While TasP was initially seen as a means of reducing individual risk when the concept was first introduced in 2006, it was only in 2010 that evidence from the HTPN 052 study suggested that it could be implemented as a population-based prevention tool.
Research Breakthrough
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 individuals on treatment were 96 percent less likely to infect their partners than participants who weren’t.
The results of the trial led many to speculate whether TasP might also 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 infected population, HIV transmission would eventually become so rare as to stop the epidemic in its tracks.
Undetectable = Untransmittable
The HTPN 052 was only the starting point in the journey to implement TasP. Between 2010 and 2018, two studies—called PARTNER1 and PARTNER2—aimed to evaluate the risk of transmission in gay and heterosexual mixed-status couples in whom the HIV-infected partner was virally suppressed
This was considered significant since only 2 percent of couples in the HTPN 052 was gay (a group statistically at highest risk of HIV infection). By contrast, nearly 70 percent of the couples in the PARTNER1 and PARTNER2 studies was gay.
Based on these results of the PARTNER1 and PARTNER2 studies, the researchers concluded that the risk of HIV transmission when the viral load is fully suppressed is zero. The results were conveyed to the public under the new public health campaign “U=U” (Undetectable = Untransmittable).
Challenges in Implementation
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 only hovered around 80 percent, 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) has placed the concept closer within reach.
While all of these facts point to TasP as a vital part of an individual-based prevention strategy, does it necessarily mean that it would on a population-based scale?
From the start, it was clear that there would be a number of strategic hurdles to overcome if TasP were to be feasible:
It would require high coverage of HIV testing and treatment, particularly in underserved, high-prevalence communities. In the U. S. , as many as 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 to 65 as part of a routine healthcare provider’s visit. It would require intensifying the follow-up of existing patients. According to the Centers for Disease Control and Prevention (CDC), only 44 percent of Americans diagnosed with HIV are linked to medical care. Research suggests that the fear of disclosure and the lack of HIV-specific care are among the reasons that so many delay treatment until the appearance of symptomatic disease. It would require the means by which to ensure population-based adherence, the success of which is highly variable and hard to predict. According to the CDC, of HIV-positive people currently on therapy, nearly one in four are unable to maintain the necessary adherence to achieve complete viral suppression. Finally, the cost of implementation is seen to be a major obstacle particularly as global HIV funding continues to be severely reduced.
Evidence in Support to TasP
The city of San Francisco may be the closest thing to a proof of concept for TasP. With gay and bisexual men comprising nearly 90 percent of the city’s infected population, consistent, targeted intervention has resulted in a low rate of undiagnosed cases.
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 key 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 percent) and the rate of sexually transmitted infections and extramarital sex was extremely low (0.04 and 0.07 percent, respectively).
According to the study, 80 percent of the HIV-positive partners, all of whom were newly treated at the start of the trial, had been placed on ART by 2012. During that time, the drop in new infection correlated to an overall reduction in risk of around 48 percent.
Moreover, as the study progressed and more HIV-positive partners were placed on ART, rates appeared to drop even further. From 2009 to 2012, the consistent use of ART reduced HIV risk by some 67 percent, nearly three times what was seen from 2006 to 2009 when it was only 32 percent.
A Word From Verywell
As compelling as these results are, TasP should not be considered an infallible strategy even among committed, serodiscordant couples. In the end, taking HIV medication is not the same thing as achieving an undetectable viral load.
With that being said, the aims of the strategy remain strong. This is especially true for couples wanting to have children or individuals at high risk of infection. In such cases, pre-exposure prophylaxis (PrEP) can also be prescribed to further protect the HIV-negative partner. When used together, TasP and PrEP can reduce the risk of infection to near-negligible rate.
Always discuss these options with your healthcare provider before embarking on any such strategy.
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