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“PrEP as a bridge to ART” strategy dramatically reduces risk of HIV transmission for heterosexual serodiscordant couples

24 March 2015

Serodiscordant couples (where one partner is HIV positive and the other HIV negative) can be at high risk of HIV transmission, but several strategies are available to reduce this risk. Highly effective HIV prevention options include the use of antiretroviral treatment (ART) by the HIV-positive partner, and the ongoing use of pre-exposure prophylaxis (PrEP) by the HIV-negative partner). PrEP involves the daily use of a pill called Truvada (a fixed-dose formulation of two anti-HIV drugs, tenofovir + FTC) and is available from Canadian healthcare providers through “off-label” prescriptions. Research shows that the consistent and correct use of ART and PrEP can each reduce the risk of HIV transmission by over 90%.

World Health Organization guidelines currently recommend the offer of ART (regardless of CD4 count) and PrEP to partners in serodiscordant relationships to prevent HIV transmission. However, not all HIV-positive partners want to start ART when offered and it can take up to six months after starting treatment for the amount of virus (viral load) in the bodily fluids to reach undetectable levels. An undetectable viral load is important to maximize the prevention benefits of ART. Combining the delivery of ART and PrEP in strategic ways could help maximize the reduction in risk of HIV transmission within serodiscordant couples.

At the recent Conference on Retroviruses and Opportunistic Infections (CROI) held in Seattle in February 2015, the preliminary results of a study evaluating an innovative ART–PrEP delivery strategy were presented. The data available so far suggest that the strategy is feasible and highly effective.

Partners PrEP Demonstration Project

The study – known as the Partners PrEP Demonstration project – enrolled heterosexual serodiscordant couples from Kenya and Uganda who were at high risk of HIV transmission and not using ART or PrEP.

The study investigators used a previously developed HIV risk-scoring tool to determine whether couples were at high risk. According to the tool, couples were considered to be at higher HIV risk if they were younger, had fewer children, were living together, were engaging in condomless sex, the HIV-negative partner was uncircumcised (if male), and the HIV-positive partner had a high blood viral load. If the couple's HIV risk score exceeded a certain threshold, they were considered high risk and eligible to enter the study.

Once enrolled, the HIV-positive partner was offered ART (regardless of their CD4 count) and the HIV-negative partner was offered PrEP. Based on whether the HIV-positive partner decided to start ART, the length of time the HIV-negative partner was offered PrEP differed:

  • If the HIV-positive partner started ART immediately, then the HIV-negative partner was asked to take PrEP for six months and then stop. This allowed time for the HIV-positive partner to achieve an undetectable viral load.
  • If the HIV-positive partner decided to delay ART, then the HIV-negative partner was asked to take PrEP until six months after the HIV-positive partner decided to start ART.

This strategy was called “PrEP as a bridge to ART” and was developed to maximize the reduction in HIV transmission risk while at the same time minimizing the use of PrEP by the HIV-negative partner. Minimizing PrEP use is important because it is expensive and – once the HIV-positive partner has been on ART for six months – the risk of HIV transmission is likely so low that PrEP no longer provides a significant added benefit (and is unlikely to be a cost-effective use of resources).

During the study, couples were asked to visit study staff every three months. At each visit, both partners received adherence and risk-reduction support, the HIV-negative partner was tested for HIV, and the HIV-positive partner received their primary HIV care.



Between November 2012 and August 2014, 1,013 heterosexual serodiscordant couples at high risk of HIV transmission were enrolled. Below is an average profile of the couples:

  • the HIV-negative partner was female in 33% and male in 67% of couples
  • age – 30 years
  • 65% reported condomless sex in the past month
  • viral load in HIV-positive partners was 37,000 copies/ml (more than 40% had a viral load of over 50,000)

The results presented at the conference included 858 person-years of follow up (in others words, they followed the equivalent of 858 couples for a year). It is important to note that the study is still ongoing and plans to continue until 2000 person-years have been accumulated.

Uptake of PrEP and ART

Uptake of PrEP and ART in the study has been high. So far, over 95% of couples have initiated PrEP and 80% have started ART (at some point in the study). For the follow-up time included in the preliminary analysis (858 person-years):

  • 48% involved the use of PrEP by itself
  • 27% involved the use of PrEP and ART
  • 16% involved the use of ART by itself
  • 9% involved neither ART nor PrEP use

This means that the HIV-negative partners were on PrEP for the majority (75%) of the follow-up time included in the analysis. However, as the study continues, the percentage of time on PrEP will decrease and the percentage of time on ART will increase.


Despite following the couples for a significant period of time, only two HIV infections have occurred so far. This is significant because researchers estimated that 40 HIV infections would have occurred if ART and PrEP had not been offered to couples. Based on these calculations, investigators concluded that the “PrEP as a bridge to ART” strategy reduced the expected rate of HIV infections by 96%.


Very little data on adherence was presented at CROI. However, study investigators briefly indicated that adherence to PrEP and ART in the study appeared to be high, and that over 90% of those on ART had achieved an undetectable viral load.

The investigators also took a closer look at the two HIV infections which occurred in the study. They did this to determine if the infections were a result of poor adherence to PrEP/ART or failure of these tools to prevent HIV infection. While both participants who became infected had accepted the offer of PrEP, neither had PrEP drugs detectable in their blood at the time their HIV infection was detected, suggesting they were not taking PrEP consistently. For one of these individuals, their HIV-positive partner had not yet started ART. For the other individual, their HIV-positive partner had started ART but the relationship had ended, suggesting this infection originated from a person who was not in the study and may not have been on ART. Therefore, it appeared that both infections were due to poor adherence, and not failure of PrEP/ART to prevent infection.


The study investigators concluded that their preliminary results “demonstrate that PrEP as a bridge to ART is not only feasible but highly effective in preventing HIV transmission in this population.” Since the HIV-negative partners were on PrEP for the majority of the time that couples were followed, the results highlight the importance of adding this strategy to prevention services for serodiscordant couples. Indeed, the vast majority of HIV-negative partners decided to start PrEP once offered, indicating a high level of interest in this option.

While the Partners PrEP demonstration project only enrolled heterosexual couples, PrEP and ART have been found to be effective at preventing HIV transmission in other populations, including gay men and other men who have sex with men, and people who use injection drugs. Therefore, there is no reason to believe that the “PrEP as a bridge to ART” strategy won't also work for other populations, as long as it is used consistently and correctly.


Consolidated guidelines on HIV prevention, diagnosis, treatment and care for key populations – The World Health Organization (WHO)

HIV viral Load, HIV treatment and sexual HIV transmission – CATIE Factsheet

Treatment and viral load: what do we know about their effect on HIV transmission? – Prevention in Focus

Pre-exposure prophylaxis (PrEP) – CATIE Fact Sheet

Moving PrEP into practice: an update on research and implementation – Prevention in Focus

—James Wilton


  1. Cohen MS, Chen YQ, McCauley M, et al. Prevention of HIV-1 infection with early antiretroviral therapy. New England Journal of Medicine . 2011;365(6):493-505.
  2. Baeten JM, Haberer JE, Liu AY, Sista N. Preexposure prophylaxis for HIV prevention: where have we been and where are we going? Journal of Acquired Immune Deficiency Syndromes . 2013;63 Suppl 2:S122-S129.
  3. Baeten J, Heffron R, Kidoguchi L, et al. Near Elimination of HIV Transmission in a Demonstration Project of PrEP and ART. In: Program and abstracts of the 22nd Conference on Retroviruses and Opportunistic Infections (CROI) , Seattle, USA, Feb 23-26, 2015. Abstract 24.

From Canadian AIDS Treatment Information Exchange (CATIE). For more information visit CATIE's Information Network at


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