It's a huge concern. A fatal adverse event always is. To some extent, too, this is a guilt by accusation thing: the report WILL go on the 'label,' even if it appears 90+% sure that the association is coincidental.
With the information we have now, it appears more likely that the association is coincidental. While it's impossible to prove or disprove causation in a single case, it will be picked over as thoroughly as possible. We may be in for a rough couple of months.
The interesting thing to watch will be FDA reaction to the p3 vs PV. It is reasonable for a patient with MF to take on a risk of PML that might be as high as 1 in 5000/yr, but not for a PV patient. I would not be surprised at all to see a mandate that all subjects be tested for the virus that, if it escapes immune control, causes PML.
There is less risk that bari development will be affected. The same property (fat/water partitioning) that keeps it mostly out of marrow also keeps it mostly out of CNS.