Factors which increase the risk

The longer the period for which an infant is breastfed, the greater the risk of HIV transmission. A meta-analysis of nine major studies of mother-to-child HIV transmission showed that the risk of HIV transmission is nearly constant throughout the period of breastfeeding. Consequently, a longer duration of breast-feeding is associated with a greater cumulative risk of transmission.¹

A higher viral load in breast milk is associated with a higher risk of transmission. The virus is present in lower concentrations than in blood, but there is a close correlation between viral loads in blood and breast milk. Viral loads are highest in the first two weeks after childbirth.²

Consequently, providing antiretroviral therapy (ART) to the mother, the infant or both during the breastfeeding period can significantly reduce the risk of HIV transmission through breastfeeding. Antiretrovirals taken by the mother reduce the viral load in breast milk, but are also consumed by the infant and provide prophylaxis against infection.

In African countries, a number of strategies have been investigated, including continuing maternal therapy for a few weeks after delivery, continuing maternal therapy throughout the period of breastfeeding, providing prophylactic therapy to the infant for a few weeks after delivery and providing prophylactic therapy to the infant throughout the period of breastfeeding. Depending on the strategy, transmission rates can be as low as 0 to 3%. Further details are given in the HIV treatments directory.

Women who themselves acquire HIV after giving birth are at an elevated risk of passing on their infection, due to the raised viral load in breast milk soon after infection.³

When the baby’s only food source is breastmilk, the risk of infection is lower than if he or she also consumes other liquids or solids. While this seems counter-intuitive (infants who are exclusively breastfed are exposed to more HIV-infected milk), it has been observed in a number of studies. For example, a large Zimbabwean study found that mixed feeding resulted in a fourfold greater risk of HIV transmission at six months and a threefold greater risk at 18 months.⁴

The reasons for the increased risk are still not fully understood. It is thought that food and water are seen by the infant gut immune system as ‘foreign’ and immunological cells are recruited to fight the foreign antigens. This brings infectious breast milk in closer proximity to target CD4+ cells and macrophages. Breast milk stimulates no such immunological mobilization, so that even with infectious breast milk, the exclusively breastfed infant is less likely to get infected.

Because of the increased risk associated with mixed feeding, breastfeeding mothers have sometimes been advised to make a rapid switch to solid foods at an early stage.

However viral load in breast milk has been observed to be considerably higher following rapid weaning than it had been beforehand. This suggests that if mothers do return to breastfeeding following weaning, the transmission risk will be greater than it was beforehand.⁵

Studies have found that male infants are at higher risk than female infants.¹ This may be due to cultural differences in feeding practices (for example, a higher rate of mixed feeding for boys than girls).

Mastitis, breast abscesses and bleeding nipples can lead to inflammation, increasing viral levels in breast milk and hence transmission risk. Moreover should milk become contaminated with blood, this also carries HIV. Should breastfeeding women develop any of these conditions, they are advised to avoid feeding from the affected breast until the problem clears up.⁶