ZURICH, Sept 28 (Reuters) - Swiss drugmaker Roche on Monday said 59% of infants with the severest form of spinal muscular atrophy could sit for at least five seconds after getting its drug Evrysdi for two years, according to trial data, up from 33% after a year of treatment.

"We are highly encouraged by the results we are seeing in the second year of treatment with Evrysdi," Roche said, adding that 18 of the 21 patients in the Firefish trial of patients with Type 1 SMA were still alive without needing permanent ventilation. Evrysdi was approved in August. (Reporting by John Miller; editing by Thomas Seythal)