Two patients have been taken off their HIV drugs after bone-marrow transplants seemed to clear the virus from their bodies, doctors report.

One of the patients has spent nearly four months without taking medication with no sign of the virus returning.

The team at Brigham and Women's Hospital, in the US, caution that it is far too soon to talk about a cure as the virus could return at any point.

The findings were presented at the International Aids Society Conference.

It is difficult to get rid of an HIV infection because it hides inside human DNA, forming untouchable "reservoirs" in body.

Anti-retroviral drugs keep the virus in check within the bloodstream - but when the drugs stop, the virus comes back.

HIV gone?

The two men, who have not been identified, had lived with HIV for about 30 years.

They both developed a cancer, lymphoma, which required a bone-marrow transplant.

Bone marrow is where new blood cells are made and it is thought to be a major reservoir for HIV.

After the transplant, there was no detectable HIV in the blood for two years in one patient and four in the other.

Analysis It is far too early to call this a cure for HIV. And even if it was a cure, it wouldn't be a very good one. It is very expensive and often leads to "graft-v-host" disease. There is a 15-20% mortality rate within the first few years after the transplant. This occurs when new immune cells produced by the graft treat the rest of the body as foreign and attack it. The two patients in this study have replaced their regimen of anti-retroviral drugs, with those to suppress the immune system. The procedure was carried out in these patients only because they had cancer that needed to be treated. The real value of this research for the majority of people with HIV will come from a deeper understanding of the virus and HIV reservoirs.

The pair came off their anti-retroviral drugs earlier this year.

One has gone 15 weeks, and the other seven, since stopping treatment, and no signs of the virus have been detected so far.

Dr Timothy Henrich told the BBC the results were exciting. But he added: "We have not demonstrated cure, we're going to need longer follow-up.

"What we can say is if the virus does stay away for a year or even two years after we stopped the treatment, that the chances of the virus rebounding are going to be extremely low.

"It's much too early at this point to use the C-word [cure]."

It is thought that the transplanted bone marrow was initially protected from infection by the course of anti-retrovirals. Meanwhile the transplant also attacked the remaining bone marrow, which was harbouring the virus.

However Dr Henrich cautioned that the virus could be still be hiding inside brain tissue or the gastrointestinal track.

"If [the] virus does return, it would suggest that these other sites are an important reservoir of infectious virus and new approaches to measuring the reservoir at relevant sites will be needed to guide the development of HIV curative strategies," he said.

Berlin patient

Timothy Brown, also known as the "Berlin patient" is thought to be the first person cured of Aids. He had a bone marrow transplant from a rare donor who was resistant to HIV.

The two US cases both received bone marrow from normal donors.

There was also a report of an HIV cure in a baby born in Mississippi, US. She was treated with anti-retroviral drugs at birth so it is thought the virus was cleared from the body before reservoirs were established.

Dr Michael Brady, the medical director of the Terrence Higgins Trust, said: "It is too early to know whether HIV has been eradicated from these men's bodies or whether it might return.

Media playback is unsupported on your device Media caption Doctors say it is far too soon to talk about a cure for HIV, as James Gallagher reports

"However, the case suggests that what happened to Timothy Brown, the Berlin Patient was perhaps not a one-off.

"A bone marrow transplant is a complex and expensive procedure, which comes with significant risks.

"For most people with HIV, it would be more dangerous to undergo a transplant than to continue managing the virus with daily medication.

"So while this is by no means a workable cure, it does give researchers another signpost in the direction of one."

The head of the Foundation for AIDS Research, Kevin Frost, said: "These findings clearly provide important new information that might well alter the current thinking about HIV and gene therapy.

"While stem-cell transplantation is not a viable option for people with HIV on a broad scale because of its costs and complexity, these new cases could lead us to new approaches to treating, and ultimately even eradicating, HIV."