Treatment as prevention (TasP) refers to HIV prevention methods that use antiretroviral treatment (ART) to decrease the risk of HIV transmission. ART reduces the HIV viral load in the blood, semen, vaginal fluid and rectal fluid to very low levels ('undetectable'), reducing an individual's risk of onwards HIV transmission.1
For a number of years now, there has been growing evidence of the benefits of HIV treatment as a prevention method. In 2011, a landmark study, HPTN 052, showed early initiation of antiretroviral treatment (in people with a CD4 count between 350 and 550) for the HIV-positive partner in a serodiscordant couple reduced HIV transmission to the HIV-negative partner by 96%.2
This has led to the idea that treatment as prevention could be utilised as part of a 'test and treat' strategy by increasing testing and treatment coverage as well as decreasing community viral load.6
Following the results of HPTN 052, Michel Sidibé, the Executive Director of UNAIDS, commented:
"This breakthrough is a serious game changer and will drive the prevention revolution forward. It makes HIV treatment a new priority prevention option." 7
HIV treatment is already being used as prevention
Prevention of mother-to-child transmission (PMTCT)
Treatment as prevention has been utilised since the mid-1990s to prevent the mother-to-child transmission (MTCT) of HIV. In 1994, research showed zidovudine (AZT) given to HIV-infected mothers and their babies reduced MTCT from 25% to 8%.8
Since then, testing pregnant women and treating HIV-positive mothers with antiretroviral drugs (ARVs) during pregnancy, delivery and breastfeeding has been found to reduce the risk of a mother transmitting HIV to her child by up to 90%.9
Pre-exposure prophylaxis (PrEP)
As a result, like TasP, it potentially has population-wide benefits. However, if not taken routinely and consistently, PrEP is much less effective. It is important that PrEP is offered as part of a combination package of prevention initiatives, and does not replace other, more effective methods like condoms.13
In 2015, the World Health Organisation (WHO) released new guidelines and a policy brief recommending that PrEP should be offered as a choice to people who are at substantial risk of HIV infection.14 15 Previously, PrEP was only recommended for certain key affected populations such as sex workers, men who have sex with men and people who inject drugs.16 However, it was recognised that this excluded people who are at substantial risk but do not belong to one of these groups.
Work is on-going to make PrEP more widely available. Australia, Brazil, Malaysia, South Africa, Thailand and the United Kingdom are considering including PrEP in their national HIV programmes.17 In the United States of America (USA), PrEP is already a recommended prevention option for individuals at substantial risk of HIV infection via sexual transmission or injecting drug use.18 In December 2015, PrEP was made available in France "to individuals who cannot, for diverse reasons, use condoms systematically and who belong to groups where HIV incidence is very high."19 Since then, both South Africa and Kenya have approved PrEP for adults at high risk of acquiring HIV via sexual transmission.20
Microbicides are gels or creams containing ARVs that are applied to the vagina to help prevent HIV infection. Vaginal microbicides containing ARVs are effective, so long as they are used consistently and correctly. One study observed 39% fewer infections, but its findings have not been replicated.21
The challenge is adherence – in other words, creating a product that women who are at high risk of HIV infection are able to use regularly. In this respect, the issues for microbicides and PrEP are comparable. In fact, a microbicide gel is essentially a different way to deliver PrEP and is sometimes referred to as ‘topical PrEP’.22
There is also work on developing a rectal microbicide, suitable for use during anal sex.23
Post-exposure prophylaxis (PEP)
Post-exposure prophylaxis (PEP) is short-term antiretroviral treatment taken after possible exposure to HIV. Since 1998, it has been used for healthcare workers who may have been exposed to HIV-infected fluids.24 More recently, it has been used to treat those who may have been exposed during a single event (for example sexual assault, unprotected sex or sharing drug injecting equipment).25
More research is needed on the effectiveness of PEP as an HIV prevention strategy. One trial from the mid-1990s, which gave AZT to healthcare workers exposed to HIV, prevented transmission in 81% of cases.26 However, the use of AZT in PEP has since been replaced by tenofovir as a component of a three-drug combination.27
Test and treat strategies
'Test and treat' programmes are based on the premise that the rate of new HIV infections can be reduced by rolling out universal HIV testing in order to diagnose all people living with HIV, and initiate antiretroviral treatment regardless of CD4 count or viral load.14
One study from South Africa estimated that the implementation of universal voluntary HIV testing in South Africa for adults over 15 years old would decrease HIV prevalence to 1% within 50 years.28
Trials testing the effectiveness of TasP for the general population in communities in sub-Saharan Africa with high HIV prevalence are currently ongoing. They aim to compare the effects of TasP on HIV transmission by offering ART to people with a CD4 count under 350.29 30
Limitations of treatment as prevention
TasP is not 100% effective
Following the results of the HPTN 052 study, in 2013 the World Health Organisation (WHO) recommended that antiretroviral treatment be offered to all people living with HIV who have uninfected partners (serodiscordant couples) to reduce HIV transmission.31
However, even if all serodiscordant couples had access to TasP, it is widely agreed this would not bring an end the epidemic. If the preventative benefits of treatment are overstated, people are more likely to engage in high-risk behaviours. For example, research from Switzerland showed how increased access to antiretroviral treatment can lead to a reduction in other HIV prevention measures such as condom use.32
A number of studies have reported much lower reductions in HIV transmission raising doubts about TasP as a public health intervention. Research from China of 38,000 serodiscordant couples reported that treating the HIV-infected partners reduced the risk of HIV transmission to the uninfected partner by a comparatively low 26%.33 Moreover, in the HPTN 052 study, 30% of HIV-positive people had an external partner.2
TasP, adherence and multiple drug resistance
The success of TasP is highly dependent upon people adhering to antiretroviral treatment (ART). It is widely agreed that once ART is initiated it should not be interrupted, as incomplete viral suppression causes the more sensitive strains of HIV to be suppressed and the resistant strains to become dominant. Resistant strains are harder to treat.34
Adherence is an issue even where ART is widely available. In 2011, one study from the USA reported that 15 years after the initiation of highly active antiretroviral therapy (HAART), and 4 years after the introduction of combination prevention, only 19% of 1.1 million people living with HIV in the country had an undetectable viral load.35 In South Africa, which has the largest ART programme in the world, a study found that only 64% of people who were initiated on treatment between 2002 and 2007 were still in care three years on.36
There are also concerns that the widespread use of antiretroviral treatment at a population level to reduce the number of new HIV infections could lead to a significant increase in multiple drug-resistant HIV (MDR) levels. The dramatic scaling up of ART could see increases in non-adherence resulting in the development of resistant strains of the virus.37 One study from Los Angeles County, USA, reported that the use of 'test and treat' among MSM could almost double the prevalence of MDR HIV cases from 4.8% to 9.1% by 2023 among this group.38
Despite legitimate arguments about ART adherence and drug resistance, many argue that TasP interventions should be implemented regardless given the prevention benefits and how existing combination treatment has proved effective in suppressing viral load. Moreover, there remains a lot of scope to improve the current delivery of treatment through improved monitoring of ART adherence as well as strengthening the links between treatment and care.39
The future of treatment as prevention
Treatment as prevention (TasP) has a lot of potential in reducing population level rates of HIV transmission by increasing uptake of HIV testing, offering ART and linking people to care.40
However, the effectiveness of TasP relies, at least in part, on the willingness and ability of people on treatment to remain in care and follow their prescribed course of antiretroviral drugs, adhering to them correctly. A number of studies have promoted a combination of cognitive, behavioural and mixed interventions including emotional support as means of improving adherence to ART.41 42 43
Others have suggested that more research is needed in order to identify the most effective way of delivering TasP. Research from Botswana has modelled the benefits of targeting such a strategy at people with the lowest CD4 counts.44
Bigger challenges and questions remain around the implementation of TasP in resource-limited settings. Its success depends much upon the ability of a country's healthcare service to deliver these services.45 46 However, with TasP trials on-going, the burden of adding treatment-based prevention to already strained healthcare systems remains unknown.47
Ethical and public health concerns have also been raised about how limited supplies of antiretroviral drugs in resource-poor countries are distributed - for treatment, prevention or both. One study concludes it is "unethical to watch patients with treatable AIDS worsen and die, even with supportive care, so that medications for treatment can be diverted for prevention."48 However, others maintain that while TasP requires large financial investments and poses significant implementation challenges, it is potentially a highly cost-effective approach to reducing both new HIV infections and the overall global HIV burden.49
Overall, there is wide support for treatment as an HIV prevention measure, especially in those with CD4 counts under 350. Treatment for this group must be scaled up, with healthcare systems working to increase adherence and retention in care. However, it is widely acknowledged that treatment alone will not end the global HIV epidemic. In order to be effective, TasP needs to be delivered as part of a comprehensive package of prevention methods including HIV and AIDS education, sexual and reproductive health education, condom use and behaviour change.50
Photo credit: ©iStock.com/Jeng_Niamwhan
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