- Patients must have anti-MV IgG titer of ≤ 0.5U/mL (Mayo clinic assay). Mayo Clinic will also assay the patients' IgM titer and perform a neutralizing antibody plaque-assay to determine recent MV exposure and the ability of the patients' circulating antibodies to inhibit MV propagation on Vero cells, respectively. While these tests are additional indicators of patient eligibility, final enrollment decision will be determined by IgG levels