A baby girl in the US born with HIV and believed cured after very early treatment has now been found to still harbour the virus.

Tests last week on the four-year-old child from Mississippi indicate she is no longer in remission, say doctors.

She had appeared free of HIV as recently as March, without receiving treatment for nearly two years.

The news represents a setback for hopes that very early treatment of drugs may reverse permanent infection.

Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told US media the new results were "obviously disappointing" and had possible implications on an upcoming federal HIV study.

"We're going to take a good hard look at the study and see if it needs any modifications," he said.

Genetic mutation

The child, nicknamed the "Mississippi baby", did not receive any pre-natal HIV care prior to birth.

Because of a greater risk of infection, she was started on a powerful HIV treatment just hours after labour.

She continued to receive treatment until 18 months old, when doctors could not locate her. When she returned 10 months later, no sign of infection was evident though her mother had not given her HIV medication in the interim.

Repeated tests showed no detectable HIV virus until last week. Doctors do not yet know why the virus re-emerged.

A second child with HIV was given early treatment just hours after birth in Los Angeles in April 2013.

Subsequent tests indicate she completely cleared the virus, but that child also received ongoing treatment.

Only one adult is currently believed to have been cured of HIV.

In 2007, Timothy Ray Brown received a bone marrow transplant from a donor with a rare genetic mutation that resists HIV. He has shown no signs of infection for more than five years.