Sera from different age cohorts in Belgium show limited cross-neutralization between the mumps vaccine and outbreak strains.

Mumps used to affect children between 2 and 15 years old. The mumps-measles-rubella (MMR) vaccine is available, with vaccine coverage rate of about 85% after two vaccine doses. Recently new mumps outbreaks have emerged in highly vaccinated populations; the causes for these new outbreaks are yet unknown. We tested if a difference in seroneutralizing capacity against the vaccine and wild-type viruses existed and if waning immunity could be detected. In this study, 570 serum samples (age group 2-3 years (n = 96), 8-9 years (n = 95), 13-14 years (n = 94), 18-20 years (n = 96), 24-26 years (n = 92) and 50 + years (n = 97)) in Belgium were tested in the rapid fluorescent foci inhibition test for their neutralizing capacity against the vaccine and wild-type viruses. Neutralizing antibodies against the vaccine strain were present in 84% (81/97) of the 2-3-year, 74% (70/95) of the 8-9-year, 81% (76/94) of the 13-14-year, 76% (73/96) of the 18-20-year, 67% (62/92) of the 24-26-year and 77% (75/97) of the 50+-year age group serum samples. For all age groups, only about half of these serum samples were also positive for the wild-type virus. The geometric mean titres for the vaccine and wild-type virus for all younger age groups, except for 24-26 years, were significantly different, demonstrating poor in vitro cross-neutralization. A possible contribution of antigenic differences between the genotype A and G mumps virus as well as other immune factors, in addition to lower-than-optimal vaccination coverage and waning immunity, could explain the poor in vitro cross-neutralization and should be further studied.