Look at the history.
When Tysabri first got early approval, you heard nothing about PML. Then two cases showed up with MS patients who had Avonex and Tysabri combined in a parallel trial. The drug was pulled and then re-approved about a year later. Biogen/Elan told us that PML only happened when you combined Tysabri with another immune system altering drug like Avonex. Then they told us that you could only get PML if you had previously taken these drugs and your immune system had been compromised. Monotherapy Tysabri was safe. Then PML started showing up in those only having taken Tysabri. We were told the incidence of PML was rare with only a 1:000 chance. Now it's a 1:500 chance between 2 and 3 year users.
Make no mistake....PML is preventing Tysabri from becoming a huge blockbuster drug for Biogen/Elan. I'm sure the market share and resulting revenue would be tripled if PML didn't exist. Biogen has had their hands slapped by the FDA for minimizing the risk of PML for Tysabri users.
The sooner they get PML under control, the better off these drug companies will be. The resulting income would be immense. I'm sure you don't have to look too deeply to see who pushed this co-operative research between the FDA and European authority.