Babies born with HIV benefit the most if treatment is started within hours or days of birth rather than waiting for them to be a little older, a study published Wednesday in the journal Science Translational Medicine found.

A Harvard-led study of 40 infected infants in Botswana found those treated within hours of birth developed a much smaller viral reservoir, the pool of virus that remains within the body during and after treatment and is responsible for later relapses. While babies who were given the medications starting at four months after birth did not fare as well.

The first group of babies also had more robust immune systems even than babies born without the virus.

The study was based on a case in the U.S. know as the “Mississippi Baby.” That case involved a baby who was treated within 30 hours of birth in July 2010. Her family stopped treatment when she was a toddler and she stunned the medical community by remaining in remission for 27 months before she relapsed and restarted treatment.

The findings of the Mississippi Baby case and the study in Botswana are particularly important to poorer nations where at-risk babies are not tested for HIV immediately after birth, as they are in the U.S., Europe and South Africa.

The availability of anti-HIV drugs can prevent infected moms from passing the virus on to their children but despite that, a study found that some 300-500 infants are thought to be infected every day in sub-Saharan Africa.

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