While rubella is usually mild in children and adults, up to 90% of infants born to mothers infected with rubella during the first trimester of pregnancy will develop congenital rubella syndrome (CRS), resulting in heart defects, cataracts, mental retardation, and deafness.

In 1964-1965, before rubella immunization was used routinely in the US, there was an epidemic of rubella that resulted in an estimated 20,000 infants born with CRS, with 2,100 neonatal deaths and 11,250 miscarriages. Of the 20,000 infants born with CRS, 11,600 were deaf, 3,580 were blind, and 1,800 were mentally retarded.

Many developing countries do not include rubella in the childhood immunization schedule. In 1996, two outbreaks of rubella in young adults occurred among recently arrived immigrants who were not protected against rubella. At least four pregnant women contracted rubella during this outbreak.

If immunity to rubella were to decline, rubella would once again return, resulting in pregnant women becoming infected and then giving birth to infants with CRS. The incidence of CRS declined dramatically with widespread use of rubella vaccine.