Immunity to mumps before and after MMR vaccination at 12 years of age in the first generation offered the two-dose immunization programme

Sweden was the first country in the world to introduce a two-dose programme of vaccination against measles, mumps and rubella with a combined vaccine (MMR). It was commenced in 1982 and the vaccination was carried out at the ages of 18 months and 12 years. In 1992-93 the first age-group vaccinated at 18 months reached the age of 12 and accordingly received a second dose of MMR. A total of 382 children participated in the present study. Sero-immunity against mumps was studied by testing neutralizing antibodies using serial dilutions inoculated into cell cultures before and after the 12-year vaccination. Of the 229 children earlier vaccinated (group A), 27% lacked demonstrable antibodies before the booster. Of those without documented vaccination records (group B), 56% were seronegative before vaccination. After vaccination, 93% of group A and 86% of group B were seropositive (titre > or = 2). In the seronegative children, whether vaccinated earlier or not, the seroconversion was ca 75%. Previously unvaccinated children positive before vaccination and thus likely to be naturally immune had a higher mean-titre both before and after vaccination than the seropositive children earlier vaccinated. So far, the two-dose programme has proceeded as expected.