During an epidemic of measles in Des Moines County, Iowa, 72% of the 541 cases involved school-age children. Sixty-three percent of children aged 1 to 9 years in the county had been vaccinated previously. A survey for cases of measles in preschool siblings of school age children suggested that previously vaccinated children, in an epidemic, do not transmit measles to their susceptible siblings.
Brandling-Bennett AD, Landrigan PJ, Baker EL. Failure of Vaccinated Children to Transmit Measles. JAMA. 1973;224(5):616-618. doi:10.1001/jama.1973.03220180036010