The brain infection risk from Biogen Idec’s multiple sclerosis drug Tysabri appears highest during the third year of treatment, U.S. health officials said in an updated warning released on Friday.

The potentially fatal infection, known as progressive multiform leukoencephalopathy (PML), occurs in an estimated 1.5 per 1,000 patients treated with Tysabri during months 25 to 36, the Food and Drug Administration said.

The FDA said it was the first time it had estimated the chances of PML for specific time intervals rather than providing a cumulative risk over years.