Researchers have achieved remission for an HIV patient through stem cell transfusion, only the second person in history to report a functional cure.

Functional cures, as opposed to eradicated cures, mean that strains of the virus could still be present in the body but are inactive and undetectable. In 2007, Timothy Ray Brown, known as the “Berlin Patient” underwent the same procedure and has remained HIV-free since. The new “London Patient” received a double stem cell transfusion.

Successful HIV treatments are rare, as drug-resistant strains have become widespread, and while stem cell transfusions have achieved positive results, they are complex and difficult to replicate on a wide scale. Blood donors are rare because they must have inherited the Δ32 mutation of the CCR5 gene from both parents, as this mutation is naturally resistant to HIV-1. The odds of having two parents with this mutation is like winning the genetic lottery.

There are other reasons for the infrequency of positive results. In Brown’s case, he already had one Δ32 mutation. Also, both functionally cured patients had cancer diagnoses in addition to HIV. Doctors believe that chemotherapy aided the blood transfusions because it temporarily destroys fast-dividing cells, leaving room for replacement.

While stem cell transfusions as an HIV treatment can be expensive, dangerous, and rare, the remission results from the “London Patient” are encouraging for doctors and researchers. And for the 38 million people currently living with HIV, any option for a life-saving cure is a reason for hope.