Doctors now have convincing evidence that they put HIV into long-term remission in a Mississippi baby born with the Aids virus.

The case was reported earlier this year but some doctors were sceptical that the baby was really infected rather than testing positive because of exposure to the virus in her mother's blood.

The new report, published online on Wednesday by the New England Journal of Medicine, makes clear that the girl, now three, was infected in the womb. She was treated with an aggressive cocktail of antiretroviral drugs and shows no active infection despite stopping treatment 18 months ago.

"We want to be very cautious here. We're calling it remission because we'd like to observe the child for a longer time and be absolutely sure there's no rebound," said Dr Katherine Luzuriaga, an Aids expert from the University of Massachusetts who is involved in the child's care.

The US government's top Aids scientist, Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, agreed.

"At minimum, the baby is in a clear remission," he said, after a scientist at the institute conducted tests that showed no active virus in the child. "It is possible that the baby has actually been cured. We don't have a definition for cure as we do for certain cancers, where after five years or so you can be relatively certain the person is not going to go and relapse."

A US government-sponsored international study starting in January aims to test early treatment in babies born with HIV to see if the results in this case can be reproduced.

HIV-infected mothers in the US generally receive antiretroviral therapy during pregnancy, which greatly reduces the chances of them passing on the virus to their babies. But the Mississippi woman had no pre-natal care and her HIV was discovered during labour. Doctors considered the baby to be at such a high risk that they started the child on three powerful medicines 30 hours after birth, rather than waiting for a test to confirm infection, as is usually done.

Within a month, the baby's virus fell to undetectable levels. She remained on treatment until she was 18 months old, when doctors lost contact with her. Ten months later, when she returned, they could find no sign of infection even though her mother had stopped giving the child Aids medicines.

Only one other person is thought to have been cured of HIV infection – a San Francisco man who had a bone marrow transplant in 2007, from a donor with natural resistance to HIV, and showed no sign of infection five years later.

But unlike the bone marrow case, the Mississippi baby has "no immune mechanism we can identify that would keep the virus in check", said another study author, Dr Deborah Persaud of the Johns Hopkins children's centre, who helped to investigate the case.

Dr Peter Havens, the paediatric HIV chief at the children's hospital of Wisconsin and a government adviser on treatment guidelines, said the child might have an undiscovered genetic trait that helped her to manage the virus.

"I'm just not convinced that her dramatic response would be replicable in a large population," he said, adding that it was too soon to recommend treating other high-risk babies with such an aggressive combination of drugs without more research.

In the upcoming study, doctors plan to give Aids medicines for at least two years and watch for signs of remission before suspending treatment and seeing whether a remission results.

The Mississippi case "did open people's eyes further" about a possible cure, Luzuriaga said. "We might be able to intervene early and spare children a lifetime of therapy. That is the potential impact of this case."