The first clinical trial of a gene therapy for inherited progressive blindness showed "very promising" initial results, according to Oxford University researchers leading the study.

The aim of the treatment was to get the gene therapy into the cells in the retina without causing damage. After six months, however, the patients actually showed improvements in their vision in dim light and two of the six were able to read more lines on the eye chart.

A total of nine patients had one eye treated with the gene therapy in operations at the Oxford Eye Hospital. The therapy was given in one eye to allow comparison with progression of the disease in the other eye.

Results at six months were reported for the first six patients in The Lancet journal. The first patient to have the operation has been followed up for over two years. Based on the success of the treatment in the first six patients, three more have recently been tested at a higher dose.

Professor Robert MacLaren, who led the development of the retinal gene therapy and this first clinical trial, said: "It is still too early to know if the gene therapy treatment will last indefinitely, but we can say that the vision improvements have been maintained for as long as we have been following up the patients."

He added the results confirmed that the virus can deliver its DNA payload without causing significant damage to the retina.

This has huge implications for anyone with a genetic retinal disease such as age-related macular degeneration or retinitis pigmentosa because results indicated gene therapy can be applied safely before the onset of vision loss.