AIDS researcher Professor Sharon Lewin. Credit:Simon O'Dwyer However, there is a key difference between the Berlin and Boston patients that could advance research towards a cure for HIV. Mr Brown received a transplant from a donor with an unusual gene mutation that resists HIV whereas the Boston patients received transplants from donors with no known resistance to the disease. Timothy Henrich and Daniel Kuritzkes, the doctors managing the Boston patients, told an HIV conference in Malaysia that this suggested the process of stem-cell transplantation was responsible for their suspected remission. In particular, they believe a common complication of transplantation, graft-versus-host disease, could be at play because it involves newly transplanted donor cells attacking the transplant recipient's body. The doctors said although it was too early to say whether their patients had been cured permanently, repeated tests of large volumes of cells, plasma and tissue had found no sign of the virus.

"We demonstrated at least a 1000 to 10,000-fold reduction in the size of the HIV reservoir in the peripheral blood of these two patients, but the virus could still be present in other tissues such as the brain or gastrointestinal tract," Dr Henrich told the International AIDS Society Conference on HIV Pathogenesis, Treatment and Prevention in Kuala Lumpur. He said the patients were being closely monitored because the virus could reappear at any moment. While the doctors have been inundated with requests for the procedure since they announced earlier results from the patients last year, they said it was not suitable for people whose virus was well controlled by treatment and who are otherwise healthy. This was because bone marrow transplantation kills up to 20 per cent of people within the first few years. It is also expensive and patients may need long-term immunosuppressive medications to minimise donor cells attacking their own tissues. Sharon Lewin, an HIV researcher and director of infectious diseases at Melbourne's Alfred hospital, said although the cases created hope that a cure was possible, more patients needed to be studied in similar circumstances to see if the mechanisms could be understood and assist ongoing research into a more universal and practical cure.

"There is a lot of excitement among patients, but we don't want to raise expectations too high. We're at the very beginning of nutting the science out," said Professor Lewin, who also works at the Burnet Institute. Last year, American doctors announced that a Mississippi baby had been cured of HIV after receiving unusually large doses of conventional treatment started 30 hours after birth. When the baby was taken off treatment at 18 months, HIV could not be detected in the child's blood or tissue. The toddler is still free of the virus today. French researchers also believe early treatment may lead to a functional cure where patients still have HIV but do not need treatment. In a widely publicised trial, 14 French patients who received treatment earlier than usual in the course of their infection have been living comfortably without treatment for a median of seven and a half years. They are now considered "post-treatment controllers" of HIV. It is unclear if they remain infectious or not. The French researchers estimate that as many as 15 per cent of people who start treatment early and continue for at least a year might be able to control the virus without treatment thereafter, relieving them of a lifelong daily drug regimen. While research into a cure for HIV, which affects about 34 million people, is gaining momentum, most doctors and scientists are reluctant to predict how long it could take to find a universal cure.