A man in London has become the second person to be clear of HIV after a stem cell transplant, doctors have said.

Until now, Timothy Ray Brown from the US was the only person thought to have been cured of HIV after undergoing a transplant in Berlin 12 years ago.

The unidentified man in the UK, who is being called "the London patient", received bone marrow stem cells from a donor with a rare genetic mutation that resists HIV infection.

Almost three years on, and more than 18 months after coming off antiretroviral drugs, highly sensitive tests show no signs 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 treated the man, Reuters reports.


Image: Timothy Ray Brown, known as the 'Berlin Patient', is the only other person known to have been 'cured' of HIV via transplant

He said that despite the patient appearing "functionally cured," it is "too early to say he's cured".

The London patient was diagnosed with HIV in 2003 but did not start to take drugs to control the infection until 2012.

He developed Hodgkin lymphoma in later 2012 and agreed to a stem cell transplant to treat the cancer in 2016.

His doctors believed that with the right kind of donor, the London patient could find an HIV cure as well as cancer treatment.

First HIV cured man reacts to UK success

Doctors found a donor with a gene mutation that confers natural resistance to HIV.

Around 1% of people descended from northern Europeans have inherited the mutation from both parents and are immune to most HIV.

The donor had this double copy of the mutation, which Dr Gupta described as "an improbable event".

"That's why this has not been observed more frequently," he added.

The transplant changed the man's immune system, giving him the donor's mutation and HIV resistance.

He then voluntarily stopped taking HIV drugs to see if the virus would come back.

Image: Antiretroviral drug Truvada is an HIV medication

HIV patients rely on life-long daily medication to suppress the virus, and when the drugs are stopped, the virus usually comes back within two to three weeks.

However the virus did not return in the London patient's case and there is still no trace of the virus after 18 months off the drugs.

Stem cell transplants are known to be hard on the body, starting with radiation or chemotherapy to damage the existing immune system and make room for a new one.

The failure rate of such transplants is high.

Dr Gero Hutter, the German doctor who treated the first cured patient Mr Brown, who is still free of the virus, described the latest case as "great news" and "one piece in the HIV cure puzzle".