Optimal age for vaccination against measles in the State of São Paulo, Brazil, taking into account the mother's serostatus.

In order to investigate if the changing levels of measles antibody in women resulting from extensive vaccination programs influence the susceptibility of children, we measured the seroprevalence of measles virus antibody of children in the first year of life and of their mothers. We compared maternal antibody decay of two groups of children: those whose mothers were 25 years old or more (mothers born in the pre-vaccination era), and less than 25 years old (mothers born in the vaccination era). Therefore, the 25-year-age cut-off was chosen to distinguish between vaccinated and non-vaccinated mothers. We also compared the immunogenicity of measles vaccine in children from 6 to 12 months of age, in these two groups and also according to their mother's serostatus. The optimal age of vaccination for a routine program was estimated by means of mathematical models. This study was carried out in a sample of 1216 mothers and their respective children. Our results indicate that the optimal age for vaccination of the whole sample was 15 months, 17 months for children born from older mothers, 14 months for children born from younger mothers, 17 months for children born from seropositive mothers and 12 months for children born from seronegative mothers. Therefore, a change to an earlier age of routine vaccination is not justified by our results.