High rates of JCV seroconversion in a large international cohort of natalizumab-treated patients.

Aims: To retrospectively assess factors associated with John Cunningham virus (JCV) seroconversion in natalizumab-treated patients.

Background: Natalizumab is highly effective for the treatment of relapsing-remitting multiple sclerosis (RRMS), but its use is complicated by opportunistic JCV infection. This virus can result in progressive multifocal leukoencephalopathy (PML).