A Mississippi toddler believed to have been cured of HIV shortly after birth now has detectable levels of the virus once again, doctors reported on Thursday.

American researchers reported in March 2013 that, following early treatment with particularly aggressive antiretroviral medication, a Mississippi baby girl showed was no longer showing signs of the virus, in the first recorded “functional cure” of its kind. She was believed to have contracted the virus from her mother, but treatment was started before that could be fully confirmed by tests.

However, U.S. health official and the child’s doctors have reported that the presence of HIV in her blood had returned to detectable levels, dashing hopes that the treatment might have cured her before the disease could take hold.

Dr Hannah Gay, the paediatric HIV specialist who treated the child when she born, described her disappointment as like “a punch to the gut”. The girl, now 4, has been returned to permanent anti-HIV drug treatment.

Dr Deborah Persaud, another paediatric HIV specialist involved in the case, said: “The fact that this child was able to remain off antiretroviral treatment for two years and maintain quiescent virus for that length of time is unprecedented. Typically, when treatment is stopped, HIV levels rebound within weeks, not years.”

Dr Anthony Fauci, Director of the National Institute of Allergy and Infectious Diseases (NIAID) within the National Institutes of Health (NIH), said in a statement: “Certainly, this is a disappointing turn of events for this young child, the medical staff involved in the child’s care, and the HIV/AIDS research community.

“Scientifically, this development reminds us that we still have much more to learn about the intricacies of HIV infection and where the virus hides in the body. The NIH remains committed to moving forward with research on a cure for HIV infection.”

These developments confirm that transmission and infection occurred during labour, and that the child’s period of good health can only have been down to the early treatment, he added.

The baby’s initial treatment was begun within 30 hours of her birth, and doctors used a more powerful cocktail of drugs than would normally be given to newborns. Within a month, the viral load had dropped to undetectable levels.

The treatment continued for the first 18 months of the child’s life, at which point her mother decided to stop the treatment. Doctors who saw the child five months later had expected the virus to have returned, but it remained below detectable levels.

Her case was reported at the 2013 Conference on Retroviruses and Opportunistic Infections, at which point she was 30 months old and still showed no signs of HIV in blood tests. Doctors believed that the virus had been caught before it had had a chance to create a ‘reservoir’ of latent cells containing the virus.

There have since been other cases in which very early and aggressive treatment seems to have reduced the presence of the virus to below detectable levels, but as these babies have been kept on drugs they cannot be described as “cured”.