The antibody function known as antibody dependent cellular cytotoxicity (ADCC) and the ADCC sensitivity of HIV strains may influence the transmission of HIV from mother to child during breastfeeding. These data imply that enhancing ADCC, through a vaccine, for example, may not be sufficient to prevent transmission because chronically infected individuals can harbor ADCC-resistant strains. The findings provide novel insights about immunologic characteristics that a vaccine may need to elicit to help block HIV transmission.