After four weeks of age, infants who breast feed from mothers infected with HIV continue to be at risk for infection with HIV for as long as they breastfeed, according to an analysis conducted and funded by the National Institute of Child Health and Human Development of the National Institutes of Health (NIH).

Previously, researchers thought the risk for being infected with the virus from breast milk diminished as an infant grew older.

The analysis determined that a significant proportion of infants - 42 per cent - were infected by breast-feeding after they were 4 weeks old.

The study also found that infants were at greater risk for contracting the virus through breastfeeding if their mothers had low levels of CD4+ cells, an immune cell targeted by the AIDS virus. Moreover, male infants were more likely to contract the virus through breastfeeding than were female infants.

"In many poor countries, mothers who are infected with HIV don't have the option of bottle feeding their infants to prevent spreading the virus to them," said Duane Alexander, director of the NICHD. "This information will help us to devise new ways to help prevent infants from becoming infected with the AIDS virus," he opined.

The analysis pooled information from a number of studies that took place in Africa, said the NICHD author of the study, Jennifer Read of NICHD's Pediatric Adolescent and Maternal AIDS Branch. Dr Read explained that one of the greatest strengths of the study was the large number of children included in the analysis.

For the analysis, researchers examined information on 4,085 children in 9 studies. A total of 3,025 children in the study had negative HIV test results at 4 weeks of age and were breastfed through at least 28 days of age. Of these 3,025 children, 223 had late postnatal transmission - testing negative for HIV at 4 weeks of age, but testing positive after that time. The remainder of the 3,025 children who were uninfected at 4 weeks of age did not become infected.

Late postnatal infections occurred throughout the duration of breastfeeding, with children becoming infected at any time, from when they were 4 weeks old until they were 18 months old. In all, late postnatal transmissions occurred among 42 per cent of the 993 children for whom timing of HIV infection was known.

The analysis also revealed that children of mothers who had low levels of CD4+ cells were more likely to become infected with HIV than were children whose mothers had higher CD4+ levels.

"The association of lower maternal CD4+ counts and a higher risk of transmission was not unexpected," Dr. Read said. "Individuals with lower CD4+ counts may have higher concentrations of HIV in the bloodstream and in breast milk," she added.

According to Dr. Read, the study findings might be useful in devising new strategies to prevent infected mothers from passing HIV to their infants through breast milk.

The researchers discussed possible strategies for preventing the virus from spreading through breast milk, such as giving anti-HIV drugs to mothers who are breast-feeding. Similarly, such drugs might be given to infants while they are breast-feeding.

"An important implication of our analyses is that, since children of HIV-1-infected mothers have a consistent and substantial risk of acquisition of HIV-1 throughout the period of breast-feeding, to be most effective, interventions to prevent transmission through breast-feeding should be continued until the cessation of breast-feeding," the researchers wrote.

The NICHD is part of the National Institutes of Health (NIH), the biomedical research arm of the federal government. NIH is an agency of the U.S. Department of Health and Human Services.