An outbreak of measles occurred among adolescents in Corpus Christi, Texas, in the spring of 1985, even though vaccination requirements for school attendance had been thoroughly enforced.
Only 4.1 percent of these students (74 of 1806) lacked detectable antibody to measles… and more than 99 percent had records of vaccination with live measles vaccine.
[T]hree seronegative students seroconverted without experiencing any symptoms.
We conclude that outbreaks of measles can occur in secondary schools, even when more than 99 percent of the students have been vaccinated and more than 95 percent are immune.