PML can have severe permanent effects on the brain of patients who survive. Given the disabling effects, travel to a central site can be difficult. I think that having the trial with investigators in or visiting multiple major cities would make participation easier. If the possibility of a cure/prevention of PML were available, most patients would want to participate. The awareness of PML is somewhat limited because PML is not widely known. For those patients in a situation or therapy that puts them at risk of PML, more information would be helpful in making people aware of the infection and what types of things to be watchful for.
Things that might discourage participation are lengthy visits to far away sites or a treatment that is more involved than simply taking a medication.