Abstract
In a Dutch population group neutralizing antibodies against mumps virus were determined by a plaque reduction technique, which proved reproducible, sensitive and specific. The results with sera of about 800 suburban children show that mumps is acquired at an early age with peak acquisition rates between the ages of four and six years. Over 90% have acquired mumps before the age of 14 years. More than 95% of about 1000 adults (18--65 years) have neutralizing antibodies. The relatively constant median titre suggests that antibodies persist during life. During a family study 77 clinical and 18 subclinical cases were observed. In families with index cases the attack rate was 26/37 = 0.71. Eleven children (excluding six babies) escaped infection. The mean attack rate during the epidemic was 0.30. The mean titre of mumps neutralizing antibody is maximal during the first year after the disease but declines during childhood. Mothers exposed to mumps in the family occasionally showed a significant rise in titre. Some seronegative mothers remained seronegative after exposure.