Effect of undocumented and excluded vaccinations on measles immunity in a university population.

Epidemic measles has occurred even in highly vaccinated university and college populations because vaccine efficacy is only 90-95% and some student bodies do not have 100% vaccine coverage. Some institutions have not adopted or enforced a one-dose, preregistration immunization requirement because of the increased administrative burden, although in 1990 a documented two-dose schedule was recommended to increase herd immunity to a critical 93.5-96.0%. Perhaps one documented immunization for all students coupled with an undocumented or excluded pre-1968 one that some students are reporting will result in adequate herd immunity. At the University of South Florida 149 students (group I), after documenting receipt of one immunization, attempted to prove immunity by having an antibody test. They were compared with 181 students who documented two doses (group II) for factors which might influence differences of immunity between the groups after one dose. Undocumented immunizations were estimated by determining the percent of students reporting recent immunizations after allowing for the expected unimmunized percent. Excluded vaccinations were calculated for students born before 1967. Group I had a 98% (95% C.I. 94.2-99.6) immunity. One-third to one-half of both groups had undocumented and excluded immunizations. Some differences identified were not important enough to prevent extrapolation of this high immunity from group I to group II students. One documented live measles vaccination for all students combined with an undocumented or excluded one for some of them may provide adequate herd immunity to prevent measles epidemics on university and college campuses.