Variation in breastmilk HIV-1 viral load in left and right breasts during the first 3 months of lactation.

The mechanism and risk factors associated with mother-to-child transmission of HIV-1 through breastfeeding remain unclear; breastmilk viral load may be an important determinant of transmission. Analysis of breastmilk cell-free viral load in samples taken from each breast at 1, 6 and 14 weeks postpartum showed that HIV-1 is shed intermittently and load may differ considerably between breasts of an individual woman at any given time. Breastmilk HIV-1 load was undetectable in approximately one-third of samples.