A mother who feeds formula or other food part of the time and breast-feeds to fill in is therefore increasing her baby's vulnerability to HIV infection.
"We already know how to prevent breast-feeding transmission: Don't breast-feed," says Lynne Mofenson of the National Institute of Child Health and Human Development in Bethesda, Md.
The researchers didn't randomly assign women to breast-feed or formula feed, but studied healthy and HIV-infected women who chose to breast-feed only, bottle-feed only, or supplement breast-feeding with formula, milk, or solid foods.
At an October 2006 meeting in Geneva, WHO members refined the breast-feeding recommendation for HIV-infected women to emphasize that the first choice for babies during their first 6 months is exclusive breast-feeding, unless the conditions for bottle-feedhag are "acceptable, feasible, affordable, sustainable, and safe." That is, women should exclusively breast-feed if they don't have access to clean water, good health care and medicines, and formula.