Measles Outbreak among Vaccinated High School Students -- Illinois
From December 9, 1983, to January 13, 1984, 21 cases of measles occurred
in Sangamon County, Illinois.* Nine of the cases were confirmed
serologically. The outbreak involved 16 high school students, all of whom
had histories of measles vaccination after 15 months of age documented in
their school health records. Of the five remaining cases, four occurred in
unvaccinated preschool children, two of whom were under 15 months of age,
and one case occurred in a previously vaccinated college student (Figure
5).

The affected high school had 276 students and was in the same building as
a junior high school with 135 students. A review of health records in the
high school showed that all 411 students had documentation of measles
vaccination on or after the first birthday, in accordance with Illinois
law.


http://www.cdc.gov/mmwr/preview/mmwrhtml/00000359.htm