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

Dilemma of concepts and strategies for the prevention of spread of HIV in relation to human behavior, law and human rights

The new prevalence data regarding the estimated global number of human immunodeficiency virus positive (HIV+) cases, i.e., including people who are either aware or unaware of their HIV infection in 2010, lead many to wonder why the increase in incidence has reached today's unprecedented level and escalated within such a short time. This, in spite of prevention campaigns in countries affected by HIV/acquired immune deficiency syndrome (AIDS) with their urgent messages aimed at preventing HIV transmission by promoting changes in individual's behavior. This article analyzes the background of the prevention strategies, in particular their political, social and legal concepts in terms of human rights, and reveals traits of human behavior not considered thus far. A radical reappraisal is necessary, at social and legislative levels, as well as options additional to current concepts. When ethical issues come up, they become blamed for outmoded moralistic positions. However, ignoring the reality has led to dire consequences from prioritizing individual human rights over society's collective need to prevent the spread of HIV.

Sexual behavior and HIV transmission risk of Ugandan adults taking antiretroviral therapy: 3 year follow-up

BACKGROUND

Long-term impact of antiretroviral therapy (ART) on sexual HIV-transmission risk in Africa is unknown. We assessed sexual behavior changes and estimated HIV transmission from HIV-infected adults on ART in Uganda.

METHODS

Between 2003 and 2007, we enrolled and followed ART-naive HIV-infected adults in a home-based AIDS program with annual counseling and testing for cohabitating partners, participant transmission risk-reduction plans, condom distribution and prevention support for cohabitating discordant couples. We assessed participants' HIV plasma viral load and partner-specific sexual behaviors. We defined risky sex as intercourse with inconsistent/no condom use with HIV-negative or unknown serostatus partners in previous 3 months. We compared rates using Poisson regression models, estimated transmission risk using established viral load-specific transmission estimates, and documented sero-conversion rates among HIV-discordant couples.

RESULTS

Of 928 participants, 755 (81%) had 36 months data: 94 (10%) died and 79 (9%) missing data. Sexual activity increased from 28% (baseline) to 41% [36 months (P < 0.001)]. Of sexually active participants, 22% reported risky sex at baseline, 8% at 6 months (P < 0.001), and 14% at 36 months (P = 0.018). Median viral load among those reporting risky sex was 122,500 [interquartile range (IQR) 45 100-353 000] copies/ml pre-ART at baseline and undetectable at follow-up. One sero-conversion occurred among 62 cohabitating sero-discordant partners (0.5 sero-conversions/100 person-years). At 36 months, consistent condom use was 74% with discordant partners, 55% with unknown and 46% with concordant partners. Estimated HIV transmission risk reduced 91%, from 47.3 to 4.2/1000 person-years.

CONCLUSIONS

Despite increased sexual activity among HIV-infected Ugandans over 3 years on ART, risky sex and estimated risk of HIV transmission remained lower than baseline levels. Integrated prevention programs could reduce HIV transmission in Africa.

HIV treatment for prevention

"No virus, no transmission." Studies have repeatedly shown that viral load (the quantity of virus present in blood and sexual secretions) is the strongest predictor of HIV transmission during unprotected sex or transmission from infected mother to child.

Effective treatment lowers viral load to undetectable levels. If one could identify and treat all HIV-infected people immediately after infection, the HIV/AIDS epidemic would eventually disappear.

Such a radical solution is currently unrealistic. In reality, not all people get tested, especially when they fear stigma and discrimination. Thus, not all HIV-infected individuals are known. Of those HIV-positive individuals for whom the diagnosis is known, not all of them have access to therapy, agree to be treated, or are taking therapy effectively. Some on effective treatment will stop, and in others, the development of resistance will lead to treatment failure. Furthermore, resources are limited: should we provide drugs to asymptomatic HIV-infected individuals without indication for treatment according to guidelines in order to prevent HIV transmission at the risk of diverting funding from sick patients in urgent need?

In fact, the preventive potential of anti-HIV drugs is unknown. Modellers have tried to fill the gap, but models differ depending on assumptions that are strongly debated. Further, indications for antiretroviral treatments expand; in places like Vancouver and San Francisco, the majority of HIV-positive individuals are now under treatment, and the incidence of new HIV infections has recently fallen. However, correlation does not necessarily imply causation. Finally, studies in couples where one partner is HIV-infected also appear to show that treatment reduces the risk of transmission.

More definite studies, where a number of communities are randomized to either receive the "test-and-treat" approach or continue as before, are now in evaluation by funding agencies. Repeated waves of testing would precisely measure the incidence of HIV infection. Such trials face formidable logistical, practical and ethical obstacles. However, without definitive data, the intuitive appeal of "test-and-treat" is unlikely to translate into action on a global scale. In the meantime, based on the available evidence, we must strive to provide treatment to all those in medical need under the current medical guidelines. This will lead to a decrease in HIV transmission while "test-and-treat" is fully explored in prospective clinical trials.

Working Group on Estimation of HIV Prevalence in Europe. HIV in hiding: methods and data requirements for the estimation of the number of people living with undiagnosed HIV

Many people who are HIV positive are unaware of their infection status. Estimation of the number of people with undiagnosed HIV within a country or region is vital for understanding future need for treatment and for motivating testing programs. We review the available estimation approaches which are in current use. They can be broadly classified into those based on prevalence surveys and those based on reported HIV and AIDS cases. Estimation based on prevalence data requires data from regular prevalence surveys in different population groups together with estimates of the size of these groups. The recommended minimal case reporting data needed to estimate the number of patients with undiagnosed HIV are HIV diagnoses, including CD4 count at diagnosis and whether there has been an AIDS diagnosis in the 3 months before or after HIV diagnosis, and data on deaths in people with HIV. We would encourage all countries to implement several methods that will help develop our understanding of strengths and weaknesses of the various methods.

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

Background

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

Methods

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

Results

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

Conclusion

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Clinical significance of transient HIV type-1 viraemia and treatment interruptions during suppressive antiretroviral treatment

BACKGROUND

Transient episodes of HIV type-1 viraemia are frequently observed in patients on suppressive combination antiretroviral therapy (cART). We studied the effect of such episodes and of treatment interruptions on clinical outcome and immunological response.

METHODS

A total of 3,321 patients from the ATHENA cohort had virological suppression (HIV type-1 RNA<50 copies/ml) after 24 weeks of cART. The association between subsequent episodes of treatment interruptions, viral suppression, low-level (50-400 copies/ml) and high-level (>400 copies/ml) viraemia and the outcomes death, AIDS or immunological response (CD4(+) T-cell count increase > or =50% from 24 weeks) was studied with Poisson regression models, including either time-updated cumulative follow-up, time spent per type of episode or modelling episodes as binary status indicators.

RESULTS

During 11,165 person-years of follow-up, 88 patients died, 111 developed AIDS and 2,019 had an immunological response. Longer follow-up time in treatment interruptions increased the risk of AIDS (relative risk [RR] 8.07, 95% confidence interval [CI] 3.98-16.4 per year longer) and impaired immunological response (RR 0.22, 95% CI 0.12-0.41). High-level viraemia was only associated with immunological response (RR 0.55, 95% CI 0.40-0.74), whereas low-level viraemia was not associated with any of the three outcomes. Status indicator models gave similar results. When also including time-updated CD4(+) T-cell counts, the observed associations diminished.

CONCLUSIONS

Treatment interruptions and high-level, but not low-level, viraemia are strongly associated with clinical outcome, mainly via their effect on CD4(+) T-cell counts.

An overview of the relative risks of different sexual behaviours on HIV transmission

PURPOSE OF REVIEW

Sexual intercourse represents the majority of HIV transmission and is preventable. Overall, the risk of HIV transmission following a single sexual exposure is low especially in comparison with other sexually transmitted infections (STIs), with estimates of the average probability of male-to-female HIV transmission only 0.0005-0.0026 per coital act. The risk of acquiring HIV from a single contact varies enormously and is dependant upon the infectiousness of the HIV-positive individual and the susceptibility to HIV of their sexual partner.

RECENT FINDINGS

Of concern, unprotected sex among men who have sex with men (MSM) has increased in recent years and HIV incidence in both MSM and heterosexuals remains a considerable public health concern. Sexual practices and health optimism about HIV have changed, which have significantly impacted HIV risk behaviour.

SUMMARY

In this review article we summarize the current evidence regarding the observed relative risks of HIV transmission for each different types of sex act, relationship type and the strategies that have been tested to interrupt transmission.

Rethinking prevention of HIV type 1 infection

Research on the prevention of human immunodeficiency virus (HIV)-1 infection is at a critical juncture. Major methodological challenges to performing prevention trials have emerged, and one after another promising biomedical interventions have failed to reduce the incidence of HIV-1 infection. Nevertheless, there is growing optimism that progress can be achieved in the near term. Mathematical modeling indicates that 2 new strategies, "test and treat" and preexposure prophylaxis, could have a major impact on the incidence of HIV-1 infection. Will our hopes be justified? We review the potential strengths and limitations of these antiretroviral "treatment as prevention" strategies and outline other new options for reducing the incidence of HIV-1 infection in the near term. By maximizing the potential of existing interventions, developing other effective strategies, and combining them in an optimal manner, we have the opportunity to bring the HIV-1 epidemic under control.

HIV transmission risk through anal intercourse: systematic review, meta-analysis and implications for HIV prevention

BACKGROUND

The human immunodeficiency virus (HIV) infectiousness of anal intercourse (AI) has not been systematically reviewed, despite its role driving HIV epidemics among men who have sex with men (MSM) and its potential contribution to heterosexual spread. We assessed the per-act and per-partner HIV transmission risk from AI exposure for heterosexuals and MSM and its implications for HIV prevention.

METHODS

Systematic review and meta-analysis of the literature on HIV-1 infectiousness through AI was conducted. PubMed was searched to September 2008. A binomial model explored the individual risk of HIV infection with and without highly active antiretroviral therapy (HAART).

RESULTS

A total of 62,643 titles were searched; four publications reporting per-act and 12 reporting per-partner transmission estimates were included. Overall, random effects model summary estimates were 1.4% [95% confidence interval (CI) 0.2-2.5)] and 40.4% (95% CI 6.0-74.9) for per-act and per-partner unprotected receptive AI (URAI), respectively. There was no significant difference between per-act risks of URAI for heterosexuals and MSM. Per-partner unprotected insertive AI (UIAI) and combined URAI-UIAI risk were 21.7% (95% CI 0.2-43.3) and 39.9% (95% CI 22.5-57.4), respectively, with no available per-act estimates. Per-partner combined URAI-UIAI summary estimates, which adjusted for additional exposures other than AI with a 'main' partner [7.9% (95% CI 1.2-14.5)], were lower than crude (unadjusted) estimates [48.1% (95% CI 35.3-60.8)]. Our modelling demonstrated that it would require unreasonably low numbers of AI HIV exposures per partnership to reconcile the summary per-act and per-partner estimates, suggesting considerable variability in AI infectiousness between and within partnerships over time. AI may substantially increase HIV transmission risk even if the infected partner is receiving HAART; however, predictions are highly sensitive to infectiousness assumptions based on viral load.

CONCLUSIONS

Unprotected AI is a high-risk practice for HIV transmission, probably with substantial variation in infectiousness. The significant heterogeneity between infectiousness estimates means that pooled AI HIV transmission probabilities should be used with caution. Recent reported rises in AI among heterosexuals suggest a greater understanding of the role AI plays in heterosexual sex lives may be increasingly important for HIV prevention.

Determinants of HIV-1 transmission in men who have sex with men: a combined clinical, epidemiological and phylogenetic approach

OBJECTIVES

To identify biological factors associated with HIV transmission in men who have sex with men (MSM).

DESIGN

A longitudinal phylogenetic analysis of HIV-1 from an MSM cohort, incorporating clinical and epidemiological data.

METHODS

Potential individuals were HIV-infected MSM attending a sexual health clinic between 2000 and 2006. Individuals were classified such that they could move from recent to chronic infection categories. HIV-1pol gene sequences were obtained from plasma virus or proviral DNA and clusters estimated by maximum likelihood and conservative genetic distance differences. The single most likely transmitter generating each recent infection was ascertained and risk factors around time of likely transmission explored using Poisson regression modelling.

RESULTS

Out of 1144 HIV-infected MSM, pol sequence data were obtained for 859 (75%); 159 out of 859 (19%) were recently HIV infected at diagnosis. A single most likely transmitter was identified for 41 out of 159 (26%), of which 11 were recently infected (27%) and 30 chronically infected. Factors associated with transmission in multivariable analysis were: younger age {rate ratio per 5 years older 0.68 [95% confidence interval (CI) 0.54-0.86], P=0.0009}, higher viral load [rate ratio per log higher 1.61 (95% CI 1.15-2.25), P=0.005], recent infection [rate ratio 3.88 (95% CI 1.76-8.55), P=0.0008] and recent sexually transmitted disease [rate ratio 5.32 (95% CI 2.51-11.29), P=0.0001]. HAART was highly protective in a univariable model, RR 0.14 (95% CI 0.07-0.27, P=0.0001).

CONCLUSION

Onward transmission of HIV among MSM is significantly associated with recent infection, sexually transmitted diseases and higher viral load, and reduced by effective HAART. The majority of new infections appear to occur from individuals whose infection was undiagnosed at the time of transmission.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Low-level exposure to HIV induces virus-specific T cell responses and immune activation in exposed HIV-seronegative individuals

HIV-specific T cells response and T cell activation are frequently seen in exposed seronegative individuals (ESN). In this study, we report HIV-specific response and level of T cell activation in ESN partners of HIV-infected patients presenting low or undetectable levels of HIV-RNA. We evaluated 24 HIV-serodiscordant couples. ESN were classified into three categories of exposure to HIV (very low, low, and moderate-high), considering levels of HIV-RNA in their infected partner and frequency of sexual high-risk practices within the last 12 mo. HIV-specific T cell responses and activation levels in T cell subsets were evaluated by flow cytometry. We reported that 54% of ESN had detectable HIV-specific T cells response, being the highest prevalence seen in the low exposure group (64%). Several T cell subsets were significantly increased in ESN when compared with controls: CD4(+)CD38(+) (p = 0.006), CD4(+)HLA-DR(-)CD38(+) (p = 0.02), CD4(+)CD45RA(+)CD27(+)HLA-DR(-)CD38(+) (p = 0.002), CD8(+)CD45RA(+)CD27(+)CD38(-)HLA-DR(+) (p = 0.02), and CD8(+)CD45RA(+)CD27(-)CD38(+)HLA-DR(+) (p = 0.03). Activation of CD8(+) T cells was increased in ESN with detectable HIV T cell responses compared with ESN lacking these responses (p = 0.04). Taken together, these results suggest that persistent but low sexual HIV exposure is able to induce virus-specific T cells response and immune activation in a high proportion of ESN, suggesting that virus exposure may occur even in conditions of maximal viral suppression in the HIV-infected partner.

Decreases in community viral load are accompanied by reductions in new HIV infections in San Francisco

Background

At the individual level, higher HIV viral load predicts sexual transmission risk. We evaluated San Francisco's community viral load (CVL) as a population level marker of HIV transmission risk. We hypothesized that the decrease in CVL in San Francisco from 2004–2008, corresponding with increased rates of HIV testing, antiretroviral therapy (ART) coverage and effectiveness, and population-level virologic suppression, would be associated with a reduction in new HIV infections.

Methodology/Principal Findings

We used San Francisco's HIV/AIDS surveillance system to examine the trends in CVL. Mean CVL was calculated as the mean of the most recent viral load of all reported HIV-positive individuals in a particular community. Total CVL was defined as the sum of the most recent viral loads of all HIV-positive individuals in a particular community. We used Poisson models with robust standard errors to assess the relationships between the mean and total CVL and the primary outcome: annual numbers of newly diagnosed HIV cases. Both mean and total CVL decreased from 2004–2008 and were accompanied by decreases in new HIV diagnoses from 798 (2004) to 434 (2008). The mean (p = 0.003) and total CVL (p = 0.002) were significantly associated with new HIV cases from 2004–2008.

Conclusions/Significance

Reductions in CVL are associated with decreased HIV infections. Results suggest that wide-scale ART could reduce HIV transmission at the population level. Because CVL is temporally upstream of new HIV infections, jurisdictions should consider adding CVL to routine HIV surveillance to track the epidemic, allocate resources, and to evaluate the effectiveness of HIV prevention and treatment efforts.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

HIV treatment optimism and unsafe anal intercourse among HIV-positive men who have sex with men: findings from the positive connections study

This study was designed to examine the impact of HIV treatment optimism on sexual risk among a racially diverse sample of HIV-positive MSM. Survey data were collected from 346 racially diverse HIV-positive MSM.

INCLUSION CRITERIA

18 years of age, male, at least one incident of unprotected anal intercourse (UAI) in the last year, currently on treatment. Other variables included demographics, sexual risk, depression, internalized homonegativity, HIV treatment history, alcohol/drug use and beliefs about HIV treatments (Susceptibility to transmit HIV, Severity of HIV infection and Condom Motivation). Those with lower income were more likely to report that HIV was less transmissible. A self-reported decrease in condom motivation was associated with being White, well-educated and increased alcohol/drug use. A decrease in Severity of HIV was associated with better mental health, being non-White and undetectable viral load. Sexual risk appears related to beliefs about how treatment affects the transmissibility of HIV. Race, socioeconomic status, alcohol/drug use, mental health and viral load were also associated with treatment beliefs.

Transmission networks of HIV-1 among men having sex with men in the Netherlands

OBJECTIVE

To obtain insight in the HIV-1 transmission networks among men having sex with men (MSM) in the Netherlands.

DESIGN

A phylogenetic tree was constructed from polymerase sequences isolated from 2877 HIV-1 subtype B-infected patients monitored as part of the AIDS Therapy Evaluation in the Netherlands (ATHENA) nationwide observational cohort.

METHODS

For MSM with a known date of infection, the most similar sequences were selected as potential transmission pairs when they clustered with bootstrap value of at least 99%. Time from infection to onward transmission was estimated as the median time between dates of infection for each transmission pair. The source of infections with a resistant strain was traced using the entire phylogenetic tree.

RESULTS

Of sequences from 403 MSM with a known date of infection between 1987 and 2007, 175 (43%) formed 63 clusters. Median time to onward transmission was 1.4 years (interquartile range 0.6-2.7). Twenty-four (6%) MSM carried a virus with resistance-related mutations, 13 of these were in eight clusters together with sequences from 28 other patients in the entire phylogenetic tree. Six clusters contained sequences obtained from 29 men all presenting the same resistance-related mutations.

CONCLUSION

From our selection of likely transmission pairs, we conclude that onward transmission of HIV-1 from infected MSM in the Netherlands happens both during and after primary infection. Transmission of resistant strains from the antiretroviral therapy-treated population is limited, but strains with resistance-related mutations have formed subepidemics.

Molecular sequence data of hepatitis B virus and genetic diversity after vaccination

The effect of vaccination programs on transmission of infectious disease is usually assessed by monitoring programs that rely on notifications of symptomatic illness. For monitoring of infectious diseases with a high proportion of asymptomatic cases or a low reporting rate, molecular sequence data combined with modern coalescent-based techniques offer a complementary tool to assess transmission. Here, the authors investigate the added value of using viral sequence data to monitor a vaccination program that was started in 1998 and was targeted against hepatitis B virus in men who have sex with men in Amsterdam, the Netherlands. The incidence in this target group, as estimated from the notifications of acute infections with hepatitis B virus, was low; therefore, there was insufficient power to show a significant change in incidence. In contrast, the genetic diversity, as estimated from the viral sequence collected from the target group, revealed a marked decrease after vaccination was introduced. Taken together, the findings suggest that introduction of vaccination coincided with a change in the target group toward behavior with a higher risk of infection. The authors argue that molecular sequence data provide a powerful additional monitoring instrument, next to conventional case registration, for assessing the impact of vaccination.

The effect of intrinsic stochasticity on transmitted HIV drug resistance patterns

Estimates of transmitted HIV drug-resistance prevalence vary widely among and within epidemiological surveys. Interpretation of trends from available survey data is therefore difficult. Because the emergence of drug-resistance involves small populations of infected drug-resistant individuals, the role of stochasticity (chance events) is likely to be important. The question addressed here is: how much variability in transmitted HIV drug-resistance prevalence patterns arises due to intrinsic stochasticity alone, i.e., if all starting conditions in the different epidemics surveyed were identical? This 'thought experiment' gives insight into the minimum expected variabilities within and among epidemics. A simple stochastic mathematical model was implemented. Our results show that stochasticity alone can generate a significant degree of variability and that this depends on the size and variation of the pool of new infections when drug treatment is first introduced. The variability in transmitted drug-resistance prevalence within an epidemic (i.e., the temporal variability) is large when the annual pool of all new infections is small (fewer than 200, typical of the HIV epidemics in Central European and Scandinavian countries) but diminishes rapidly as that pool grows. Epidemiological surveys involving hundreds of new infections annually are therefore needed to allow meaningful interpretation of temporal trends in transmitted drug-resistance prevalence within individual epidemics. The stochastic variability among epidemics shows a similar dependence on the pool of new infections if treatment is introduced after endemic equilibrium is established, but can persist even when there are more than 10,000 new infections annually if drug therapy is introduced earlier. Stochastic models may therefore have an important role to play in interpreting differences in transmitted drug-resistance prevalence trends among epidemiological surveys.

HIV testing factors associated with delayed entry into HIV medical care among HIV-infected persons from eighteen states, United States, 2000-2004

Despite the importance of timely entry into care after HIV diagnosis, the timing of care entry has not been described recently in a large, diverse population of persons with HIV. Dates of HIV diagnosis and entry into HIV care were obtained by interview of HIV-infected adults, most of whom had entered care for HIV, in 18 U.S. states from 2000 through 2004. Time to care entry was analyzed as a dichotomous variable; delayed care entry was defined as care entry greater than 3 months after HIV diagnosis. Multivariable logistic regression models were used to describe HIV testing-related factors associated with delayed care entry. Among 3942 respondents, 28% had delayed care entry. Diagnostic testing-related characteristics associated with delayed care entry included anonymous and first-time HIV testing. Providers of HIV testing should be aware that those who test positive anonymously and those whose first HIV test is positive may have increased risk for delayed HIV care entry. Developing programs that reinforce timely linkage to HIV care, targeted at those at increased risk for delaying care entry, should be a public health priority.

Potential impact of early antiretroviral therapy on transmission

PURPOSE OF REVIEW

In this review, we will discuss the potential of early highly active antiretroviral therapy (HAART) to reduce the sexual transmission of HIV on an individual and population level. We will focus on the biological plausibility and behavioural factors associated with HAART use and interventions that might influence such a strategy.

RECENT FINDINGS

Empiric and phylogenetic studies support the view that recent HIV infection is a highly infectious disease stage. Evidence increasingly demonstrates that individuals on fully suppressive HAART are significantly less likely to transmit HIV to sexual partners and some even suggest that such individuals cannot transmit HIV. Changes in risk behaviour are associated with the availability of HAART but behavioural studies offer contradictory observations regarding the direction and magnitude of these changes. This in turn makes the intricate assumptions and therefore outcomes of many complex mathematical modelling studies less secure. Furthermore, there is evidence that it is those individuals with undiagnosed HIV infection who contribute significantly to onward sexual transmission.

SUMMARY

At an individual level, HAART will reduce viral load and, therefore, infectiousness, although whether to zero or not remains controversial. At a population level, if recent infection and undiagnosed infections continue to drive onward transmission, earlier treatment will only alter the pandemic if accompanied by an improvement in diagnosing the undiagnosed and recognizing recent infection.

Mucosal immunity and protection against HIV/SIV infection: strategies and challenges for vaccine design

To date, most HIV vaccine strategies have focused on parenteral immunization and systemic immunity. These approaches have not yielded the efficacious HIV vaccine urgently needed to control the AIDS pandemic. As HIV is primarily mucosally transmitted, efforts are being re-focused on mucosal vaccine strategies, in spite of complexities of immune response induction and evaluation. Here, we outline issues in mucosal vaccine design and illustrate strategies with examples from the recent literature. Development of a successful HIV vaccine will require in-depth understanding of the mucosal immune system, knowledge that ultimately will benefit vaccine design for all mucosally transmitted infectious agents.

Relation between HIV viral load and infectiousness: a model-based analysis

BACKGROUND

A consensus statement released on behalf of the Swiss Federal Commission for HIV/AIDS suggests that people receiving effective antiretroviral therapy-ie, those with undetectable plasma HIV RNA (<40 copies per mL)-are sexually non-infectious. We analysed the implications of this statement at a population level.

METHODS

We used a simple mathematical model to estimate the cumulative risk of HIV transmission from effectively treated HIV-infected patients (HIV RNA <10 copies per mL) over a prolonged period. We investigated the risk of unprotected sexual transmission per act and cumulatively over many exposures, within couples initially discordant for HIV status.

FINDINGS

Assuming that each couple had 100 sexual encounters per year, the cumulative probability of transmission to the serodiscordant partner each year is 0.0022 (uncertainty bounds 0.0008-0.0058) for female-to-male transmission, 0.0043 (0.0016-0.0115) for male-to-female transmission, and 0.043 (0.0159-0.1097) for male-to-male transmission. In a population of 10 000 serodiscordant partnerships, over 10 years the expected number of seroconversions would be 215 (80-564) for female-to-male transmission, 425 (159-1096) for male-to-female transmission, and 3524 (1477-6871) for male-to-male transmission, corresponding to an increase in incidence of four times compared with incidence under current rates of condom use.

INTERPRETATION

Our analyses suggest that the risk of HIV transmission in heterosexual partnerships in the presence of effective treatment is low but non-zero and that the transmission risk in male homosexual partnerships is high over repeated exposures. If the claim of non-infectiousness in effectively treated patients was widely accepted, and condom use subsequently declined, then there is the potential for substantial increases in HIV incidence.

FUNDING

Australian Research Council.