described relationship power as stemming from numerous sources including sexual positioning, gender roles, education, income, prior relationship experiences, and internalized homophobia. The findings have a number of implications for service providers and program design. Interventionists and other researchers need to consider power dynamics and other contextual elements of IPV before effective interventions can be developed for YMSM and other sexual minority populations.

HIV treatment as prevention: contradictory perspectives from dynamic mathematical models

The preventative effects of antiretroviral therapy for people with HIV have been debated since they were first raised. Models commenced studying the preventive effects of treatment in the 1990s, prior to initial public reports. However, the outcomes of the preventive effects of antiretroviral use were not consistent. Some outcomes of dynamic models were based on unfeasible assumptions, such as no consideration of drug resistance, behavior disinhibition, or economic inputs in poor countries, and unrealistic input variables, for example, overstated initiation time, adherence, coverage, and efficacy of treatment. This paper reviewed dynamic mathematical models to ascertain the complex effects of ART on HIV transmission. This review discusses more conservative inputs and outcomes relative to antiretroviral use in HIV infections in dynamic mathematical models. ART alone cannot eliminate HIV transmission.

Clinical effectiveness and cost-effectiveness of HIV pre-exposure prophylaxis in men who have sex with men: risk calculators for real-world decision-making

BACKGROUND

Oral pre-exposure prophylaxis (PrEP) can be clinically effective and cost-effective for HIV prevention in high-risk men who have sex with men (MSM). However, individual patients have different risk profiles, real-world populations vary, and no practical tools exist to guide clinical decisions or public health strategies. We introduce a practical model of HIV acquisition, including both a personalized risk calculator for clinical management and a cost-effectiveness calculator for population-level decisions.

METHODS

We developed a decision-analytic model of PrEP for MSM. The primary clinical effectiveness and cost-effectiveness outcomes were the number needed to treat (NNT) to prevent one HIV infection, and the cost per quality-adjusted life-year (QALY) gained. We characterized patients according to risk factors including PrEP adherence, condom use, sexual frequency, background HIV prevalence and antiretroviral therapy use.

RESULTS

With standard PrEP adherence and national epidemiologic parameters, the estimated NNT was 64 (95% uncertainty range: 26, 176) at a cost of $160,000 (cost saving, $740,000) per QALY--comparable to other published models. With high (35%) HIV prevalence, the NNT was 35 (21, 57), and cost per QALY was $27,000 (cost saving, $160,000), and with high PrEP adherence, the NNT was 30 (14, 69), and cost per QALY was $3,000 (cost saving, $200,000). In contrast, for monogamous, serodiscordant relationships with partner antiretroviral therapy use, the NNT was 90 (39, 157) and cost per QALY was $280,000 ($14,000, $670,000).

CONCLUSIONS

PrEP results vary widely across individuals and populations. Risk calculators may aid in patient education, clinical decision-making, and cost-effectiveness evaluation.

SI infection on a dynamic partnership network: characterization of R0

We model the spread of an SI (Susceptible → Infectious) sexually transmitted infection on a dynamic homosexual network. The network consists of individuals with a dynamically varying number of partners. There is demographic turnover due to individuals entering the population at a constant rate and leaving the population after an exponentially distributed time. Infection is transmitted in partnerships between susceptible and infected individuals. We assume that the state of an individual in this structured population is specified by its disease status and its numbers of susceptible and infected partners. Therefore the state of an individual changes through partnership dynamics and transmission of infection. We assume that an individual has precisely n 'sites' at which a partner can be bound, all of which behave independently from one another as far as forming and dissolving partnerships are concerned. The population level dynamics of partnerships and disease transmission can be described by a set of (n +1)(n +2) differential equations. We characterize the basic reproduction ratio R0 using the next-generation-matrix method. Using the interpretation of R0 we show that we can reduce the number of states-at-infection n to only considering three states-at-infection. This means that the stability analysis of the disease-free steady state of an (n +1)(n +2)-dimensional system is reduced to determining the dominant eigenvalue of a 3 × 3 matrix. We then show that a further reduction to a 2 × 2 matrix is possible where all matrix entries are in explicit form. This implies that an explicit expression for R0 can be found for every value of n.

The syndemic of AIDS and STDS among MSM

The spread of HIV and other STDs among men who have sex with men (MSM) has been labeled a syndemic because in this population a number of different and interrelated health problems have come together and interact with one another. The various elements of the syndemic have an additive effect, each one intensifying the others. These factors include the number of infectious diseases endemic in this population, the high rate of substance abuse problems and psychological disorders, and the significant percentage of MSM who have experienced childhood sexual abuse and other adverse events. While MSM are disproportionately affected by HIV, syphilis, and other STDs, health activists from the gay community have systematically resisted the application of the full range of public health strategies traditionally used to prevent their spread. In the more than three decades since the beginning of the HIV/AIDS epidemic, there have been substantial advances in testing and treatment, yet the infection rate among MSM, and particularly young MSM, remains high, even as it has been dropping among other risk groups. This paper deals with the history of the syndemic, the failure of various risk reduction strategies, and treatment as prevention.

Strong propensity for HIV transmission among men who have sex with men in Vietnam: behavioural data and sexual network modelling

OBJECTIVES

Survey data from men who have sex with men (MSM) in Asian cities indicate ongoing and drastic increases in HIV prevalence. It is unknown which behavioural factors are most important in driving these epidemics. We aimed to analyse detailed sexual behaviour data among MSM in Vietnam and to model HIV transmission using improved assumptions on sexual network structure.

SETTING

Vietnam.

PARTICIPANTS

Internet-using men who had ever had sex (any type) with a man, aged ≥18 years and living in Vietnam. The study was cross-sectional, population-based and performed in 2012, using online respondent-driven sampling. The Internet-based survey instrument was completed by 982 participants, of which 857 were eligible. Questions included sociodemography and retrospective sexual behaviour, including number of unprotected anal sex (UAS) acts per partner.

PRIMARY AND SECONDARY OUTCOME MEASURES

Estimated basic reproductive number over 3 months as a function of transmission risk per UAS act; frequency distributions of number of UAS partners and UAS acts during last 3 months.

RESULTS

36% (CI 32% to 42%) reported UAS at least once during the last 3 months. 36% (CI 32% to 41%) had ever taken an HIV test and received the result. UAS partner numbers and number of UAS acts were both highly skewed and positively correlated. Using a weighted configuration model, taking into account partner numbers, frequency of UAS and their correlations, we estimated the basic reproductive number (R0) over 3 months. The results indicated rapid transmission over a wide range of values of per-act transmissibility.

CONCLUSIONS

Men with multiple partners had unexpectedly high UAS frequency per partner, paired with low HIV testing rates. The study highlights the importance of collecting data on frequency of UAS acts and indicates the need to rapidly scale-up HIV prevention services and testing opportunities for MSM in Vietnam.

Gay male couples' attitudes toward using couples-based voluntary HIV counseling and testing

Many men who have sex with men acquire HIV from their primary male partners while in a relationship. Studies with gay couples have demonstrated that relationship characteristics and testing behaviors are important to examine for HIV prevention. Recently, couples-based voluntary HIV counseling and testing (CVCT) has become available to male couples throughout the U.S. However, HIV-negative couples' attitudes toward using CVCT and how their relationship characteristics may affect their use of CVCT remain largely unknown. This information is particularly relevant for organizations that offer CVCT. To assess couples' attitudes, and associated factors toward using CVCT, a cross-sectional study design was used with a novel Internet-based recruitment method to collect dyadic data from a national sample of 275 HIV-negative gay couples. Multivariate multilevel modeling was used to identify factors associated with differences between and within couples about their attitudes towards using CVCT. Findings revealed that couples were "somewhat" to "very likely" to use CVCT. More positive attitudes toward using CVCT were associated with couples who had higher levels of relationship satisfaction and commitment toward their sexual agreement and among those who had at least one partner having had sex outside of the relationship. Less positive attitude toward using CVCT was associated with couples who had higher levels of trust toward their partners being dependable. Differences within couples, including age between partners, whether sex had occurred outside of the relationship, and value toward a sexual agreement also affected their attitudes toward using CVCT. Providing additional testing methods may help HIV-negative gay couples better manage their HIV risk.

Actor-partner effects of demographic and relationship factors associated with HIV risk within gay male couples

Recent research has investigated the association of relationship factors and dynamics with sexual behaviors and HIV risk among gay male couples. However, few studies with gay male couples have used the Actor-Partner Interdependence Model framework to examine whether factors influence an individual and his partner's sexual risk behaviors. None of these studies analyzed whether relationship factors had influenced the sexual risk behaviors of both partners within the couple. Our cross-sectional study used dyadic data from 142 gay male couples to assess actor-partner effects of relationship commitment, trust, and investment in one's sexual agreement for HIV risk. Multilevel modeling was used to examine which actor-partner effects of these factors were predictive of individuals and their partners having had UAI within and outside the relationship. Results indicated that participants' likelihood of having had UAI within and outside of the relationship significantly decreased with: (1) actor effects of value in and commitment to a sexual agreement, and quality of alternatives to the relationship and (2) partner effects of participant's age, dependability of trust, quality of alternatives to the relationship, and investment of relationship commitment. No significant actor-partner effects were detected for having had UAI within the relationship. Our findings suggest that future HIV prevention strategies should take into account how relationship factors influence an individual and his main partners' sexual risk behaviors and in turn, the couple's risk for HIV. However, more research is needed to examine how actor-partner effects of relationship factors influence a variety of sexual risk behaviors within gay male couples.

Distinguishing sources of HIV transmission from the distribution of newly acquired HIV infections: why is it important for HIV prevention planning?

OBJECTIVE

The term 'source of HIV infections' has been referred to as the source of HIV transmission. It has also been interpreted as the distribution of newly acquired HIV infections across subgroups. We illustrate the importance of distinguishing the two interpretations for HIV prevention planning.

METHODS

We used a dynamical model of heterosexual HIV transmission to simulate three HIV epidemics, and estimated the sources of HIV transmission (cumulative population attributable fraction) and the single-year distribution of new HIV infections. We focused an intervention guided by the largest transmission source versus the largest single-year distribution of new HIV infections, and compared the fraction of discounted HIV infections averted over 30 years.

RESULTS

The single-year distribution of newly acquired HIV infections underestimated the source of HIV transmission in the long term, when the source was unprotected sex in high-risk groups. Under equivalent and finite resources, an intervention strategy directed by the long-term transmission source was shown to achieve a greater impact than a distribution-directed strategy, particularly in the long term.

CONCLUSIONS

Impact of HIV prevention strategies may vary depending on whether they are directed by the long-term transmission source or by the distribution of new HIV infections. Caution is required when interpreting the 'source of HIV infections' to avoid misusing the distribution of new HIV infections in HIV prevention planning.

Modelling the impact of chlamydia screening on the transmission of HIV among men who have sex with men

BACKGROUND

Recent studies have found high prevalences of asymptomatic rectal chlamydia among HIV-infected men who have sex with men (MSM). Chlamydia could increase the infectivity of HIV and the susceptibility to HIV infection. We investigate the role of chlamydia in the spread of HIV among MSM and the possible impact of routine chlamydia screening among HIV-infected MSM at HIV treatment centres on the incidence of chlamydia and HIV in the overall MSM population.

METHODS

A mathematical model was developed to describe the transmission of HIV and chlamydia among MSM. Parameters relating to sexual behaviour were estimated from data from the Amsterdam Cohort Study among MSM. Uncertainty analysis was carried out for model parameters without confident estimates. The effects of different screening strategies for chlamydia were investigated.

RESULTS

Among all new HIV infections in MSM, 15% can be attributed to chlamydia infection. Introduction of routine chlamydia screening every six months among HIV-infected MSM during regular HIV consultations can reduce the incidence of both infections among MSM: after 10 years, the relative percentage reduction in chlamydia incidence would be 15% and in HIV incidence 4%, compared to the current situation. Chlamydia screening is more effective in reducing HIV incidence with more frequent screening and with higher participation of the most risky MSM in the screening program.

CONCLUSIONS

Chlamydia infection could contribute to the transmission of HIV among MSM. Preventive measures reducing chlamydia prevalence, such as routine chlamydia screening of HIV-infected MSM, can result in a decline in the incidence of chlamydia and HIV.

Prospects of elimination of HIV with test-and-treat strategy

Recently, there has been much debate about the prospects of eliminating HIV from high endemic countries by a test-and-treat strategy. This strategy entails regular HIV testing in the entire population and starting antiretroviral treatment immediately in all who are found to be HIV infected. We present the concept of the elimination threshold and investigate under what conditions of treatment uptake and dropout elimination of HIV is feasible. We used a deterministic model incorporating an accurate description of disease progression and variable infectivity. We derived explicit expressions for the basic reproduction number and the elimination threshold. Using estimates of exponential growth rates of HIV during the initial phase of epidemics, we investigated for which populations elimination is within reach. The concept of the elimination threshold allows an assessment of the prospects of elimination of HIV from information in the early phase of the epidemic. The relative elimination threshold quantifies prospects of elimination independently of the details of the transmission dynamics. Elimination of HIV by test-and-treat is only feasible for populations with very low reproduction numbers or if the reproduction number is lowered significantly as a result of additional interventions. Allowing low infectiousness during primary infection, the likelihood of elimination becomes somewhat higher. The elimination threshold is a powerful tool for assessing prospects of elimination from available data on epidemic growth rates of HIV. Empirical estimates of the epidemic growth rate from phylogenetic studies were used to assess the potential for elimination in specific populations.

Random modelling of contagious diseases

Modelling contagious diseases needs to include a mechanistic knowledge about contacts between hosts and pathogens as specific as possible, e.g., by incorporating in the model information about social networks through which the disease spreads. The unknown part concerning the contact mechanism can be modelled using a stochastic approach. For that purpose, we revisit SIR models by introducing first a microscopic stochastic version of the contacts between individuals of different populations (namely Susceptible, Infective and Recovering), then by adding a random perturbation in the vicinity of the endemic fixed point of the SIR model and eventually by introducing the definition of various types of random social networks. We propose as example of application to contagious diseases the HIV, and we show that a micro-simulation of individual based modelling (IBM) type can reproduce the current stable incidence of the HIV epidemic in a population of HIV-positive men having sex with men (MSM).

Patterns of HIV and sexually transmitted infection testing among men who have sex with men couples in the United States

BACKGROUND

Most men who have sex with men (MSM) within the United States acquire human immunodeficiency virus (HIV) while in a same-sex relationship. Few studies have examined HIV and sexually transmitted infection (STI) testing rates among MSM couples. Interestingly, the patterns that MSM test for HIV while in their relationships remain largely unknown. The current study helps fill this gap in knowledge by assessing HIV testing patterns and HIV and STI testing rates from a large convenience sample of Internet-using MSM couples.

METHODS

The current study used a cross-sectional study design to collect dyadic data from 361 MSM couples who lived throughout the United States. A novel recruitment strategy that included placing paid targeted advertisements on Facebook enrolled both men in the couple to independently complete the confidential electronic survey.

RESULTS

Nearly half of the HIV-negative men indicated either not having been tested for HIV since their relationship started or only testing if they believed they were at risk. Few men reported testing every 3 to 4 months. HIV/STI testing rates varied among the sample of couples. Few men reported having been diagnosed with a recent STI. Testing patterns and rates were mostly similar, irrespective of whether unprotected anal intercourse was practiced within and/or outside the relationship.

CONCLUSIONS

HIV testing and prevention services must target men who are at risk for acquiring HIV within MSM couples. To help accomplish this goal, additional research is needed to examine the specific barriers and facilitators to HIV and STI testing among MSM in couples.

Measuring and modelling concurrency

This article explores three critical topics discussed in the recent debate over concurrency (overlapping sexual partnerships): measurement of the prevalence of concurrency, mathematical modelling of concurrency and HIV epidemic dynamics, and measuring the correlation between HIV and concurrency. The focus of the article is the concurrency hypothesis - the proposition that presumed high prevalence of concurrency explains sub-Saharan Africa's exceptionally high HIV prevalence. Recent surveys using improved questionnaire design show reported concurrency ranging from 0.8% to 7.6% in the region. Even after adjusting for plausible levels of reporting errors, appropriately parameterized sexual network models of HIV epidemics do not generate sustainable epidemic trajectories (avoid epidemic extinction) at levels of concurrency found in recent surveys in sub-Saharan Africa. Efforts to support the concurrency hypothesis with a statistical correlation between HIV incidence and concurrency prevalence are not yet successful. Two decades of efforts to find evidence in support of the concurrency hypothesis have failed to build a convincing case.

What drives the US and Peruvian HIV epidemics in men who have sex with men (MSM)?

In this work, we estimate the proportions of transmissions occurring in main vs. casual partnerships, and by the sexual role, infection stage, and testing and treatment history of the infected partner, for men who have sex with men (MSM) in the US and Peru. We use dynamic, stochastic models based in exponential random graph models (ERGMs), obtaining inputs from multiple large-scale MSM surveys. Parallel main partnership and casual sexual networks are simulated. Each man is characterized by age, race, circumcision status, sexual role behavior, and propensity for unprotected anal intercourse (UAI); his history is modeled from entry into the adult population, with potential transitions including HIV infection, detection, treatment, AIDS diagnosis, and death. We implemented two model variants differing in assumptions about acute infectiousness, and assessed sensitivity to other key inputs. Our two models suggested that only 4–5% (Model 1) or 22–29% (Model 2) of HIV transmission results from contacts with acute-stage partners; the plurality (80–81% and 49%, respectively) stem from chronic-stage partners and the remainder (14–16% and 27–35%, respectively) from AIDS-stage partners. Similar proportions of infections stem from partners whose infection is undiagnosed (24–31%), diagnosed but untreated (36–46%), and currently being treated (30–36%). Roughly one-third of infections (32–39%) occur within main partnerships. Results by country were qualitatively similar, despite key behavioral differences; one exception was that transmission from the receptive to insertive partner appears more important in Peru (34%) than the US (21%). The broad balance in transmission contexts suggests that education about risk, careful assessment, pre-exposure prophylaxis, more frequent testing, earlier treatment, and risk-reduction, disclosure, and adherence counseling may all contribute substantially to reducing the HIV incidence among MSM in the US and Peru.

Rates of testing for HIV in the presence of serodiscordant UAI among HIV-negative gay men in committed relationships

We examined testing rates for HIV-negative men (N = 752) from a sample of gay male couples. Approximately half (52 %) tested in the past year. Among men who had engaged in sexual risk behavior in the past 3 months, 27 % tested within that period and 65 % within the past year. For men in concordant relationships these rates were 25 and 60 %, for men in serodiscordant relationships they were 34 and 72 %. MSM in primary relationships are testing at lower rates than the general MSM population, even after potential exposure to HIV. Testing and prevention messages for MSM should factor in relationship status.