Seroconversion of JCV antibodies is strongly associated to natalizumab therapy.

Previous infection with John Cunningham virus (JCV) increases the risk of progressive multifocal leukoencephalopathy in patients with multiple sclerosis (MS) undergoing treatment with natalizumab. Patients who test negative for JCV antibodies must be assessed every six months due to the risk of seroconversion. Data from the United States of America, Portugal, Holland, France, United Kingdom and Sweden have shown a strong correlation between the use of natalizumab and JCV seroconversion.