Live, attenuated measles virus vaccine was licensed for use in the United States in 1963. During the past 13 years, more than 80 million children have been immunized. During the prevaccine era, major epidemics occurred in most cities at two- or threeyear intervals, and the number of reported cases in the United States was approximately 500,000 each year.
The extensive use of measles vaccine during the past decade has had a profound effect on the epidemiology of the disease in this country. The number of reported cases has declined to levels of approximately 35,000 per year, a decrease exceeding 90%. In addition, there has been a corresponding reduction in the number of cases of encephalitis following measles cases, and major nationwide epidemics have not occurred. However, despite this encouraging trend, outbreaks of measles have been observed in adolescents as well as in young children during the past four or five