Undetectable = Untransmittable (U=U) is a human rights issue, Dr Carrie Foote told the Conference on Retroviruses and Opportunistic Infections (CROI 2019) in Seattle last week. “All people living with HIV have a right to accurate information about their social, sexual and reproductive health,” she said.

Foote has been living with HIV since 1988, is one of the founding members of the U=U campaign and is an Associate Professor of Sociology in the School of Liberal Arts at Indiana University – Purdue University Indianapolis.

“Stigma is killing us,” she added. “HIV stigma is a public health emergency and U=U is an immediate and effective response to begin to dismantle stigma.”


Undetectable = Untransmittable (U=U)

U=U stands for Undetectable = Untransmittable. It means that when a person living with HIV is on regular treatment that lowers the amount of virus in their body to undetectable levels, there is zero risk of passing on HIV to their partners. The low level of virus is described as an undetectable viral load. 


Social attitudes that suggest that having a particular illness or being in a particular situation is something to be ashamed of. Stigma can be questioned and challenged.


In HIV, usually refers to legal jurisdictions which prosecute people living with HIV who have – or are believed to have – put others at risk of acquiring HIV (exposure to HIV). Other jurisdictions criminalise people who do not disclose their HIV status to sexual partners as well as actual cases of HIV transmission. 

post-exposure prophylaxis (PEP)

A month-long course of antiretroviral medicines taken after exposure or possible exposure to HIV, to reduce the risk of acquiring HIV.

detectable viral load

When viral load is detectable, this indicates that HIV is replicating in the body. If the person is taking HIV treatment but their viral load is detectable, the treatment is not working properly. There may still be a risk of HIV transmission to sexual partners.

She told a CROI symposium that the U=U campaign was launched by Bruce Richman of the Prevention Access Campaign in 2016 and reflects the finding that HIV-positive individuals on successful treatment cannot infect their sexual partners. As Dr Pietro Vernazza from St Gallen Hospital in Switzerland explained, this finding came about after a series of studies demonstrating no documented linked cases of sexual transmission when the HIV-positive partner was on successful antiretroviral treatment and virally suppressed. For more information regarding the basics of U=U, see our factsheet here.

U=U is a gamechanger

Dr Foote said that U=U is an incredibly significant finding, but this "amazing science" is not as well-known as it should be. Millions of people living with HIV are still unaware of the facts and implications of U=U and what it means for their lives. While it is encouraging that countries such as Canada, cities such as Paris, some public health jurisdictions and international organisations (including UNAIDS, the World Health Organization and the International AIDS Society) all support the U=U message, the transformative power of this concept is still limited.

For people living with HIV, U=U has the potential to transform their social, sexual and reproductive lives while also working to dismantle stigma. Additionally, it encourages early testing and treatment and provides a strong public health argument for eliminating barriers to universal care. It is clear that for those living with HIV, widespread knowledge of U=U has far-reaching effects.

Dr Foote shared some quotes from HIV-positive people from around the world, illustrating the impact of U=U. In the words of Mark from Baltimore: “When I finally internalised this message… something suddenly lifted off of me that is hard to describe. It was almost as if someone wiped me clean. I no longer feel like this diseased pariah.”

For this reason, Dr Foote emphasised that resources such as patient-information brochures, HIV factsheets and treatment guidelines need to be updated to reflect the current science.

Viral load does not equal value

Dr Foote said it was crucial to avoid making a link between undetectability and moral judgements. Ideally, all people living with HIV would have an undetectable viral load; however, structural barriers and social injustices prevent some individuals from starting treatment or adhering to it. Factors affecting already marginalised communities such as racism and transphobia might result in barriers to accessing treatment and therefore will impact upon undetectability.

Messaging should not isolate anyone living with HIV, even if they have a detectable viral load. “Treatment is a personal decision and not a public health responsibility,” she said.

While U=U has definite implications for HIV criminalisation and law reform, and could be used as a defence in individual cases, Dr Foote emphasised that HIV criminalisation is fundamentally indefensible. U=U is just one advocacy tool when it comes to demonstrating how these unjust laws are out of step with scientific advances, but they require urgent reform for all those living with HIV, regardless of U=U. A potential danger is that those who do not have an undetectable viral load could become targets for criminalisation, setting up a new good vs bad HIV dichotomy.

Language matters

This was one of the most important points raised by Dr Foote; often the language used around undetectability implies that there is still some level of risk when engaging in condomless sex. However, the research findings are clear: there is zero risk of infection.

Our language needs to reflect this in order to move us forward. Thus, instead of using phrases such as ‘greatly reduces’, ‘close to zero’ or ‘extremely unlikely’, it is important to definitively state that those who have an undetectable viral load cannot pass on the virus and that successful treatment eliminates onward transmission.

One of the physicians in the audience added that attitudes influence language use and it was important for physicians to spread the U=U message without adding interpretation. Another audience member echoed this sentiment and urged health professionals not to make moral judgements regarding their patients’ sex lives and instead simply share the most up-to-date scientific information with them.

Clinical questions

Dr Nneka Nwokolo from the Chelsea and Westminster Hospital in London focused on some of the clinical conundrums and questions that frequently come up in clinical settings around U=U.

Regarding serodiscordant couples (where one partner is HIV positive and the other is HIV negative), the research clearly shows that the risk of infection comes from sexual encounters outside the primary relationship and not from the HIV-positive undetectable partner. Dr Nwokolo said it was important to emphasise this when counselling patients and their partners.

Another clinical question related to treatment adherence and viral load ‘blips’ – when the virus briefly becomes detectable. When an individual has been virally suppressed for a long period of time, an occasional missed dose, or even a viral load ‘blip’ does not negate U=U. This was emphasised by both Drs Nwokolo and Vernazza.

However, it is not known exactly how many missed does do affect the risk of transmission.

This is also linked to the exact threshold at which it can be said an individual has an undetectable viral load. The studies that have provided the bedrock of evidence have generally used below 200-400 copies/ml as a definition of undetectability. However, viral load tests with a lower limit of detection of 1000 copies/ml are used in some resource-limited settings. She said it was not certain whether a viral load below 1000 copies/ml equates to no transmission risk.

Additionally, while current guidelines state that an individual must be undetectable for six months before it can be assumed they can safely engage in condomless sex, Dr Nwokolo stated that this area requires more research as the actual duration might be less.

Drs Nwokolo and Vernazza both stated that the presence of other sexually transmitted infections (STIs) does not affect U=U, as there were high rates of STIs in PARTNER and other studies, which did not affect infection rates.

Various speakers emphasised that U=U is specifically related to sexual transmission of HIV. An undetectable viral load does not prevent other STIs, and is not necessarily as effective when considering other HIV infection routes. Thus, there is still a chance – albeit very small, at around 0.3-0.6% – of infection via other routes such as breastfeeding, needlestick injury and needle sharing among people using intravenous drugs even when the HIV-positive individual is undetectable. There are various clinical questions around managing these other possible routes of infection, with different responses in different settings.

For instance, guidelines differ from country to country regarding whether post-exposure prophylaxis (PEP) should be made available after a needlestick injury from a patient with an undetectable viral load. In the US, it is routinely offered, whereas in the UK it is not. This is indicative of different ways of managing low levels of risk from patients with an undetectable viral load. Patient choice and attitudes towards potential infection are important factors, as PEP is occasionally prescribed as a means of alleviating anxiety instead of being based on the actual risk of infection. Dr Nwokolo disagreed with this approach and felt that education about U=U and the low risk of transmission was important.

In relation to breastfeeding, high-resource countries such as the US, tend to recommend formula-feeding for HIV-positive undetectable mothers, whereas in low-resource settings, the benefits of breastfeeding are judged to outweigh any potential risks. In these settings, diarrhoeal diseases, pneumonia and malnutrition are common, as access to clean water may be limited, with infant formula also being expensive or inaccessible. Thus, the very small risk of infection from an HIV-positive undetectable mother is seen as an acceptable level of risk in these settings.

U=U in low-resource settings

Dr Nwokolo raised the question of challenges with U=U in low-resource settings. Competing priorities (such as for food or transport) might compromise adherence. While viral load testing in the US may be as frequent as every three to four months, it may only happen annually or less frequently in low-resource contexts, with results not readily available.

This concern was echoed by a physician in the audience from Zimbabwe, who stated that if only 20% of patients receive viral load testing, how can doctors confidently support the U=U message? Dr Foote believed that this was an important reason why advocating for universal access to testing, treatment and monitoring services was so important: so that everyone could benefit from U=U, especially in low-resource settings where it could be so impactful.


Foote C Me and U: community perspectives. Conference on Retroviruses and Opportunistic Infections, Seattle, abstract 118, 2019.

View the abstract on the conference website.

Watch the webcast of this presentation on the conference website.

Nwokolo N Caring for U: clinical conundrums. Conference on Retroviruses and Opportunistic Infections, Seattle, abstract 117, 2019.

View the abstract on the conference website.

Watch the webcast of this presentation on the conference website.

Vernazza P The story of U: scientific underpinnings. Conference on Retroviruses and Opportunistic Infections, Seattle, abstract 116, 2019.

View the abstract on the conference website.

Watch the webcast of this presentation on the conference website.