A 32-year-old who had never heard a sound. A toddler who could now perceive a whisper. In the largest and longest trial of gene therapy for inherited deafness, both scenarios played out — and the results have held for up to two and a half years.

The study, published Wednesday in Nature, treated 42 children and adults born with autosomal recessive deafness 9 (DFNB9), a rare condition caused by mutations in a single gene called OTOF. Without a working copy, hair cells in the inner ear cannot pass sound signals to the brain. People with the condition are born completely deaf.

Researchers delivered a functional copy of the gene using a harmless virus injected into the inner ear. About 90% of participants gained hearing — most within weeks — and continued improving over roughly six months. Younger patients saw the greatest gains, with some reaching near-normal hearing thresholds, but two of three adults treated also showed measurable recovery.

“It is very encouraging to see meaningful improvements in some adult patients,” said Zheng-Yi Chen, an associate scientist at Mass Eye and Ear who co-led the trial with Yilai Shu of Fudan University. “It suggests there may be more flexibility in the human auditory system than we expected.”

Participants who gained hearing also began developing speech perception — a meaningful quality-of-life shift. No serious treatment-related side effects were observed, though researchers emphasize that longer follow-up is needed. About 10% of participants did not respond.

DFNB9 is rare, affecting roughly 50 babies born each year in the US. But the trial’s success is accelerating interest in gene therapies for other forms of genetic hearing loss, which collectively account for a significant share of pediatric deafness. Regeneron Pharmaceuticals is developing a similar treatment that could become the first gene therapy for deafness to receive FDA approval.

As for whether these gains truly last a lifetime — that question will take years to answer. For now, the data runs two and a half years and counting.

Sources