A girl in Mississippi who was thought to have been cured of HIV with aggressive drug treatment immediately after birth is showing signs of infection with the virus, federal health officials said Thursday, a serious setback to hopes for a cure for AIDS.

The report in March 2013 that the child had apparently been cured raised the possibility that aggressive early treatment might be able to reverse infections in newborns — and perhaps in newly infected adults. About 2.3 million people worldwide were newly infected with HIV in 2012, the last year for which figures were available; 260,000 were infants infected at birth or immediately afterward.

During a telephone news conference held by the National Institutes of Health, Dr. Hannah Gay, the pediatrician at the University of Mississippi Medical Center in Jackson who first put the child on large doses of antiretroviral drugs, said it was “like a punch in the gut.”

The girl is now nearly 4. As recently as March, doctors had said she seemed free of HIV. But Thursday, doctors said they were surprised last week to find the virus in her blood, and there were signs it was harming her immune system. She is now back on treatment and is responding well, they said.

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With hopes raised by the Mississippi case, doctors had made plans for a worldwide clinical trial in which about 450 babies — chosen because their infected mothers had no testing or treatment before the births — would be put on the three-drug regimen called triple therapy. If those who were infected with HIV showed no virus after 48 weeks of treatment, the plan was to stop their drugs and see if they had been cured.

In light of the failure to cure the Mississippi girl, “We’ve got to go back and look at the trial’s design,” said Dr. Anthony Fauci, a leading AIDS expert who is director of the National Institute of Allergy and Infectious Diseases.

In March, a second baby, born in Long Beach, Calif., appeared to have been cured after early and aggressive treatment.

Most HIV-infected mothers in the United States get AIDS medicines during pregnancy, which greatly cuts the chances they will pass the virus to their babies. The Mississippi baby’s mother received no prenatal care and her HIV was discovered during labor. Doctors started the infant on a powerful three-drug treatment 30 hours after birth, even before tests could determine whether she was infected. The girl was treated 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 the mother had stopped giving her AIDS medicines.

The Mississippi case stirred worldwide excitement last year when it was described in The New England Journal of Medicine. Dr. Deborah Persaud, an associate professor at Johns Hopkins Children’s Center and the lead author of the report, said at the time that it was “proof of principle that we can cure HIV infection if we can replicate this case.”

On Thursday, Persaud said the fact that the child had remained virus-free for two years was “unprecedented.” Normally, the virus rebounds in a few weeks.

The child’s virus was identical to the mother’s, so there was no doubt that it was the virus passed at birth, not a later infection.

Before the Mississippi baby, one person had been considered cured of HIV. That was Seattle native Timothy Brown, who had a transplant of blood stem cells to treat his leukemia after his bone marrow was wiped out with drugs and radiation. The new stem cells were from a donor with a natural resistance to HIV.

Material from The Associated Press is included in this report.