The evidence supporting U=U

On this page you'll learn about the research that underpins the Can't Pass It On/Undetectable = Untransmittable (U=U) message.

By the end of this section you'll be able to:

• Explain the context for the research that proved that people living with HIV who have an undetectable viral load can’t pass it on.
• Understand the evidence.
• Explain the evidence to patients in terms of:
  • How each of the four main studies built on the previous work.
  • How they confirmed that anyone who acquired HIV in the study got it from someone outside their primary relationship.
• Explain to patients what being virally suppressed means and how this is tested for.
• Answer questions from your patients relating to how long it takes to become virally suppressed and how to maintain an undetectable viral load.

• The context for the research that proved that people living with HIV who are on effective treatment can’t pass the virus on to their sexual partners.
• The evidence base that has built over two decades.
• Why it took so long to scientifically and categorically prove that people living with HIV who have an undetectable viral load can’t pass it on.
• How strong the research evidence to support U=U is.
• A discussion of the questions that can be asked to challenge the U=U message.
• The definition of U=U.

In this video, Professor Alison Rodger talks you through the research that underpins the Can't Pass It On/Undetectable = Untransmittable (U=U) message.

Alison is Professor of Infectious Diseases and HIV at University College London, and lead author on the Partner 1 and Partner 2 studies.

This video is divided into the following chapters, which you can access within the video using the scroll bar:

• Introduction, definitions and risk of HIV transmission (0:00).
• The evidence leading up to the Partner studies (1:48)
• The Partner studies (6:04).
• How the evidence weighs up (16:09).
• 'Yes, but...'s, including genital shedding, viral load rebound or blips (16:50).
• When does U=U apply? (19:54).

'Yes, but...'s

‘Yes, but...’s are the questions or challenges we sometimes get about the U=U message.

If a patient has an undetectable viral load in the blood, then STIs and potential intermittent low-level detection of HIV RNA in semen do not impact on the U=U message.

Viral load blips in the region of 20-200 copies/ml are not a concern and the U=U message still applies. These blips are resuppressed very quickly and viral load rebounds are not common in the UK setting: if someone is adherent to treatment their viral load remains suppressed.

Based on 2019 figures, 98% of people living with HIV in the UK are on antiretroviral therapy (ART) and 97% of those have an undetectable viral load. If someone is on treatment and taking it as prescribed, they can be confident they will not pass the virus on to their sexual partners as there is a very low risk of viral load rebound.

Does U=U apply in the first six months on ART once the virus is suppressed in blood?

The definition of U=U is that an individual has a zero risk of passing the virus on to their sexual partner once they have been on treatment for six months and have an undetectable viral load. It usually takes between three and six months for the viral load to become undetectable after starting treatment. However, if an individual’s adherence is good and they have a documented undetectable viral load sooner than six months, they can be confident they will not pass the virus on to their sexual partner(s).

Evidence base

The evidence base and how it built over two decades:

1. Rakai Study: Quinn T et al N Engl J Med 2000; 342: 921-9 Viral Load and Heterosexual Transmission of Human Immunodeficiency Virus Type.

2. Swiss Statement [PDF]: Vernazza P et al. HIV positive individuals not suffering from any other STD and adhering to an effective antiretroviral treatment do not transmit HIV sexually. Bulletin des Médecins Suisses 89 (5), 30 January 2008 (English translation).

3. HPTN052: Cohen M et al N Engl J Med 2011; 365: 493-505 Prevention of HIV-1 Infection with Early Antiretroviral Therapy.

4. Opposites Attract: Bavinton B et al. Strategies used by gay male serodiscordant couples to reduce the risk of HIV transmission from anal intercourse in three countries. Journal of the International AIDS Society J Int AIDS Soc. 2019;  22( 4):e25277.

5. Partner 1: Rodger A et al JAMA. 2016;316(2): 171-181 Sexual Activity Without Condoms and Risk of HIV Transmission in Serodifferent Couples When the HIV-Positive Partner Is Using Suppressive Antiretroviral Therapy.

6. Partner 2: Rodger A et al Lancet 2019; 393: 2428–38 Risk of HIV transmission through condomless sex in serodifferent gay couples with the HIV-positive partner taking suppressive antiretroviral therapy (PARTNER): final results of a multicentre, prospective, observational study.

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