A single jab of a breakthrough gene therapy could reverse hearing loss in people within weeks, according to new research.

The cutting-edge therapy improved hearing in children and adults with congenital deafness or severe hearing impairment, with a 7-year-old regaining almost full hearing in a clinical trial, researchers from Sweden’s Karolinska Institutet said.

The clinical trial, detailed in the journal Nature Medicine, showed that a healthy copy of the OTOF gene injected in the inner ear improved hearing of all 10 participants.

The small-scale trial included people who had a genetic form of deafness or severe hearing impairment caused by mutations in a gene called OTOF.

These mutations cause a deficiency of the protein otoferlin, which plays a key role in transmitting sound signals from the ear to the brain.

While the therapy seemed to work best in children, researchers said, it could benefit adults as well.

In the trial, a synthetic, harmless version of the adeno-associated virus was used to deliver a properly functional OTOF gene to the inner ear via a single injection.

The effects of the therapy were evident in the majority of patients, whose hearing recovered rapidly after just a month.

After six months, researchers noted considerable hearing improvement in all participants, with their average volume of perceptible sound improving from 106 decibels to 52.

Those between the ages of five and eight responded best to the treatment, the study found.

One seven-year-old girl quickly recovered almost all her hearing, and she was able to hold daily conversations with her mother four months afterwards.

“This is the first time that the method has been tested in teenagers and adults,” Maoli Duan, an author of the study from Karolinska Institutet, said.

“Hearing was greatly improved in many of the participants, which can have a profound effect on their life quality. We will now be following these patients to see how lasting the effect is.”

Researchers also found that the treatment was safe and well-tolerated. Participants did not report any serious adverse reactions in the follow-up period of 6-12 months.

The most common reaction was a reduction in the number of the immune system’s neutrophils, a type of white blood cell.

“OTOF is just the beginning,” Dr Duan said, adding that researchers were working on other common genes behind deafness such as GJB2 and TMC1.

“These are more complicated to treat, but animal studies have so far returned promising results. We are confident that patients with different kinds of genetic deafness will one day be able to receive treatment.”