My doc told me that statistically people with a titer below a certain level have NO increased risk of
PML. Above that level the risk of
PML goes way up, possibly even higher than the 1 in 1000 which is the standard risk quoted.
Interesting enough, it is not a linear risk, meaning that your risk does not seem to go up as your titer goes up from zero. These are the numbers that my neuro gave me, but I don't know the exact units.