HIV, breastfeeding and being ‘undetectable’

Antiretroviral treatment (ART) keeps people living with HIV healthy – but it’s also a powerful prevention tool. We now know that with effective HIV treatment, people living with HIV can suppress the virus to an undetectable level which makes the risk of sexual transmission of HIV equal to zero (known as ‘Undetectable = Untransmittable’ or U=U).

The U=U movement has unburdened thousands of people living with HIV with the knowledge that they cannot give HIV to their partners through sex if they are virally suppressed. But there has so far been no consensus on declaring U=U for breastfeeding. Why is that?

Why are breastfeeding guidelines different?

First of all, it’s important to note that not all HIV clinical guidelines for infant feeding are the same.

In high-income countries, women are advised not to breastfeed their infants because of the potential risk of HIV transmission. But in low-income countries, replacement feeding is often neither safe nor feasible, as mothers may not have access to clean water or formula feed. In these cases, the immunological benefits of exclusive breastfeeding far outweigh the risk of HIV transmission for the child in the context of malnutrition, diarrhoea, disease or pneumonia.

What does the science say about breast milk and HIV transmission?

30 years into the response, there are still a number of unanswered questions around the exact mechanism by which a baby can become infected via breast milk. There also remain questions about the viral load of HIV in blood versus the viral load in breast milk, and what a ‘safe’ threshold is for transmission. This is why scientists and policymakers have yet to declare U=U for breastfeeding.

It is known that HIV particles and HIV-infected cells are present in breast milk, and there is a clear link between the mother having a high viral load and the baby becoming infected, which is why it is important for the mother to be on treatment while breastfeeding to reduce her viral load.

But a number of factors could also contribute to a baby becoming infected – including the high percentage of latently-infected CD4 cells and other tissue cells such as macrophages and dendritic cells in breastmilk compared to blood. These cells can hide inactivated HIV in ‘viral reservoirs’ which treatment cannot get to. These cells may be associated with mother-to-child transmission in women with or without suppressed viral loads, but we require more research to know exactly how they contribute to infection.

What is the ‘real’ risk of HIV transmission?

In scenarios where women do not have access to antiretroviral treatment, up to 20% of infants will become infected with HIV through breastfeeding if they weren’t already infected during pregnancy or childbirth. Where treatment is available, this risk can be reduced to below 1% in the real world.

So despite the many unanswered scientific questions, we do know the actual risk of HIV transmission is extremely low when the mother has access to suppressive ART.

The PROMISE trial in southern Africa found the transmission risk when the mother was on combination ART to be as low as 0.3% at 6 months, and 0.7% at 12 months. In another meta-analysis of six studies in low-income settings where mothers started treatment before or during pregnancy, post-natal HIV transmission rates of 1.08% were estimated at 6 months, with higher rates from mothers who started ART in the later stages of pregnancy (when they would be less likely to have achieved viral load suppression during breastfeeding).

According to researchers in a recent Lancet viewpoint, most of the cases of mother-to-child transmission during breastfeeding can be partly explained by either detectable virus or poor adherence.

Are breastfeeding guidelines changing for mothers with undetectable viral load?

In light of U=U, researchers and clinicians in high-income countries are calling for a better understanding of HIV infection via breastfeeding. There also remain concerns around treatment toxicity in infants, as antiretroviral treatment is present in breast milk of mothers, as well as the effect of inflammation in the mammary gland, such as mastitis, which can increase HIV shedding in breast milk.

But we know that breastfeeding has multiple benefits over formula feeding for the baby, such as anti-inflammatory effects, improvements in the baby’s immune system, improvement in the development of the gut microbiome, and better overall health and psychological outcomes in the long-run.  

While no high-income countries explicitly recommend breastfeeding, treatment bodies in the US and in European countries such as the UK and Switzerland now call for a human-rights centred approach, which advocates for mothers to be counselled about breastfeeding and the risks, and supported if they wish to exclusively breastfeed and are virally suppressed.

So until our understanding of the science improves, guidelines are unlikely to recommend breastfeeding for mothers with an undetectable viral load in high-income countries any time soon.