Boston: Two men who became free of HIV infection after bone marrow transplants have been off Aids drugs for several months without any sign of the virus returning, according to scientists.

Talk of a cure is premature, the doctors told the International Aids Society conference in Kuala Lumpur, but the news will raise hopes that progress can be made towards a treatment that will not only keep the virus at bay but eventually eradicate it.

The two men, who had longstanding HIV infections, were given bone marrow transplants at the Dana-Farber/Brigham and Women’s Cancer Centre in Boston because they were suffering from cancers of the blood. Tests showed that as their own cells were replaced by cells from the bone marrow donor, the levels of virus in their blood steadily dropped to undetectable.

One of the men stopped taking antiretroviral medication to suppress HIV almost four months ago and the other seven weeks ago. They still have no detectable HIV in their blood cells.

“While these results are exciting, they do not yet indicate that the men have been cured,” said Timothy Henrich, from the division of infectious diseases at Brigham. “Long-term follow up of at least one year will be required to understand the full impact of a bone marow transplant on HIV persistence.”

One of the patients became infected with HIV in the early years of the epidemic, and the other was infected as a baby. They are both monitored frequently. Scientists have conducted extensive tests to try to find HIV not only in their blood cells but also in tissue and have not found anything, although they cannot be sure it is not there. HIV is able to hide and remain at low levels but remain capable of becoming active.

The virus could still be present in an organ such as the brain or in the gastrointestinal track, Henrich said. “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.”

Even if the virus does not reappear, bone marrow transplants are not feasible for everybody. There is a 15-20% mortality rate and the patient has to take drugs to suppress their immune system thereafter. But the experiences of the two men will teach scientists a great deal more about how to combat the virus.

The International Aids Society, which brings together researchers in the field worldwide, has led efforts towards a hunt for a cure for HIV infection as successive attempts to design a vaccine have failed.

The case of the “Berlin patient”, later identified as American Timothy Ray Brown, was proof that a cure was possible. Brown underwent a stem cell transplant because of cancer, but his doctors managed to find a donor who had genetic resistance to HIV — a rare thing. Brown has been cured of both HIV infection and cancer, but the treatment carries major risks and is hugely expensive, and it would be very unlikely that many donors with genetic resistance to the virus could be found.

In March it was announced that a child born with HIV in Mississippi had been “cured” by being given very prompt antiretroviral therapy after birth. The child’s mother stopped giving the baby drugs at around the age of 18 months and no longer went for regular checks to the hospital. When they returned five months later, the child had no detectable virus. Scientists are uncertain why it happened, but speculated that the early use of drugs within 30 hours of the birth may have prevented the virus taking hold.

Henrich said he was cautious but excited about the scientific developments. “Five years ago if you’d mentioned a cure, research people wouldn’t have taken you seriously,” he said. “We’re not there yet. Are we close? Probably not, but who knows? This could be a rapid learning curve in the next few years.”