Out of interest- but the concern is a greater exposure of jcv...

Out of interest- but the concern is a greater exposure of jcv active users of natalizumab (Tysabri)

http://multiple-sclerosis-research.blogspot.co.uk/2014/07/clinic-speak-natalizumab-pml-update.html

"The mortality associated with PML in this setting is currently 23%, i.e. 109 MSers have died as result of PML. Please note that the majority of the PML survivors have a poor functional outcome. You need to keep these figures in context of well over 125,800 MSers been treated with natalizumab worldwide with over 347,000 years of natalizumab exposure."
"The following graph that I have put together from the monthly updates demonstrates the number of new PML cases per month seems to be going down, despite a gradual and linear increase in number of exposed MSers. Clearly the ratio is decreasing which indicates that the PML de-risking programme is working; in other words less MSers at risk of PML are staying on the natalizumab. Herein lies the problem. Let me explain. This means that the proportion of JCV-ve MSers on natalizumab is increasing. However the total number of MSers on treatment is used in the denominator to calculate the risk of getting PML."