A child in the US who appeared to have been cured of HIV now has detectable levels of the virus in her blood again, doctors have confirmed.

The child, known as the ‘Mississippi baby', was born in 2010 to a HIV-positive mother who had not received any HIV treatment, known as antiretroviral medication, during her pregnancy.

Because of the high risk of HIV exposure, the baby was started on antiretroviral medication when she was just 30 hours old. One week later, tests confirmed that she was HIV positive and she continued on the medication.

The little girl received this treatment until she was 18 months old, at which point she stopped attending hospital for reasons unknown. When she re-entered the hospital system five months later, blood samples revealed ‘undetectable HIV levels and no specific HIV antibodies'.

"The child continued to do well in the absence of antiretroviral medicines and was free of detectable HIV for more than two years. However, during a routine clinical care visit earlier this month, the child, now nearly four years of age, was found to have detectable HIV levels in the blood," the US National Institute of Allergy and Infectious Diseases (NIAID) said.

Repeat blood tests carried out 72 hours later confirmed the findings.

"Additionally, the child had decreased levels of CD4+ T-cells, a key component of a normal immune system, and the presence of HIV antibodies - signals of an actively replicating pool of virus in the body," the NIAID noted.

As a result, the child was placed back on antiretroviral medication.

The NIAID said that to date, the little girl is tolerating the medication with no side-effects ‘and treatment is decreasing virus levels'.

It described the news as a ‘disappointing turn of events'. However, according to Prof Deborah Persaud, one of two paediatric HIV experts involved in the child's care, the fact that this child was able to remain off antiretroviral treatment for two years ‘is unprecedented'.

"Typically, when treatment is stopped, HIV levels rebound within weeks, not years," she pointed out.

Meanwhile, according to NIAID director, Dr Anthony Fauci, this case indicates that while early treatment did not completely eliminate HIV, it may have ‘considerably limited its development and averted the need for antiretroviral medication over a considerable period'.

"Now we must direct our attention to understanding why that is and determining whether the period of sustained remission in the absence of therapy can be prolonged any further," Dr Fauci added.