Women with HIV are often told by health care providers to refrain from breastfeeding for fear their breast milk will transmit the virus to their infants. But a new study released Thursday in the journal PLoS Pathogens suggests breast milk may kill the virus and protect against its transmission.

The study was done on mice, adding to the growing confusion as to whether it's ever safe for women with HIV to breastfeed.

Researchers at the University of North Carolina School of Medicine fed mice whose immune systems had been engineered to mimic those of humans breast milk from healthy human donors that had been injected with HIV.

The researcher found that the virus could not be transmitted to the mice through the breast milk, and that the virus died when it entered the breast milk.

"We reinforced the belief, and we have solid data that milk is not a vehicle for transmission but may offer protection," said Victor Garcia, a professor of medicine at the University of North Carolina, and a co-author of the study. "Milk should not be withheld from children."

More than 15 percent of new HIV infections occur in children, according to the World Health Organization. If untreated, only 65 percent of HIV-infected children will live to see their first birthdays, and fewer than half will make it to the age of 2, WHO estimates.

For years, HIV experts have linked the virus in babies to breastfeeding. But most infants who are breastfed by HIV-infected mothers, even for long periods of time, do not become infected.

WHO recommends that infected mothers in some countries breastfeed their infants, and that both mother and infant take antiretroviral medication to avoid HIV transmission.

"One of the big breakthroughs of having this model is to look at what is affecting transmission," said Garcia. "If milk isn't it, then how is it being transmitted?"

Garcia said one way to answer that question might be to learn what it is in breast milk that kills the HIV and study whether it can be used to protect against other forms of transmission.

But the findings don't mean that infected mothers should breastfeed their children without taking antiretroviral medications just yet, Garcia said. Future studies will look at milk from infected donors to see if the outcome will be the same, he said.

"Milk itself has an inhibitory effect," said Garcia. "But from what we know so far, it seems like there won't be a big difference."