In what researchers say is a serious setback in the quest for an AIDS cure, a child believed cured of H.I.V. with drugs administered after her birth in Mississippi is now displaying signs of the virus.

Reports following the girl's 2013 birth suggested she was the first person in whom drugs had completely eliminated the virus, raising hopes that other babies infected in the womb could be cured after they were born.

Around 260,000 babies carrying the virus are born around the world every year, and the Mississippi case had doctors planning worldwide clinical trials of multiple-drug therapies like that used in the case of the apparently cured child.

However the child, whose name has not been released, has now tested positive for the human immunodeficiency virus, federal officials are saying.

That news "felt very much like a punch to the gut," says Hannah Gay, a physician who treated the baby.

The Mississippi doctors made the unprecedented decision to begin drug treatments immediately after the child's birth because the mother was determined to be HIV positive and had received no treatment for the virus during her pregnancy.

Infants are not usually treated until as least 6 weeks after birth when an HIV infection can be confirmed.

Gay said the child had been examined recently and although in apparently good health, blood tests results showed the presence of the virus.

She is receiving anti-viral treatments and is responding well to them, doctors say.

DNA sequencing has confirmed it is not a later infection but is the original virus the mother passed to the child during pregnancy, Gay says.

"We knew it was a possibility," Gay said. "We had no way to predict if and when that might occur."

In the light of an apparent failure of the cure, researchers say they are reexamining plans for the global trials.

"We've got to go back and look at the trial's design," says AIDS expert Dr. Anthony S. Fauci, head of the National Institute of Allergy and Infectious Diseases.

The tests on the Mississippi girl showed a viral load of 16,000 copies of the virus per cubic milliliter of blood.

"You sometimes get a blip of 100 copies or 500 copies, but 16,000 is not a blip," Fauci says. "That is an unequivocal relapse."

 "It's obviously disappointing, but I was not surprised," he says of the news. "I've been chasing these reservoirs for the last 25 years, and I know this virus has a really uncanny way of hiding itself."