Does U=U for breastfeeding mothers and infants? Researchers highlight unanswered questions

Research is urgently needed to provide clear recommendations on breastfeeding to mothers with HIV who have an undetectable viral load on antiretroviral treatment, a group of researchers say in the journal The Lancet HIV.

They say that “There is insufficient evidence to state that U=U [undetectable equals untransmittable] in the context of breastfeeding”. Although the risk of transmission is probably small if women have suppressed viral load, the authors highlight a number of research questions that still need to be addressed in order to give evidence-based advice.

In resource-limited settings, the World Health Organization recommends that women with HIV should breastfeed if formula feeding is not safe and feasible and that mothers should receive antiretroviral therapy and infants should receive antiretroviral prophylaxis.



The fluid portion of the blood.

deoxyribonucleic acid (DNA)

The material in the nucleus of a cell where genetic information is stored.

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.

CD4 cells

The primary white blood cells of the immune system, which signal to other immune system cells how and when to fight infections. HIV preferentially infects and destroys CD4 cells, which are also known as CD4+ T cells or T helper cells.


A healthcare professional’s recommendation that a person sees another medical specialist or service.

In high-income settings, guidelines have discouraged breastfeeding in women on antiretroviral treatment. However, recent guideline updates by the British HIV Association, the European AIDS Clinical Society and the US Department of Health and Human Services each acknowledge that women with undetectable viral loads may choose to breastfeed, and if they do so, regular viral load and adherence monitoring should take place to minimise the risk of transmission. But the authors of The Lancet HIV review say there is still a lack of evidence to determine how women who breastfeed should be monitored and what risks might remain, despite an undetectable viral load.

The authors of The Lancet HIV paper say that several important research questions still need to be answered to give women the fullest possible information about the risks of breastfeeding, and to determine what package of monitoring and support is necessary during the breastfeeding period.

They highlight questions that still need to be answered.

Is there a level of HIV in plasma or breast milk below which the virus cannot be transmitted?

Several studies have shown that women can have detectable HIV in breast milk even when they have undetectable HIV in plasma.

Investigators on the Breastfeeding, Antiretrovirals and Nutrition (BAN) study concluded that maintaining a plasma viral load below 100 copies/ml may prevent breast milk transmission, based on the observation that all mothers who transmitted HIV during breastfeeding in that study had at least one plasma viral load measurement above 100 copies/ml. However, the Mma Bana study in Botswana identified two cases of transmission that probably occurred during the breastfeeding period and where each mother had a plasma viral load below 50 copies/ml both one month and three months after delivery (infant infection was detected in each case around 90 days after delivery). In one case the mother reported adherence difficulties.

Furthermore, one case has been documented in Malawi where HIV was transmitted through breastfeeding even though the mother had an undetectable viral load (< 37 copies/ml) in both plasma and breast milk.

A mathematical modelling exercise led by UNAIDS, based on all available data from clinical trials and observational cohorts up to 2012, estimated that there was a 0.16% (approximately one in 750) risk of HIV transmission for every month of breastfeeding if the mother had already started antiretroviral treatment prior to delivery.

These findings show that the risk of transmission is small, but cannot be ruled out. The researchers recommend the establishment of an international registry that will record the outcomes of all breastfed infants of mothers with HIV. As well as looking at drug safety in infants, a registry would provide detailed information on any cases of transmission that might occur, and improve estimates of the risk of transmission.

Can antiretroviral drugs suppress cell-associated virus?

The persisting risk of transmission through breast milk despite viral suppression in plasma is probably a consequence of cell-associated virus in breast milk, experts have concluded.

HIV may be cell-free or it may be cell-associated, that is, contained within an immune system cell such as a CD4 lymphocyte in the form of viral DNA. Breast milk contains several types of cells potentially infected with HIV. These cells include long-lived latently infected CD4 cells containing proviral DNA that is not replicating and so is not susceptible to antiretroviral drugs.

The researchers say that more research is needed to establish whether long-term antiretroviral treatment prior to breastfeeding reduces levels of cell-associated virus in breast milk.

What virological monitoring should breastfeeding women receive?

The British HIV Association (BHIVA) recommends that women who are breastfeeding while on antiretroviral treatment should attend their clinic once a month for a viral load test for themselves and their infant. US guidelines indicate testing every one to two months.

Although draft BHIVA guidelines recommend against breastfeeding, they acknowledge that some women will choose to do so and that it is better that women can be open about this choice and receive support from their clinic to minimise the risk of transmission.

The BHIVA draft guidelines recommend a harm reduction approach: minimise the amount of time that breastfeeding takes place, stop breastfeeding if the infant has any gastrointestinal symptoms or if the mother has mastitis or a breast infection, and test viral load in mother and baby once a month during breastfeeding.

BHIVA guidelines emphasise that “Women who breastfeed with a known detectable HIV viral load should be referred to social care as they are putting their infant at significant risk of HIV infection.”


Waitt C et al. Does U=U for breastfeeding mothers and infants? Breastfeeding by mothers on effective treatment for HIV in high-income settings. The Lancet HIV, advance online publication, 27 June 2018.