Cohen, Myron S.a,b,c; Muessig, Kathryn E.a; Smith, M. Kumib; Powers, Kimberly A.a,b; Kashuba, Angela D.M.d
Antiviral agents can be used to prevent HIV transmission in three ways: before exposure as preexposure prophylaxis (PrEP), after exposure as postexposure prophylaxis (PEP), and as treatment of infected people for secondary prevention [1–3]. PEP for HIV prevention has been well established but is not well suited to clinical research investigation. However, recent research developments in PrEP and secondary prevention provide a unique opportunity to highlight areas of advancement that have galvanized changes in HIV treatment and prevention and to highlight topic areas that remain undecided or controversial.
Assessing the prevention benefits of antiretrovirals: pharmaceutical and observational studies
With the development of antiviral agents in the early 1990s, a confluence of reasoning from two disciplines emerged. Clinical pharmacological studies demonstrated that HIV drugs penetrate the genital tract but with variable success [4–8]. Epidemiologic observational studies suggested that antiretroviral therapy (ART) might play a role in reducing the sexual transmission of HIV.
Intensive investigations of the pharmacology of antiretrovirals in genital secretions have demonstrated that several drugs in different therapeutic classes of antiretrovirals reliably concentrate in the male and female genital tract (Table 1). Drug penetration can be predicted for semen by the degree of antiretroviral protein binding in blood. However, predictors of drug penetration into the female genital tract remain unknown.
The relationship between the penetration of drugs into the genital tract, suppression of genital tract replication with treatment, and the relevance of persistent HIV shedding to HIV transmission remains only partially understood. First, it is clear that persistent intermittent ‘shedding’ of HIV in both male [9–14] and female [15–17] genital secretions can be expected even when treatment has reduced the blood plasma viral load . Several groups have reported detection of HIV in semen resistant to protease inhibitors [19,20], reflecting poor penetration of this class of agents. Conversely, Ghosn et al. reported complete suppression of HIV in semen over 48 weeks with the lopinavir–ritonavir combination, even though these agents were only detectable in blood plasma and not in seminal plasma (<30 ng/ml). Inability to completely and consistently suppress replication of HIV in the genital tract in men and women demonstrates that these compartments do not get the full benefit of antiretrovirals. Indeed, quite recently, investigators have argued that the persistent low copy HIV replication might be ascribed to poor penetration or metabolism of ART in lymphoid tissues . After a single dose of tenofovir, we have demonstrated discrepant penetration into cervical, vaginal, and rectal tissues, with cervical and vaginal tissue levels 10–100 times lower for tenofovir and tenofovir diphosphate than those achieved in rectal tissue . Sexually transmitted diseases (STDs) can stimulate shedding of HIV in spite of antiviral therapy [24,25]. However, the results of observational reports and the HIV Prevention Trials Network (HPTN) clinical trial 052 (see below) suggest that HIV shedding (so readily observed in spite of treatment with antiviral agents) may not actually contribute much to HIV transmission.
HIV transmission among HIV-serodiscordant couples has been the subject of several observational studies [26,27]. A subset of these studies with information on index partner ART status (Table 2) allows us to consider the effects of these drugs on transmission risk [28–37]. Reduction of HIV transmission with treatment has been reported in all but one report from China. In the study from China, 1927 infected people in stable HIV-discordant couples were offered free ART . Over 4918 person-years of follow-up time, similar proportions of participants who were taking antiretrovirals and participants who were not taking antiretrovirals infected their susceptible sexual partners (4.8 vs. 3.2%, P = 0.12). However, there is no way to judge whether, or how, the infected persons were taking their medications, and genetic tests were not performed to confirm linked transmissions within the pairs.
Attia et al. undertook a meta-analysis that emphasized the limited data (five studies total, with a combined 1098 person-years of follow-up) available from observational studies for demonstrating transmission prevention related to ART. Nonetheless, these results, and modeling of transmission probabilities with ART (reviewed in ), led to the rather controversial 2007 ‘Swiss Statement’ . In this declaration, the investigators concluded – based on available data – that under particular circumstances, treated, HIV-infected persons could engage in unprotected sex acts with minimal risk of transmission to an HIV-negative partner. The Swiss Statement requirements included informed consent from the HIV-negative partner, HIV-positive partner on ART suppressed to undetectable levels of blood plasma HIV for at least 6 months, and lack of any other STDs. The Swiss people took these recommendations seriously. In an analysis conducted among 7309 HIV-infected persons in the Swiss HIV Cohort Study, after the Swiss Statement was issued, participants were 1.24–2.04 (varied by risk group) times more likely to report unprotected sexual contacts .
Clinical trials in stable heterosexual couples and generalizability to other contexts
To determine the magnitude and durability of a prevention effect from ART, a randomized controlled trial was conducted by the HPTN . In the HPTN 052 trial, 1763 sexually active HIV-discordant couples (in a stable relationship over the past 3 months reporting vaginal or anal intercourse at least three times during that period) were recruited from 13 sites in nine countries. At enrollment, the HIV-positive participant was required to be ART-naive and have a CD4 cell count between 350 and 550 cells/μl; individuals with active tuberculosis were excluded. Participants were randomized to start ART at study enrollment (early therapy) or after two consecutive CD4 cell count measures less than or equal to 250 cells/μl and more than 200 cells/μl (delayed therapy). The history of the HPTN 052 trial has been previously reported . The HPTN 052 study is ongoing and scheduled for completion in 2015. However, because of the overwhelming benefits of ART observed in the first 1.7 years of average follow-up, the National Institutes of Health (NIH) Multinational Data Safety Monitoring Board (DSMB) recommended on 28 April 2011 that the sponsor (NIH) make the interim results available. The individuals in the early treatment arm had greater than 96% protection from HIV acquisition from their HIV-infected partner . The single HIV transmission event in the treated group was ascribed to transmission before HIV suppression was possible (Jabara, Ping, Swanstrom, personal communication). Individuals in the early arm had reduced episodes of opportunistic infection, especially extrapulmonary tuberculosis. Further analysis has demonstrated that patients receiving early ART had delayed time to a primary HIV endpoint and significantly reduced secondary clinical endpoints (Grinsztejn, et al., abstract submitted, IAS 2012). Following the DSMB decision, all participants in the delayed treatment arm who had not already initiated treatment (due to CD4 cell count <250 cells/μl) were offered antiretroviral drugs. HPTN 052 is continuing so as to determine the durability of the prevention benefit and to monitor individuals in the delayed arm for adverse clinical events realized even after ART has been initiated; an observational study has suggested that delayed initiation of ART even at higher CD4 cell counts could lead to cardiovascular and other complications [43–45].
A key question from HPTN 052 is the generalizability to other contexts: heterosexual couples with CD4 cell counts lower and higher than those studied in HPTN 052, high-risk heterosexual individuals (e.g., sex workers and their clients), MSM, and intravenous drug users (IDUs). There are no data to address this issue directly. The PEPFAR Scientific Advisory Board concluded that for heterosexual transmission, there is no reason to believe that ART will not suppress HIV transmission regardless of pretreatment viral burden or stage of disease . For MSM, a WHO Expert Committee concluded that there is no reason to presume that treated, HIV-infected MSM will not be rendered less contagious . However, the biology of HIV transmission is sufficiently different in IDUs and MSM as compared with heterosexual transmission, warranting further consideration and study (K.E. Muessig, et al., under review).
For example, the number of HIV variants acquired and the efficiency of transmission are higher in MSM and IDU than in heterosexual transmission . Additionally, whether ART reduces infectivity through anal sex by the same order of magnitude as for vaginal sex remains uncertain (K.E. Muessig, et al., under review). Although such an effect is widely assumed, this is a key missing piece of evidence. Although an individual-level clinical trial among MSM may be unethical in light of the results of HPTN 052, at least two observational studies are underway to assess the effects of ART on HIV transmission among MSM. The Partners of People on ART: A New Evaluation of the Risks (PARTNER) Study  and the ‘Opposites Attract’ study  will recruit serodiscordant male–male couples in Europe and Australia/Thailand, respectively. In these studies, serodiscordant couples who are engaging in anal intercourse, and in whom the HIV-infected partner is on ART, will be followed longitudinally to estimate the rate of HIV transmission.
Antiretroviral treatment and population-level benefits
The individual benefits of ART may translate to a population-level effect in some parts of the world; however, the data used to support this conclusion have been controversial in large part because of the limitations of the methods employed (Smith, et al., under review). Eight published ecological studies have examined trends in the HIV epidemic – in some cases using measures of HIV incidence – and concomitant availability of antiretrovirals (Table 3) [51–58]. These studies are interesting and provocative, but they are inevitably subject to considerable confounding and bias. The measurement of antiretroviral use is also problematic, as HIV transmission can only be prevented with reliable and durable suppression of HIV.
Ecological studies have had mixed results. In San Francisco, investigators argue that new diagnoses of HIV have fallen as a result of broader availability of ART and demonstrable reduction in viremia in some members of the ‘community’, broadly defined as people with recognized HIV who are receiving treatment . In British Columbia, antiretrovirals have been related to reductions in new diagnoses of HIV among IDUs , a population that has received many other successful interventions [59,60] and in whom adherence to antiretrovirals may be poor . HIV incidence among other groups such as MSM in the USA, Australia, France and Amsterdam may be rising, in spite of the wide availability of antiretroviral drugs [62–66]. In Canada, the portion of MSM making up the estimated number of incident infections increased between 2005 and 2008 . Conversely, HIV incidence worldwide has been falling dramatically; between 2001 and 2009, HIV incidence decreased by more than 25% in 33 countries, 22 of which were in sub-Saharan Africa . These declines are most readily ascribed to a complex set of interventions including antiretroviral drugs. So, although it is certainly possible that ART is reducing the incidence of HIV in communities with broad access to ART, the methods used in ecological studies, along with a large number of unaddressed issues and incongruent results, preclude a definitive answer to this hypothesis.
Most recently, investigators in South Africa reported results from a study in KwaZulu-Natal in which they compared the density of ART coverage in different communities with HIV sero-conversion in a longitudinal cohort of 16 667 people. They adjusted their analysis to consider sexual behavior, socio-demographic variables associated with HIV acquisition, and HIV prevalence in the surrounding community (range <10 to >40%). In the adjusted analysis, each percentage point increase in ART coverage of all HIV-positive persons in the surrounding community resulted in a 1.7% (P < 0.001) reduction in risk of HIV acquisition for HIV-negative persons living in that community . These results present the most compelling population-level evidence to date that treatment with ART can reduce incidence of HIV.
Community randomized trials
Given the need to understand the proper balance between antiretrovirals and other modes of HIV prevention, as well as the magnitude of benefit of combination prevention, population-level clinical trials are planned. At least 50 studies are ongoing or planned , some of which are quite ambitious. HPTN 065 is a study designed to determine whether people with HIV in New York City and Washington, District of Columbia, can be detected and efficiently linked to care . Three studies are being supported by a variety of agencies including the Office of the Global AIDS Coordinator (PEPFAR), USAID, the Bill and Melinda Gates Foundation, the US Centers for Disease Control (CDC) and the US NIH, and one study by the French National Agency for Research on AIDS and viral hepatitis (ANRS). In a study in Botswana, the investigators intend to find and treat people with the highest viral loads, as they may be most contagious . All of these studies are likely to use molecular phylogeny to better understand the spread of HIV, and new cases introduced from outside the target communities.
Treatment as prevention, though exciting, cannot be guaranteed success at the population level. Can enough people be detected, linked to care, and properly treated to make a difference [73–75]? Also, current HIV detection strategies cannot find people with acute and early HIV disease [74,75], who may or may not contribute greatly to the spread of HIV [76–79]. The biology and epidemiology of acute HIV infection have been extensively reviewed; the relative importance of acute HIV infection to the spread of HIV is the subject of current debate .
PrEP is used to prevent many infectious diseases (e.g., endocarditis, malaria). However, usage of PrEP is subject to universal questions: does the agent work (biological plausibility); can the agent be given at the right time to work (pharmacokinetics and dynamics); will at-risk individuals use the agent properly and reliably; will cost and toxicity outweigh the benefit(s) of the intervention; and is usage sufficiently limited? In essence, the right drugs must be used at the right time for the right duration.
Attempts to develop PrEP regimens for HIV have been challenging, and the results confusing. We have summarized the work from seven trials (Table 4) [41,81–86]. To date, a limited number of agents have been used, selected primarily because they are well tolerated and because they provided protection in a macaque model using either rectal or vaginal exposure to SIV [87,88]. However (as discussed below), the relative safety of daily oral tenofovir for HIV-negative people has been questioned .
Two trials have used 1% tenofovir gel intravaginally with different results. Although the populations were similar, the dosage schedules were different. In the CAPRISA 004 study  heterosexual women at high risk of infection, ages 18–40 years, used the gel in a coitally dependent manner: one dose of gel up to 12 h before sex and one dose of gel up to 12 h after sex, with no more than two doses in 24 h. In one arm of the VOICE trial , women ages 18–45 used a dose of gel daily, regardless of sexual activity. The CAPRISA 004 study demonstrated 39% protection against HIV acquisition by tenofovir gel, whereas the tenofovir gel arm of the VOICE trial was stopped for futility. The reasons for the failure of tenofovir gel in VOICE trial after the success of CAPRISA 004 have not yet been determined.
Five studies of oral PrEP have been undertaken (Table 4) [82–86]. iPrEx is the only study devoted to MSM. In this trial , 2499 MSM in South America and the USA were provided a daily fixed-dose combination pill of tenofovir with emtricitabine. The investigators reported a 44% reduction in HIV acquisition compared with placebo controls. Data were also analyzed to consider self-reported pill usage. Incidence was reduced by 73% if self-reported adherence was high (>90% of doses taken), 50% if adherence was intermediate (>50% of doses), and 32% if adherence was low (<50% of doses). Among those who reported good adherence (taking study drug ≥50% of days), 46% of men who remained HIV-negative and 92% of men who seroconverted had no drug detected in selected blood and cell samples (Grant et al., 2010, Supplemental Table 8). The iPrEx investigators also used a case–control design to measure the association between detectable antiretrovirals in blood plasma and peripheral blood mononuclear cells and incident HIV infection. The majority of incident infections in the study drug arm had pharmacokinetic data available (34/36), and only 9% of these individuals had detectable study drug levels in selected plasma and cell specimens as compared with 51% of those who did not become infected. On the basis of this result, the authors argue that PrEP resulted in a relative risk reduction of 92% [95% confidence interval (CI) 40–99%] comparing patients with detectable study drug levels to those without detectable drug levels. Despite these promising findings, both self-report and pharmacologic markers raise concerns for accurately and reliably measuring drug adherence, an issue we discuss below.
Three studies of oral PrEP have involved women. The TDF2 study  enrolled 540 women and 660 men randomized to receive a daily fixed dose combination pill of tenofovir–emtricitabine or placebo. Study participants were predominantly unmarried adults ages 21–29 years living in Botswana. In this study, tenofovir–emtricitabine offered 64% protection against HIV infection. However, the study numbers were too small to draw definitive conclusions about protection in men and women separately, and 30% of those enrolled did not complete the study. In the FEM-PrEP study [85,90], 2120 heterosexual women aged 18–45 years living in high-prevalence areas in Kenya, South Africa, and Tanzania were randomized to receive either daily tenofovir–emtricitabine (FTC/TDF) or placebo. This study was discontinued for futility in April 2011. The investigators recently reported an HIV incidence rate of 4.7/100 person-years among the FTC/TDF group and 5.0/100 person-years in the placebo group for a hazard ratio for infection of 0.94 (95% CI 0.59–1.52, P = 0.81) . Adherence may have been a critical contributing factor as less than 50% of infected cases and uninfected matched controls had detectable study drug in their blood plasma. Finally, the VOICE (MTN-003) study  enrolled heterosexual women aged 18–45 years in high-prevalence areas of Uganda, South Africa, and Zimbabwe. Women were randomized to daily oral tenofovir or daily oral tenofovir–emtricitabine. In September 2011, the daily oral tenofovir arm was stopped for futility, whereas the daily oral tenofovir–emtricitabine arm continues.
The largest trial, Partners in Prevention, focused on 4758 discordant heterosexual couples in Kenya and Uganda (38% negative women and 62% negative men) who reported intensive condom use and received counseling, in addition to the seronegative partner receiving daily tenofovir or daily tenofovir–emtricitabine. At the 2011 International AIDS Society meeting, the investigators reported that daily tenofovir conferred 62% protection against HIV acquisition and daily tenofovir–emtricitabine conferred 73% protection against HIV acquisition . At the 2012 Conference for Retroviruses and Opportunistic Infections, the authors updated their results reporting 67% protection from daily tenofovir and 75% protection from the combined regimen . Only two individuals who were infected at randomization and one individual infected after randomization developed resistance (K65R or M184V mutation).
These trials differ in many ways. However, clearly, poor adherence would limit success as reported for FEM-PrEP and iPrEx. It is also possible that the drugs employed are not perfectly suited to PrEP, especially in women. Substantial differences in antiretroviral drug concentrations in mucosal tissues have been reported [23,92], which may help explain these discordant study findings . As noted above, after the first dose of oral FTC/TDF, rectal tissue concentrations are far greater than cervical or vaginal tissue concentrations . This could explain why tenofovir–emtricitabine conferred protective efficacy in iPrEx , despite only modest adherence. In the VOICE 003 trial , the lack of protection with oral TDF could reflect low tissue concentrations of the drug in the cervix and vagina. Furthermore, differences between study populations in risk behaviors and underlying rate of infection deserve consideration.
Two new studies (HPTN 067 and HPTN 069) sponsored by the HIV Prevention Trials Network (HPTN), one study sponsored by ANRS (IPERGAY), and an extension of the iPrEx study (iPrEx OLE) are addressing some of the limitations of the previous PrEP studies. Due to the adherence issues with daily PrEP dosing, the behavioral study HPTN 067 (the ADAPT study: Alternative Dosing to Augment PrEP Pill-Taking) is designed to test the hypothesis that recommending intermittent usage of oral tenofovir–emtricitabine, compared with recommending daily usage, will be associated with equivalent coverage of sex events before and after exposure dosing, lower number of pills needed for coverage, and decreased severity and frequency of self-reported side effects . HPTN 067 is enrolling 180 MSM and 180 heterosexual women and aims to identify dosing regimens that foster healthy sexual practices and pill-taking behavior in people at high risk of infection. The study includes a 6-week lead-in period, which includes directly observed therapy at enrollment and weeks 1 through 4, followed by 1 week without dosing to determine individual steady-state pharmacokinetics. Participants are then randomly assigned to one of three dosage groups in a 1 : 1 : 1 ratio: daily dosing, time-driven dosing, and event-driven dosing to be completed over 24 weeks.
iPrEx OLE (open-label extension) is a continuation of the iPrEx study that is enrolling participants in 11 sites in Peru, Ecuador, Brazil, the USA, South Africa, and Thailand . It is hoped that knowing with certainty that one is on the study drug (and not placebo) will lead to higher medication adherence. The 72-week study will also assess the long-term efficacy and safety of PrEP, changes in sexual behavior, drug resistance, changes in bone mineral density and fat distribution, and the impact on hepatitis infection.
The IPERGAY study sponsored by ANRS is a randomized controlled trial among HIV-negative MSM testing the prevention efficacy of Truvada (tenofovir–emtricitabine; Gilead Sciences Inc., Foster City, California, USA) in combination with regular HIV/STD testing, immunization against hepatitis A and B, postexposure treatment as needed, and condom distribution . Enrollment is currently ongoing in France and will also be extended to Canada.
Due to the possible limitations related to efficacy and toxicity (see below) of tenofovir–emtricitabine, HPTN 069 (NEXT-PrEP: Novel Exploration of Therapeutics for PREP) is assessing the safety and tolerability of maraviroc-containing PrEP regimens . HPTN 069 is enrolling 400 MSM and 200 heterosexual women and will compare 48-week safety and tolerability of daily maraviroc, maraviroc–emtricitabine, maraviroc–tenofovir, or tenofovir–emtricitabine. Secondary objectives will include evaluation of electronically monitored adherence, pharmacokinetics in systemic and genital tract compartments, and efficacy of ex-vivo HIV challenge in tissue biopsy explants.
Alternative drugs and delivery systems more appropriate to PrEP are also being pursued. Vaginal rings for women containing antiretrovirals would be similar in concept to vaginal rings currently used for contraception and hormone replacement therapy . These rings could maintain long-term, sustained antiretroviral release for local efficacy. Due to the long-term drug release, rings can be used in a coitally independent manner and inserted monthly, which could have an adherence benefit over gels or pills.
Dapivirine [TMC120; Janssen R&D Ireland (previously Tibotec)] has been formulated into a ring and is currently undergoing phase III studies . Two phase I studies evaluating dapivirine 25 or 200 mg delivered from a vaginal ring over 7 days in 25 healthy women found the ring to be well tolerated, with adverse effects similar to placebo. Mucosal fluid sampled at up to 7 days after insertion in all women from the introitus, cervix, and ring area had mean drug concentrations 1000 times the 50% effective concentration (EC50) against wild-type HIV-1 [100,101]. A combination dapivirine and maraviroc ring is currently undergoing a phase 1 safety and pharmacokinetic study.
An alternate formulation for better adherence is a long-acting injectable product that could be administered every 30–90 days. A rilpivirine (RPV) (TMC278 LA) nanosuspension is in early phase of development for this purpose [102,103]. In an exploratory study among 32 HIV-negative participants, a single intramuscular injection with either 300, 600, or 1200 mg of RPV showed varying, prolonged plasma, genital tract, and male rectum concentrations over 84 days . Optimal dosage, differences in drug concentrations in various biological compartments, and long-term safety of multiple doses all require further study.
Although a potentially strong addition to the biomedical technologies available for HIV prevention, PrEP has given rise to a number of concerns . First, HIV resistance is an important consideration while using oral antiviral agents. Among the 10 individuals enrolled in iPrEx with unrecognized HIV infection at baseline (and therefore subsequent unwitting exposure to ART directed at PrEP rather than treatment), three developed FTC-resistant mutations; this resistant mutation would be expected from the double-drug therapy employed . However, among 36 men who became HIV infected during the trial in the FTC-TDF group, no ART resistance was observed. These results have sometimes been interpreted to indicate that PrEP does not threaten the utility of the ART agents used . However, given the low adherence rates recorded in iPrEx, these results could also suggest that the study individuals used no ART product during the time after HIV was acquired and diagnosed. Detection of ART resistance markers represents a surrogate for failed PrEP usage, and ART resistance might compromise future management of people using PrEP who acquire HIV .
Second, the long-term biological impacts of FTC-TDF taken for PrEP purposes (as compared with HIV treatment purposes) will require additional study. Specifically, tenofovir has been linked with renal injury [89,107] and loss of bone mineral density (BMD)  when used for HIV treatment. Individuals with preexisting renal conditions have been excluded from PrEP studies [109,110]. In the iPrEx study, a nonsignificant trend toward elevated creatinine levels was found among the intervention arm , and a substudy showed a small but significant (up to 1%) loss in BMD .
An additional concern is that the use of ART as PrEP could affect sexual behavior . In the iPrEx study, the investigators reported no increase in sexual risk behaviors. However, an individual's behavior within an unproven medical trial as compared with behavior of an individual under the belief of effective PrEP in a real-world setting must also be considered. ‘Risk compensation’, wherein individuals alter their behaviors in response to perceptions of risk, has been documented in relation to the availability of antiretroviral drugs and other biomedical HIV-prevention approaches [113–115]. Furthermore, even within a controlled trial setting such as the iPrEx study, self-reported adherence to ART was less than perfect, and other STDs were detected in both groups, challenging the veracity of the behavioral data collected. Finally, PrEP has generated discussions about resource distribution and how to balance the current HIV treatment coverage gap with expansion of preexposure prevention measures .
Taking antiretroviral treatment to scale: modeling, observation, and empirical data
Although investigators have been evaluating the potential utility of antiviral agents for prevention, a virtual parallel universe of researchers have been making the case that the benefits of ART are both inevitable, and already visible. This work has been conducted through evaluation of observational and ecologic data and mathematical modeling.
Mathematical modelers take the best available data and make assumptions about biology and behaviors to provide predictions of the future that are often provocative. A large number of models have focused on the usage of ART by HIV-infected persons to reduce the spread of HIV (Table 5) [117–125], as described in numerous review articles [2,126–128]. The results of several such modeling studies have suggested that expanded ART use could result in substantial reductions in HIV incidence under certain, optimistic conditions [120,129]. However, modeling studies have also shown that the population-level transmission prevention benefits of ART could be severely compromised by such factors as increased risk behavior [130–132], ongoing transmission during acute and early HIV infection , antiretroviral drug resistance , concentration of risk in population subgroups , and sub-optimal ART coverage, effectiveness, or adherence [125,134] (Table 5). Considerable uncertainty surrounds the fundamental assumptions and parameter values used in these models; more empirical data about risk behavior patterns, STD co-transmission, ART uptake and adherence, and effects of ART on infectivity (especially for anal contact and parenteral transmission) will facilitate more reliable model projections.
One area of special interest is the potential balance between PrEP and treatment, as well as the targeted use of antiretroviral-based prevention strategies within discordant couples. Using available data to look at management of HIV-discordant couples, Hallett et al. reported that use of PrEP by the uninfected partner could be at least as cost-effective as earlier ART initiation by the infected partner, provided that the annual cost of PrEP is less than 40% the cost of ART and the effectiveness of PrEP exceeds 70%. At the population level, Pretorius et al. used a mathematical model to examine the impact and cost-effectiveness of PrEP relative to ART in South Africa. These investigators concluded that PrEP use would be most cost-effective if utilized before ART reaches 65% of HIV-positive persons; as ART coverage increased beyond this level, the cost-effectiveness of PrEP was predicted to decrease rapidly. El-Sadr et al. evaluated treatment of HIV-infected partners within serodiscordant couples as a strategy for reducing HIV incidence at the population level and found that the predicted effectiveness in a given setting depended on HIV prevalence and the degree of HIV discordancy. In general, these results do not support a focus on HIV-discordant couples for public health purposes. As with the modeling studies of treatment as prevention described above, these studies indicate that PrEP effectiveness will depend on many factors and that the choice of intervention must take into account the epidemiological context in a given setting.
Combination prevention: the way forward
Implementation of ART as prevention faces substantial challenges, including logistic limitations [73,75], potential changes in risk-taking behaviors, and cost. Indeed, ART usage will need to be part of a combination strategy . In a remarkable 2011 speech at the NIH, Secretary of State Hillary Clinton expressed hope for an ‘AIDS Free Generation’ . On World AIDS Day 2011, President Barack Obama pledged an additional US$ 35 million for state AIDS drug assistance programs based on the HPTN 052 findings . PEPFAR and the WHO have recommended the use of ART to prevent HIV transmission among heterosexual partners [46,141], and UNAIDS is looking into similar guidelines . But attention to broader use of ART cannot ignore other parts of a prevention package . Secretary Clinton concluded that broader prevention of mother-to-child transmission, more circumcision, and optimal and broader use of ART will point us in the right direction . Additionally, combination prevention strategies  will need the continued efforts of behavioral interventions to increase condom use, reduce high-risk behaviors, and address suboptimal antiretroviral adherence and risk compensation. The community-based clinical trials described in this article, though focused on ART, all embrace a similar combination prevention strategy. And all of these trials and advances must also recognize the important discoveries in the field of HIV vaccine [143,144]. It seems reasonable to expect that – after 30 years of work – the tools now available in the HIV prevention toolbox and those that will become available from ongoing research can be expected to control the spread of HIV.
The authors wish to thank David Burns, MD, of the National Institute of Allergies and Infectious Diseases and Ward Cates, MD, of FHI360 for their reviews of this manuscript.
K.E.M. and M.K.S. are supported by an NIH institutional training grant (5T32AI007001-35). K.A.P. is supported by the NIH (R01 AI083059, R01 DA025885). M.S.C. is supported by the Center for AIDS Research (CFAR) and the NIH HIV Prevention Trials Network (HPTN-052).
Conflicts of interest
There are no conflicts of interest.
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