A HIV-positive man in Britain has become the second known adult to be cleared of the AIDS virus after he received a bone marrow transplant from a HIV-resistant donor, his doctors say.

Key points: Medical experts said it is too early to call the procedure a cure but it was a sign that it could be possible to eliminate AIDS

Medical experts said it is too early to call the procedure a cure but it was a sign that it could be possible to eliminate AIDS Further gene therapy research, focusing on a mutation that confers resistance to HIV, will be undertaken

Further gene therapy research, focusing on a mutation that confers resistance to HIV, will be undertaken About 37 million people suffer from HIV and 35 million have died from AIDS-related illnesses since the 1980s

Almost three years after receiving stem cells from a donor with a rare genetic mutation that resists HIV infection — and more than 18 months after coming off antiretroviral drugs — highly sensitive tests show no trace of the man's previous HIV infection.

"There is no virus there that we can measure. We can't detect anything," said Ravindra Gupta, a professor and HIV biologist who co-led a team of doctors treating the man.

The case is a proof of the concept that scientists will one day be able to end AIDS, the doctors said, but does not mean a cure for HIV has been found.

Professor Gupta described his patient as "functionally cured" and "in remission", but cautioned: "It's too early to say he's cured."

The man is being called the London patient, in part because his case is similar to that of American man Timothy Brown, who became known as the Berlin patient when he underwent similar treatment in Germany in 2007, which also cleared his HIV.

Some 37 million people worldwide are currently infected with HIV and the AIDS pandemic has killed around 35 million people worldwide since it began in the 1980s.

Scientific research into the complex virus has in recent years led to the development of drug combinations that can keep it at bay in most patients.

'His last chance of survival'

Professor Gupta said the man had contracted HIV in 2003, and in 2012 was also diagnosed with Hodgkin's lymphoma, a blood cancer.

In 2016, when he was very sick with cancer, doctors decided to seek a transplant match.

"This was really his last chance of survival," Professor Gupta said.

The donor, who was unrelated, had a genetic mutation known as CCR5 delta 32, which confers resistance to HIV.

The transplant went smoothly but there were some side-effects, including the patient suffering a period of graft versus host disease — a condition in which donor immune cells attack the recipient's immune cells.

Most experts have said it is inconceivable such treatments could be used to cure all patients.

The procedure is expensive, complex and risky.

To do this in others, exact match donors would have to be found in the tiny proportion of people — most of them of northern European descent — who have the CCR5 mutation that makes them resistant to the virus.

Specialists said it is also not clear whether the CCR5 resistance is the only key, or whether the graft versus host disease may have been just as important.

Both the Berlin and London patients had this complication, which may have played a role in the loss of HIV-infected cells, Dr Gupta said.

Sharon Lewin, an expert at Australia's Doherty Institute and co-chair of the International AIDS Society's cure research advisory board, said the London case points to new avenues for study.

"We haven't cured HIV, but [this] gives us hope that it's going to be feasible one day to eliminate the virus," she said.

Professor Gupta said his team plans to use these findings to explore potential new HIV treatment strategies.

"We need to understand if we could knock out this (CCR5) receptor in people with HIV, which may be possible with gene therapy," he said.

The London patient, whose case was set to be reported in the journal Nature and presented at a medical conference in the US on Tuesday (local time), has asked his medical team not to reveal his name, age, nationality or other details.

Reuters