The Facts About HIV Treatment as Prevention

Strategy Aims to Reverse Transmission Rates in High-Risk Communities

Photo credit: Ramsey Jehms

HIV Treatment as Prevention (TasP) is an evidence-based approach by which persons with an undetectable viral load are far less likely to transmit the virus to an uninfected or untreated partner.

While TasP was initially seen as an advocacy tool when the concept was first introduced in 2006), it was only in 2010 that evidence from the HTPN 052 Trial suggested that it could be implemented as a population-based prevention tool.

The HTPN 052 Trial As Game Changer

The HTPN 052 Trial—which studied the impact of antiretroviral therapy (ART) on transmission rates in serodiscordant heterosexual couples—was stopped nearly four years early when it was shown that participants on ART were 96% less likely to infect their partners than participants who weren't.

The definitive results of the trial led many to speculate whether TasP might slow—if not altogether stop—the spread of HIV by reducing the so-called "community viral load." In theory, by reducing the average viral load within an entire infected population, transmission would eventually become so rare as to stop the epidemic in its tracks.

Evidence in Support of TasP

Prior to the introduction of newer-generation antiretroviral drugs, TasP was considered inconceivable due to high levels of drug toxicities and viral suppression rates that hovered around 80%, even for those with perfect adherence.

The picture has largely changed in recent years, with the introduction of more effective, cheaper medications. Even in heavily hit countries like South Africa, the availability of low-priced generics (as little as $10 per month for a tenofovir-based regimen) has placed the concept closer within reach.

Furthermore, observational, real world evidence has shown that people on ART are ten times less likely to transmit the virus to partners not on treatment.

While all of these facts point to TasP as an important component to an individual-based prevention strategy, does it necessarily suggest a viable population-based one?

Challenges in Implementation

From the start, it was clear that there would be a number of strategic hurdles to overcome if TasP were to be feasible:

  1. It would require high coverage of HIV testing and treatment, particularly in under-served, high-prevalence communities. In the U.S., currently one in five people with HIV are fully unaware of their status. In response, the U.S. Prevention Services Task Force is now recommending the once-off testing of all Americans, ages 15-65, as part of a routine doctor's visit.
  2. It would require intensifying the follow-up of existing patients. According to the Centers for Disease Control and Prevention (CDC), only 44% of Americans diagnosed with HIV are linked to medical care. Research suggests that the fear of disclosure, lack of HIV appropriate care, and perceptions about HIV itself are among the reasons that so many delay treatment until the appearance of symptomatic disease.
  1. It would require the means by which to ensure population-based adherence, the success of which is highly variable and difficult to predict. According to the CDC, of HIV-positive people in the U.S. on therapy, nearly one in four are unable to maintain the necessary adherence to achieve complete viral suppression.
  2. Finally, the cost of implementation is seen to be a major obstacle—particularly as global HIV funding is either being reduced or re-purposed, and even some middle-income nations are facing rationing within their HIV drug programs.

Does TasP Actually Work?

The city of San Francisco may be the closest thing to a proof of concept for TasP. With men who have sex with men (MSM) comprising nearly 90% of the infected population, consistent targeted intervention has resulted in a low rate of undiagnosed cases. The widespread coverage of antiretrovirals has directly resulted in a 33% drop in new infections from 2006-2008. In 2010, the introduction of universal ART at the time of diagnosis further resulted in a six-fold increase in the number of people able to maintain full viral suppression.

But most agree that San Francisco has a unique dynamic to other HIV populations. There is still insufficient evidence to support whether TasP will bring down infection rates in the same fashion elsewhere.

In fact, a 2015 study from the University of North Carolina has suggested that real-world efficacy of TasP may fall short in certain populations. The study, which looked at 4,916 serodiscordant couples in the Henan province of China from 2006 to 2012, studied the impact of ART on transmission rates in a population where consistent condom usage was relatively high (63%) and the rate of sexually transmitted infections and extramarital sex was low (0.04% and 0.07%, respectively).

According to the investigators, 80% of the HIV-positive partners, all of whom were treatment naïve at the start of the study, had been placed on ART by 2012. During that time, the drop in HIV incidence before ART was started and after ART was started correlated to an overall reduction in risk of around 48%.

Moreover, as the study progressed and more HIV-positive partners were placed on ART, efficacy was seen to rise sharply. From 2009 to 2012, the efficacy of ART to reduce transmission was 67%, up sharply from 2006 to 2009 when it was only 32%.

And while the lack of viral load testing left investigators blind to issues of virologic failure and treatment adherence, the results are similar to those seen in smaller studies and may be closer to what is currently achievable in resource-limited settings.

What Does TasP Mean for You Personally?

Firstly, it's important to note that, on an individual basis, TasP was never considered a preventive strategy for anyone outside of a committed, serodiscordant relationship. It was never meant to replace condom use or to provide free license to abandon safer sex practices.

With that being said, the aims of the strategy remain strong. This is especially true for serodiscordant couples planning families or those at higher risk of infection. In such cases, pre-exposure prophylaxis (PrEP) can also be prescribed to reduce the viral susceptibility of the HIV-negative partner. Discuss these options with your doctor before embarking on any such strategy.

In the meantime, complete adherence to antiretroviral medications combined with consistent condom use is still considered the most reliable form of HIV prevention.

Sources:

Cohen, M.; Chen, Y.; McCauley, M.; et al. "Prevention of HIV-1 infection with early antiretroviral therapy." New England Journal of Medicine. August 11, 2011; 365(6):493-505.

Gill, V.; Lima, V.; Zhang, W.; et al. "Improved Virological Outcomes in British Columbia Concomitant with Decreasing Incidence of HIV Type 1 Drug Resistance Detection." Clinical Infectious Diseases. January 1, 2010; 50(1): 98-110.

U.S. Centers for Disease Control and Prevention (CDC). "Vital Signs: HIV Prevention Through Care and Treatment - United States." Morbidity and Mortality Weekly Report (MMWR). December 2, 2011; 60(47):1618-1623.

Charlebois, B.; Das, M.; Porco, T.; and Havlir, D. "The Effect of Expanded Antiretroviral Treatment Strategies on the HIV Epidemic among Men Who Have Sex with Men in San Francisco." Clinical Infectious Diseases. April 15, 2011; 52(8):1046-1049.

Smith, K.; Westreich, D.; Liu, H.; et al. "Treatment to Prevent HIV Transmission in Serodiscordant Couples in Henan, China, 2006 to 2012." Clinical Infectious Diseases. March 13, 2015; pii: civ200. [Epub ahead of print].

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