WASHINGTON — Three young adults barely able to see because of a congenital and progressive form of blindness have regained modest amounts of vision after getting genetically engineered viruses injected into their eyes, the leaders of two independent studies reported Saturday.

The results were something short of miraculous. Three other patients showed no increase in visual acuity, and the three who improved are still legally blind.

But to get any improvement at all during such an early phase of the research suggests that the approach has promise for people with gene-based forms of blindness, researchers said.

The work could also provide a boost for the field of gene therapy, which for 15 years has tried and mostly failed to treat diseases by giving people new genes — and in a few high-profile cases has sickened or even killed them.

“We’re learning things as we go along,” said Jean Bennett of the University of Pennsylvania, who with Albert Maguire led one of the two studies.

“We were not really going for a cure. But we do view it as a success in that their vision has improved. I’d call it a dramatic response,” she said.

“If we can get any benefits in people with such advanced disease, then when we start to treat children, and when we treat a greater part of the retina and give a higher dose, the expectation is we can get even better results,” Robin Ali of University College London, who led the other study, said.

The experiments involve patients with Leber’s congenital amaurosis, a rare disease that is caused by a defect in a single gene that is crucial to the retina’s ability to convert light into signals to the brain. Affected children have impaired vision from birth and are typically blind by age 30.