Philippe Leboulch, a professor of medicine at the University of Paris, told the BBC News website: "So far the patient has no sign of the disease, no pain, no hospitalisation. He no longer requires a transfusion so we are quite pleased with that.
"But of course we need to perform the same therapy in many patients to feel confident that it is robust enough to propose it as a mainstream therapy."
'Given his life back'
Prof Leboulch is nervous about using the word "cure" as this is just the first patient to come through clinical trials.
But the study does show the potential power of gene therapy to transform the lives of people with sickle cell.
"I think it's very significant, essential they've given him his life back," said Dr Deborah Gill from the gene medicine research group at the University of Oxford.
She told the BBC: "I've worked in gene therapy for a long time and we make small steps and know there's years more work.
"But here you have someone who has received gene therapy and has complete clinical remission - that's a huge step forward."
However, the expensive procedure can only be carried out in cutting-edge hospitals and laboratories, while most sickle cell patients are in Africa.
The next big challenge will be to transform this pioneering science into something that really can help millions of people.
Sickle cell disease is a lifelong condition caused by a faulty gene that affects how red blood cells develop
SCD mainly affects people of African, Caribbean, Middle Eastern, Eastern Mediterranean and Asian origin
People with sickle cell are often at an increased risk of contracting serious infections or they could become anaemic, which is when red blood cells cannot carry enough oxygen around the body. This can cause tiredness and shortness of breath
Some patients have regular blood transfusions - usually every three to four weeks - as a form of treatment for the condition