Before measles immunizations were available, nearly everyone in the U.S. go measles. There were approximately 3 to 4 million measles cases each year. An average of 450 measles-associated deaths were reported each year between 1953 and 1963.

In industrialized countries, up to 20% of persons with measles are hospitalized, and 7% to 9% suffer from complications such as pneumonia, diarrhea, or ear infections. Some persons with measles develop encephalitis, resulting in brain damage. It is estimated that as many as 1 of every 1,000 persons with measles will die.

Widespread use of measles vaccine has led to a greater than 95% reduction in measles, compared with the pre-vaccine era.

Measles virus is common throughout the world and is frequently imported into the U.S.. In 1996, 47 cases were known to have been imported by people traveling to the US from other countries. In the first three months of 1997, all US measles cases reported have been linked to imported cases.

According to the World Health Organization, 1.1 million deaths occurred worldwide from measles in 1995. If vaccinations were stopped, 2.7 million measles deaths could be expected.

Stopping measles vaccination would be expected to lead to massive epidemics similar to those that occurred in the pre-vaccine era. Between 1989 and 1991, the number of reported measles cases in the US increased sharply, with more than 55,000 cases, 11,000 hospitalization, and 120 deaths reported. The major cause of this epidemic was low rates of vaccination among preschool children. The risk of measles in preschool-age African-American and Hispanic children was 8 to 10 times higher than that of white children, due to lower vaccination rates in these children.