Rubella immunization of teenage girls in Iceland and follow-up after a severe rubella epidemic.

Of 730 Icelandic schoolgirls, 12 years of age, screened for rubella antibodies by the haemagglutination-inhibition (HI) technique, 372 (50.9%) were found to be seronegative. Rubella vaccine was administered to 355 members of this group, and to 12 girls with an HI titre of 20. Blood samples were taken 6 weeks and 1 year after immunization from 336 previously seronegative girls and from the 12 low-positive vaccinees, and tested for HI antibodies. The seroconversion rate was 99.15%. One year after immunization, 94.8% of the girls tested had an HI antibody titre of 40 or more. The geometric mean titre (GMT) of the previously seronegative girls was 62.07 at 6 weeks and 61.69 at one year after vaccination. Of the girls with an initially low positive titre, only 3 (25%) showed a significant rise. The GMT was 40 at 6 weeks, but fell to 25.2 after 1 year. Re-immunization had no effect on antibody levels in either group.A severe rubella epidemic occurred 2 years after immunization, following which 345 vaccinees and 193 naturally immune controls were studied. All subjects had been exposed to wild rubella virus at school, and rubella cases had occurred in the homes of 116 vaccinees (33.6%) and 41 control subjects (21.2%). Only 5 (1.5%) primarily seronegative vaccinees and 2 unimmunized controls (1%) showed serological evidence of infection during the epidemic.