An HIV-positive positive patient in London has been cleared of the virus after getting a bone marrow transplant to treat lymphoma, raising hopes for possible cure through stem cell transplant.

He has remained in HIV remission despite being taken off antiretroviral therapy, 19 months after the bone marrow transplant from a CCR5 negative donor for Hodgkin’s lymphoma.

The man simply identified yet as “London Patient” is the second person to have the undetectable viral load.

The first was Berlin Patient, Timothy Brown, who has remained free of HIV and off ART since a similar bone marrow transplant 12 years ago.

He’s being the only one thought to have been cured of HIV.

Researchers from University College London announced the finding at the annual conference of retroviruses and opportunistic infections (CROI) ongoing in Seattle, USA this week.

But they have stopped short of calling it a cure.

The International AIDS Society has welcomed the announcement.

“Although it is not a viable large-scale strategy for a cure, it does represent a critical moment in the search for an HIV cure,” IAS president Anton Pozniak.

“These new findings reaffirm our belief that there exists a proof of concept that HIV is curable. The hope is that this will eventually lead to a safe, cost-effective and easy strategy to achieve these results using gene technology or antibody techniques.”

The London Patient mirrors the Berlin Patient Brown but there are differences.

Brown in 2007 was an HIV-positive man with acute myeloid leukaemia, a type of blood cancer that begins in the very cells that become the white blood cells.

He received a bone marrow transplant from a donor who was naturally resistant to HIV infection. The transplant involved stem cells—cells that are capable of producing more of itself and then grouping to become any other type of cell or tissue through a process called differentiation.

The donor was resistant because of a mutation in his CCR5 gene. The CCR5 is a critical protein that must be present for HIV to be able to enter and infect cells.

A mutation in the door’s CCR5 gene stops HIV entering. Bone marrow from a similarly CCR5 negative donor was given to the “London Patient”.

He had Lodgkin’s lymphoma, a type of cancer that begins in lymphocytes—the white blood cells that help fight off disease.

Brown stopped taking ART after the transplant and has now been free of readily detectible virus for 12 years—meaning: he is cured.

“The so-called London Patient has now been off ART for 19 months with no viral rebound which is impressive, but I would still be closely monitoring his viral load,” Sharon Lewin, IAS Governing Council Member and Co-Chair of the Towards an HIV Cure initiative, said.

“However, this is a long time to be in remission off ART’’.

“Coming 10 years after the successful report of the Berlin Patient, this new case confirms that bone marrow transplantation from a CCR5-negative donor can eliminate residual virus and stop any traces of virus from rebounding.”

Experts say two factors are likely at play.

One, the new bone marrow given to the London Patient is resistant to HIV

Two, the new bone marrow—which is where blood cells are made—is actively eliminating new HIV-infected cells.

The London Patient has not been named yet, but Brown hopes both men will meet one day, but just not yet on account of publicity concerns.

“I would like to meet the London Patient. I would say, ‘take your time and if you want to become public, do it’. It is not really a burden that I am going through, with media and stuff like that. It is actually fun, you can share in the fun,” he told the Guardian UK.

He noted his own journey with HIV and cancer has been “useful for science and giving hope to HIV-positive people.”