Laboratory markers for over-attenuation of mumps vaccine virus.

The properties of mumps vaccine virus (Leningrad-3 strain) gradually changed upon passaging in quail embryo fibroblasts, the substrate normally used for mumps vaccine production in the USSR. Alterations were extremely noticeable in the over-attenuated (38th passage) virus variant, and involved (a) poor, if any, antibody response in guinea-pigs, (b) turbid plaque formation, (c) lack of expression in cell culture of fusion protein and reduced expression of polymerase protein, and (d) enrichment by abnormally small, fusion-protein-deficient virus particles. Two other laboratory strains exhibited a similar trend to over-attenuation, though after variable passage numbers. Due to a good inter-correlation, every test (namely, inoculation of guinea-pigs, plaque assay, protein analysis, or immune electron microscopy) is indicative of mumps vaccine over-attenuation and hence might be valuable in seed virus quality control.