A second man has been “functionally cured” of HIV, but don’t get too excited just yet

An HIV-positive cancer patient in London has become the second person in history ever to be “functionally cured” of HIV after having his immune system wiped out by chemotherapy and receiving a bone marrow transplant.

“Functionally cured” in this case means that his tests have come up HIV-negative so far—it’s a bit like saying that a cancer is “in remission” while doctors continue to test for its presence over the coming years.

While this is great news, it isn’t the same as a cure, mostly because the unnamed London patient’s treatment is specific, rare, life-threatening and expensive.

Vox explains the man’s situation:

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The new patient had a form of cancer and received a treatment involving chemotherapy to wipe out his immune system and replace it, via a stem cell transplant, with non-malignant donor cells. In both cases, the donor cells also carried an added benefit: a genetic mutation that leads to HIV immunity.

In September 2017, 16 months receiving the transplant, the patient — a London man who prefers to remain anonymous—went off his antiretroviral drugs for HIV yet still tested negative for the virus. So far, he’s remained HIV-free.

Doctors have tried the same course of treatment on other patients who have either died from cancer or complications related to the transplant.

Related: US government concludes that HIV is untransmittable at undetectable levels

The first person to have been functionally cured of HIV was Timothy Brown (aka “the Berlin patient”). Brown had HIV and leukemia, a type of cancer in which a person’s bone marrow can form cancerous cells.

When Brown’s leukemia stopped responding to chemotherapy, Berlin doctors suggested a bone marrow transplant from a cancer-free donor with a special genetic mutation that made them resistant to HIV. After receiving two bone marrow transplants in 2007 and 2008, Brown became functionally cured of HIV. Now he’s 52 and lives in Palm Springs, California.

Despite the success of the two patients, doctors still don’t know why the functional cure succeeded in them and not other patients: It could have to do with their genetics, the specific strain of HIV they had or the ways their bodies adapted to the bone marrow transplants. As such, it’s hard to replicate and repeat en masse to the general public.

In short, we’re still not close to a full cure, but this makes the news no less amazing.