Data are lacking for quantifying HIV transmission risk in the presence of effective antiretroviral therapy

Brief Report: Per Sex-Act Risk of HIV Transmission Under Antiretroviral Treatment: A Data-Driven Approach

BACKGROUND

Before the completion of HPTN 052, PARTNER, and Opposites Attract studies, data were lacking to directly estimate HIV transmission risk under effective combined antiretroviral treatment (cART). Rather, estimates were obtained by extrapolating a dose-response relationship between viral load and risk of HIV transmission, observed among untreated individuals, to treated individuals. Presently, data have accumulated from 9 clinical studies for a direct validation of this extrapolation.

METHODS

Using estimates of per sex-act risk of HIV transmission on effective cART obtained by extrapolation, sexual behavior data, and a simple mathematical model, we estimated the number of seroconversions that should have been observed in HIV-serodiscordant couples where the HIV-positive partner was on cART across the 9 studies. We compared this with the number of seroconversions actually observed. Next, we directly estimated the risk of HIV transmission on effective cART, using Bayesian statistics to combine all available data.

RESULTS

We found that at least 4.7 (uncertainty bounds: 1.7-12.6) and 35.1 (uncertainty bounds: 13.2-92.0) seroconversions should have been observed among, respectively, heterosexual and men who have sex with men (MSM) serodiscordant couples. This is not validated by observations across the studies, which reported at most 1 seroconversion among heterosexual couples and 0 for MSM. Combining all available data, we found that the maximum per sex-act risk of HIV transmission under effective cART is 3.9:100,000 for heterosexuals and 4.4:100,000 for MSM.

CONCLUSIONS

Data have accumulated to render obsolete estimates of the risk of HIV transmission on cART obtained by extrapolation. Direct estimates are substantially lower and should be used in practice.

Projecting the epidemiological effect, cost-effectiveness and transmission of HIV drug resistance in Vietnam associated with viral load monitoring strategies

OBJECTIVES

The objective of this study was to investigate the potential epidemiological impact of viral load (VL) monitoring and its cost-effectiveness in Vietnam, where transmitted HIV drug resistance (TDR) prevalence has increased from <5% to 5%-15% in the past decade.

METHODS

Using a population-based mathematical model driven by data from Vietnam, we simulated scenarios of various combinations of VL testing coverage, VL thresholds for second-line ART initiation and availability of HIV drug-resistance tests. We assessed the cost per disability-adjusted life year (DALY) averted for each scenario.

RESULTS

Projecting expected ART scale-up levels, to approximately double the number of people on ART by 2030, will lead to an estimated 18 510 cases (95% CI: 9120-34 600 cases) of TDR and 55 180 cases (95% CI: 40 540-65 900 cases) of acquired drug resistance (ADR) in the absence of VL monitoring. This projection corresponds to a TDR prevalence of 16% (95% CI: 11%-24%) and ADR of 18% (95% CI: 15%-20%). Annual or biennial VL monitoring with 30% coverage is expected to relieve 12%-31% of TDR (2260-5860 cases), 25%-59% of ADR (9620-22 650 cases), 2%-6% of HIV-related deaths (360-880 cases) and 19 270-51 400 DALYs during 2015-30. The 30% coverage of VL monitoring is estimated to cost US$4848-5154 per DALY averted. The projected additional cost for implementing this strategy is US$105-268 million over 2015-30.

CONCLUSIONS

Our study suggests that a programmatically achievable 30% coverage of VL monitoring can have considerable benefits for individuals and leads to population health benefits by reducing the overall national burden of HIV drug resistance. It is marginally cost-effective according to common willingness-to-pay thresholds.

Improved virologic suppression with HIV subspecialty care in a large prison system using telemedicine: an observational study with historical controls

Correctional populations have an elevated human immunodeficiency virus (HIV) prevalence, yet many individuals lack access to subspecialty care. Our study showed that HIV-infected inmates had significantly greater virologic suppression and higher CD4 T-lymphocyte counts when managed by a multidisciplinary team of subspecialists conducting clinics via telemedicine. In other studies, these outcomes have been associated with reductions on HIV-related morbidity and mortality, as well as HIV transmission.

Antiretroviral therapy for prevention of HIV transmission: implications for Europe

The aim of this review is to summarise the evidence on the population-level effect of antiretroviral therapy (ART) in preventing HIV infections, and to discuss potential implications in the European context of recommending starting ART when the CD4 count is above 350 cells/mm3. The ability of ART to reduce the risk of HIV transmission has been reported in observational studies and in a randomised controlled trial (HPTN 052), in which ART initiation reduced HIV transmission by 96% within serodiscordant couples. As yet, there is no direct evidence for such an effect among men having sex with men or people who inject drugs. HPTN 052 led international organisations to develop recommendations with a higher CD4 threshold for ART initiation. However, there remains a lack of strong evidence of clinical benefit for HIV-positive individuals starting ART with CD4 count above 350 cells/mm3. The main goal of ART provision should be to increase ART coverage for all those in need, based on the current guidelines, and the offer of ART to those who wish to reduce infectivity; increased HIV testing is therefore a key requirement. Other proven prevention means such as condom use and harm reduction for people who inject drugs remain critical.

The detection and management of early HIV infection: a clinical and public health emergency

This review considers the detection and management of early HIV infection (EHI), defined here as the first 6 months of infection. This phase is clinically important because a reservoir of infected cells formed in the individual renders HIV incurable, and the magnitude of viremia at the end of this period predicts the natural history of disease. Epidemiologically, it is critical because the very high viral load that typically accompanies early infection also makes infected individuals maximally contagious to their sexual partners. Future efforts to prevent HIV transmission with expanded testing and treatment may be compromised by elevated transmission risk earlier in the course of HIV infection, although the extent of this impact is yet unknown. Treatment as prevention efforts will nevertheless need to develop strategies to address testing, linkage to care, and treatment of EHI. Cost-effective and efficient identification of more persons with early HIV will depend on advancements in diagnostic technology and strengthened symptom-based screening strategies. Treatment for persons with EHI must balance individual health benefits and reduction of the risk of onward viral transmission. An increasing body of evidence supports the use of immediate antiretroviral therapy to treat EHI to maintain CD4 count and functionality, limit the size of the HIV reservoir, and reduce the risk of onward viral transmission. Although we can anticipate considerable challenges in identifying and linking to care persons in the earliest phases of HIV infection, there are many reasons to pursue this strategy.

Australian prescribers' perspectives on ART initiation in the era of "treatment as prevention"

This study explores Australian prescribers' attitudes towards Treatment as Prevention (TasP) and their practices around initiating combination antiretroviral treatment (cART) for HIV. A brief online survey was conducted nationally amongst antiretroviral treatment (ART) prescribers in Australia. The sample broadly represented ART prescribers in Australia (N = 108), with 40.7% general practitioners (GPs), 25.9% sexual health clinic-based physicians and 21.3% hospital-based infectious diseases physicians. About 60% of respondents had been treating HIV-positive patients for more than 10 years. Respondents estimated that about 70-80% of all their HIV-positive patients were receiving ART. Over half of the prescribers agreed very strongly that their primary concern in recommending cART initiation was clinical benefit to individual patients rather than any population benefit. A majority of the prescribers (68.5%) strongly endorsed cART initiation before CD4+ T-cell count drops below 350 cells/mm(3), and a further 22.2% strongly endorsed cART initiation before CD4+ T-cell count drops below 500 cells/mm(3). Regarding the optimal timing of cART initiation, this study shows that prescribers in Australia in 2012 focus primarily on the benefits for their individual patients. Prescribers may need more convincing evidence of individual health benefits or increased knowledge about the population health benefits for a TasP approach to be effective in Australia.

Notions of synergy for combinations of interventions against infectious diseases in heterogeneously mixing populations

In public health programmes interventions are frequently combined with hoped for 'synergies'[22]. However, there is not yet a precise definition for synergy between interventions that captures the idea that there is added benefit at the population-level in using them together. To explore the synergy between interventions in the context of endemic disease, we consider a general model of infection spread in a heterogeneously mixing population. We consider interventions which may alter individuals' infectiousness, susceptibility, profile of infectiousness through time and survival while infected. Allowing general patterns of overlap and targeting in those receiving the interventions, we show how to compute changes to epidemiological indices such as R(0), and introduce a simple technique for calculating equilibrium prevalences and incidences via an iterated map. We argue for a particular definition of synergy and investigate its behaviour, both analytically and numerically, concluding that it is easiest to achieve synergy between interventions which perform poorly in isolation; implementation strategies that minimize the overlap of different interventions in the population tend to achieve more synergy; and that in populations with heterogeneous risk, interventions that are redundant when universally targeted can regain substantial synergy when applied in a targeted manner.

Postexposure prophylaxis, preexposure prophylaxis or universal test and treat: the strategic use of antiretroviral drugs to prevent HIV acquisition and transmission

This review considers the use of antiretroviral drugs specifically to prevent HIV transmission. Antiretroviral therapy (ART) can be implemented for the protection of uninfected individuals both before (preexposure prophylaxis) and after (postexposure prophylaxis) exposure to HIV infection. Preexposure prophylaxis may be used coitally dependently when individuals are intermittently exposed or by continuous daily dosing for those constantly exposed; postexposure prophylaxis is used in 28-day courses. Alternatively, ART can be used strategically to reduce the viral load and consequent infectiousness of an HIV-infected individual, thereby limiting the risk of onward viral transmission. A policy of universal HIV testing to enhance the identification of all HIV-positive individuals followed by immediate treatment of all HIV-positive individuals, irrespective of their CD4 cell counts (universal test and treat), has been postulated as a potential tool capable of reducing HIV incidence at a population level. This concept represents a paradigm shift in the use of ART, targeting infectious individuals for prevention rather than protecting uninfected exposed populations. This strategy could have the advantage of preventing transmission and reducing HIV incidence at a population level, as well as delivering universal access to therapy for all people living with HIV and AIDS, potentially eliminating mother-to-child HIV transmission and limiting concomitant diseases such as tuberculosis. This review critically examines the scientific basis of ART for HIV prevention, summarizing the risks and opportunities of the potential expansion of ART for prevention. Specifically, we consider the evidences for and against targeting HIV-uninfected individuals compared with enhanced HIV testing and treatment of HIV-infected individuals in terms of impact on viral transmission.

Estimating the risk of HIV transmission from homosexual men receiving treatment to their HIV-uninfected partners

Objective

To determine how the risk of HIV transmission from homosexual men receiving antiretroviral treatment is related to patterns of patient monitoring and condom use.

Methods

A stochastic mathematical simulation model was developed of cohorts of men in the Netherlands who have sex with men (MSM), defining the parameters of the model using observational cohort data. The model incorporates viral load trends during first-line treatment, patient monitoring and different scenarios for the way in which condom use may depend on recent viral load measurements. The model does not include the effect of sexually transmitted infections on HIV transmission.

Results

For MSM receiving treatment, the risk of transmitting HIV to their long-term partner is 22% (uncertainty interval: 9–37%) if condoms are never used. With incomplete use (in 30% of sex acts) the risk is reduced slightly, to 17% (7–29%). However, the risk is as low as 3% (0.2–8%) when men receiving treatment use condoms only 6 months beyond their last undetectable viral load measurement. The risk is further reduced when 3 months is the time period beyond which condoms are used.

Conclusions

When condom use by HIV-infected men receiving combination treatment with antiretroviral agents is based on their last viral load measurement, the transmission risk is much lower than with incomplete condom use. The key message for patients is that although always using condoms during treatment is the best way to protect partners from the risk of HIV transmission, when such use cannot be achieved, the second best strategy is to use condoms whenever the last undetectable viral load was measured more than 3 months ago.

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

OBJECTIVE

The objective of this study is to estimate per-contact probability of HIV transmission in homosexual men due to unprotected anal intercourse (UAI) in the era of HAART.

DESIGN

Data were collected from a longitudinal cohort study of community-based HIV-negative homosexual men in Sydney, Australia.

METHODS

A total of 1427 participants were recruited from June 2001 to December 2004. They were followed up with 6-monthly detailed behavioral interviews and annual testing for HIV till June 2007. Data were used in a bootstrapping method, coupled with a statistical analysis that optimized a likelihood function for estimating the per-exposure risks of HIV transmission due to various forms of UAI.

RESULTS

During the study, 53 HIV seroconversion cases were identified. The estimated per-contact probability of HIV transmission for receptive UAI was 1.43% [95% confidence interval (CI) 0.48-2.85] if ejaculation occurred inside the rectum, and it was 0.65% (95% CI 0.15-1.53) if withdrawal prior to ejaculation was involved. The estimated transmission rate for insertive UAI in participants who were circumcised was 0.11% (95% CI 0.02-0.24), and it was 0.62% (95% CI 0.07-1.68) in uncircumcised men. Thus, receptive UAI with ejaculation was found to be approximately twice as risky as receptive UAI with withdrawal or insertive UAI for uncircumcised men and over 10 times as risky as insertive UAI for circumcised men.

CONCLUSION

Despite the fact that a high proportion of HIV-infected men are on antiretroviral treatment and have undetectable viral load, the per-contact probability of HIV transmission due to UAI is similar to estimates reported from developed country settings in the pre-HAART era.

Examining the promise of HIV elimination by 'test and treat' in hyperendemic settings

BACKGROUND

It has been suggested that a new strategy for HIV prevention, 'Universal Test and Treat', whereby everyone is tested for HIV once a year and treated immediately with antiretroviral therapy (ART) if they are infected, could 'eliminate' the epidemic and reduce ART costs in the long term.

METHODS

We investigated the impact of test-and-treat interventions under a variety of assumptions about the epidemic using a deterministic mathematical model.

RESULTS

Our model shows that such an intervention can substantially reduce HIV transmission, but that impact depends crucially on the epidemiological context; in some situations, less aggressive interventions achieve the same results, whereas in others, the proposed intervention reduces HIV by much less. It follows that testing every year and treating immediately is not necessarily the most cost-efficient strategy. We also show that a test-and-treat intervention that does not reach full implementation or coverage could, perversely, increase long-term ART costs.

CONCLUSION

Interventions that prevent new infections through ART scale-up may hold substantial promise. However, as plans move forward, careful consideration should be given to the nature of the epidemic and the potential for perverse outcomes.