A three-year-old American girl born with HIV and treated with a combination of antiviral drugs unusually early "continues to do well" and remains free of the virus 18 months after all treatment ceased, US doctors said Wednesday.

The girl in the US state of Mississippi, born to an HIV- infected mother, remains negative to tests for HIV-specific antibodies, the standard clinical indicator of HIV infection, the doctors reported in the New England Journal of Medicine.

The doctors, however, are hesitant to declare the child fully cured but said they now have "compelling evidence" that HIV- infected infants could be functionally cured if anti-retroviral therapy begins within hours or days of infection.

"Our findings suggest that this child's remission is not a mere fluke but the likely result of aggressive and very early therapy that may have prevented the virus from taking a hold in the child's immune cells," lead author Deborah Persaud, a virologist and pediatric HIV expert at the Johns Hopkins University, said in a statement.

The little girl began combination anti-retroviral treatment 30 hours after birth owing to high-risk exposure. A series of tests in the subsequent days and weeks showed progressively diminishing viral presence in her blood, until it reached undetectable levels 29 days after birth. The child remained on antivirals until 18 months of age, at which point she was lost to follow-up for a while and stopped treatment.

Upon return to care, about 10 months after treatment stopped, the child underwent repeated standard HIV tests, none of which detected virus in the blood.

A federally funded study set to begin in early 2014 will test the early-treatment method used in the Mississippi case to determine whether the approach could be used in all HIV-infected newborns, the doctors said.

Previously, Timothy Brown, also known as "the Berlin Patient," is thought to be the only individual functionally cured of HIV that causes AIDS, a disease with no cure at present. Originally from Seattle in the US state of Washington, Brown was pursuing his studies in Berlin, Germany, when he was diagnosed with HIV in 1995. Brown was diagnosed with acute myeloid leukemia (AML) and in 2007 underwent a bone marrow transplant.

The bone marrow cells came from a donor with a rare genetic mutation of the white blood cells that renders some people resistant to HIV, a benefit that transferred to the recipient. Such a complex treatment approach, however, is neither feasible nor practical for the 33 million people worldwide infected with HIV, the doctors said.

In the Mississippi girl, ultrasensitive tests designed to sniff out trace amounts of virus "intermittently" detected viral footprints but this "leftover" HIV appears incapable of forming new virus and reigniting infection, they said.

Importantly, the child exhibits none of the immune characteristics seen in the so-called "elite controllers," a tiny percentage of HIV infected people whose immune systems allow them to naturally keep the virus in check without treatment, said the doctors.

Such people's immune systems are revved up to suppress viral replication but this is not the case with the Mississippi child, they said.

"The absence of immune system characteristics seen in elite controllers in this child is an indicator that early therapy, rather than natural immune mechanisms, led to the child's remission," the doctors said.

Currently, high-risk newborns, those born to mothers with poorly controlled infections or whose mothers' HIV status is discovered around the time of delivery, receive a preemptive combination of antivirals to prevent infection. They do not start treatment at full antiviral doses until infection is confirmed.

In an editorial accompanying the study, Scott Hammer of the Columbia University, cautioned that the child may be unique and thus doctors have to be careful before inferring general principles from this case report.