OBJECTIVES:

Mumps used to affect children between two and 15 years old. The MMR (mumps-measles-rubella) vaccine is available with vaccine coverage rate of about 85% after two vaccine doses. Recently, new mumps outbreaks emerged in highly vaccinated populations and the causes for these new outbreaks are yet unknown. We tested if a difference in seroneutralising capacity against the vaccine and wild type viruses exists and if waning immunity could be detected.

METHODS:

In this study, 570 serum samples (age group 2-3y (N=96), 8-9y (N=95), 13-14y (N=94), 18-20y (N=96), 24-26y (N=92) and 50+ (N=97)) in Belgium were tested in the rapid fluorescent focus inhibition test (RFFIT) for their neutralizing capacity against the vaccine and wild type viruses.

RESULTS:

Neutralising antibodies against the vaccine strain were present in 81% (84/97) of the 2-3y, 70% (70/95) of the 8-9y, 76% (76/94) of the 13-14y, 73% (73/96) of the 18-20y, 62% (62/92) of the 24-26y and 75% (75/97) of the 50+ 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 24-26y, were significantly different, demonstrating poor in vitro cross-neutralisation.

CONCLUSIONS:

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-neutralisation and should be further studied.