There have been two reports of mothers with an undetectable, or at least very low, viral load transmitting HIV to their babies through breast feeding, aidsmap reports.

Findings of these cases were presented in a poster at the 22nd International AIDS Conference in Amsterdam (AIDS 2018) in July. They came from the large randomized PROMISE study conducted in 14 low- and middle-income nations that is testing the effectiveness of antiretroviral (ARV) treatment in preventing mother-to-child transmission of HIV before delivery and during and after the breast-feeding period.

One infant tested positive for HIV DNA 14 weeks after birth, at a time when the infant’s mother had an undetectable viral load. The mother’s viral load was also tested six weeks postpartum, when it was between 40 and 1,000.

Another infant tested positive for HIV DNA at about 36 weeks after birth; the baby’s HIV status was then confirmed shortly afterward and again 50 weeks postpartum. That infant’s mother had an undetectable viral load according to tests conducted at weeks 14 and 26 and at both of the visits when the infant tested positive for the virus.

The researchers have two theories about how HIV could have transmitted to these infants. The first is that the sheer volume of breast milk that babies consume may send enough HIV-infected cells into the baby to establish an infection, even when the mother’s plasma viral load is undetectable. The second is that, at least in the case of the first infant, the mother’s virus was too slow to decline to an undetectable level after she started ARVs and she transmitted the virus to her infant when she had a detectable viral load that was below 1,000.

To read the aidsmap article, click here.