(This is a stock photo and not a child treated in the trial described below.) (Image credit: Xuanyu Han via Getty Images) In the largest and longest trial of its kind, 90% of people who received an experimental gene therapy for congenital deafness showed marked improvements in their hearing over the next several years. The trial, which involved 42 people and was conducted across eight sites in China , mostly involved children but also included three adults, two of whom responded well to the therapy. All of the participants started out with complete hearing loss. Although the children's hearing improved more than the adults' did, the trial results still suggest adults could benefit from the treatment. The trial was described Wednesday (April 22) in the journal Nature . These improvements in hearing appear to progress over time and then plateau and stabilize by around the one-year mark, Chen said.…