TasP

• Treatment as prevention (TasP) refers to HIV prevention methods that use antiretroviral treatment (ART) to decrease the risk of HIV transmission

Updated May 11, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Treatment as Prevention (or TasP) is an evidence-based approach by which HIV -infected persons with anundetectable 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 first introduced in 2006 (by Dr. Julio Montaner of the British Columbia Centre for Excellence in HIV/AIDS), it was only in 2010 that evidence from the HTPN 052 Trial suggested that it could be implemented as a public health measure to reduce transmission rates from a population-based perspective.

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.

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

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 some form of Universal Testing and Treatment (UTT), particularly in under-served, high-prevalence populations. In the U.S., currently one in five people with HIV are unaware of their status. In response, the U.S. Prevention Services Task Force is now recommending the once-off testing of the general population, 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 U.S. 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.
3. It would require raising the universal threshold by which ART is initiated. In July, 2013, the World Health Organization (WHO) issued updated guidelines recommending the use of ART when a person's CD4 count fall below 500 cells/µL. Currently, the U.S. is the only country to have recommended initiation with a CD4 over 500 cells/µL, based on mounting evidence that early intervention can increase in the proportion of patients able to achieve viral suppression in a high-prevalence population.
4. It would require the means to ensure population-based adherence levels, the success of which is highly variable and difficult to predict. According to the CDC, of HIV-positive people in the U.S. on antiretroviral therapy, nearly one in four are unable to maintain the necessary adherence to attain viral suppression. Meanwhile, only 28% of all HIV-infected persons in the U.S. (on treatment or not) have a fully suppressed viral load.
5. Finally, the cost of implementation is seen to be a major obstacle—particularly as global AIDS funding is either being reduced or re-purposed, and even some middle-income nations are facing rationing within their antiretroviral drug programs.

The balancing act of these and other issues may ultimately relegate TasP to only those resource-rich populations able to implement (and afford) sustainable UTT strategies.

Does TasP Actually Work?

The city of San Francisco may be the closest thing to a "proof of concept." 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 most other HIV populations. There is still not sufficient evidence to support that TasP is starting to bring down infection rates elsewhere.

What Does TasP Mean for You?

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

Most importantly, there are currently no national or global guidelines in place governing the proper implementation of TasP (although an international committee led by the World Health Organization is expected release recommendations soon).

That said, ensuring the viral suppression of an HIV-infected partner in a serodiscordant relationship is seen to be key to reducing transmission risk. This is especially important for couples planning families. In these cases, pre-exposure prophylaxis (PrEP) is sometimes added 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, full adherence to antiretroviral medications combined with consistent condom use is considered the most reliable form of HIV prevention.