Boosting effect of a second dose of measles vaccine given to 12-year-old children.

In Sweden, a two-dose programme of vaccination against measles, mumps and rubella (MMR) was introduced in 1982 and the target groups were children aged 18 months and 12 years. In 1993 the first age group of 12-year-olds that were vaccinated with MMR at 18 months will appear. The majority of the 12-year-old vaccines for many years, however, had already been vaccinated against measles and the MMR measles component was thus a booster dose. As the benefit of a booster dose against measles has been questioned, this was studied in a group of 163 12-year-old children, 122 of whom had been vaccinated against measles as young children. Of the 41 unvaccinated children, 23 had a history of clinical measles. The mean neutralizing titre level of the earlier vaccinated children, prior to the booster, was lower than that of the naturally immune children (reciprocal titre level 8 versus 20). After the booster the corresponding titre levels were 13 and 23. Among the seronegative children receiving their first dose, this figure amounted to 14. A moderate rise in titre was seen in those with low prevaccination titres. As the antibody protection afforded by vaccination was slightly lower than that of natural infection, even after a booster, follow-up studies must be recommended to evaluate the long-term protection of both a single and a two-dose programme.