vaccine was licensed in the United States in 1963, some 400,000
cases of measles were reported on average each year. Yet because
virtually all children contracted measles, the actual number of
cases probably approached that of the entire birth cohort (3.5
million to 4 million cases per By the early 1980s, however, the
annual incidence of reported measles cases had been reduced by
more than 99 percent, to less than Unfortunately, later in that
decade, the vaccination rate decreased and an outbreak of
measles occurred between 1989-1991, when more than 55,000 cases
and greater than 120 measles-related deaths were reported in the
United States. This increase was attributable to two major types
of out-breaks: those among unvaccinated preschool-age children
and those among vaccinated school-age children; this latter
group was susceptible because approximately 5 percent of those
receiving a single dose of measles-containing vaccine do not
mount a protective immune response. As a result of this
resurgence, a two-dose