The gene therapy had been worked on in a laboratory for 15 to 20 years, said Thrasher, from University College London.
He developed it alongside experts from Great Ormond Street, and added: “Eventually we felt we were ready to take that technology to patients, and Rhys was the first.”
Thrasher said they were confident it would work as they had done a lot of research.
It involved taking 100ml of Evans’ bone marrow, putting the new gene into it, then injecting the bone marrow back into his body.
“Then we waited – we didn’t do anything else,” Thrasher said.
“[Evans’] own bone marrow regenerated his immune system with the new gene in it.”
Within weeks, Evans’ immune system began to develop, and shortly after his second birthday he got the flu.
“That was a good test to see if [the treatment] had worked. I dealt with it OK and survived, so that was good news,” Evans said.
Thanks to developments in gene therapy, today, many people who previously had a life expectancy of a few years are able to live fully active lives, according to Great Ormond Street.
Most patients treated with gene therapy trials at the children’s hospital have had different forms of SCID, but they have also included those with leukaemia, spinal muscular atrophy and Leber’s Congenital Amaurosis, an inherited disorder that causes children to lose their sight.