Whether or not someone remains on @Tecfidera or @Tysabri for that matter after testing positive for the JC Virus depends on whether or not the doctor also did an assessment test against the JCV Antibody Index which would indicate one's level of risk for contracting PML. The lower you fall on the JCV antibody index the lower the risk of developing PML versus the higher you're on the index the greater the risk.

Best to discuss your level of risk in relation as to where you fall on the JC Virus Antibody Index relative to the risk of contracting PML with your doctor rather than seeking information from this forum.