I'm not sure where you are reading that, but what I see is:

"The risk for developing PML appears to increase with the number of Tysabri infusions received. The number of monthly infusions of Tysabri in the 13 patients who developed PML ranged from 12 to 35 infusions. The average number of infusions received before the diagnosis of PML was 25. There is minimal experience in patients who have received more than 35 infusions of Tysabri."

So, while they seem to be recognizing that the more infusions a person gets, the more the risk there is for them to develop PML ... they don't have enough patient experience to determine how high the risk might go beyond 35 infusions.

Cherie