The live attenuated measles vaccine was introduced in 1967 and by 1985 had prevented about 52 million cases of measles, 5,200 deaths, and 17,400 cases of mental retardation attributable to measles [3]. During the years 1989 to 1991, measles cases started to increase again, and the United States Public Health Service responded by recommending a two-dose immunization schedule [4]. The rationale for the second dose of the measles vaccine was not to serve as a booster but rather to immunize the five to 20 percent of people who had not responded to the first dose of the vaccine.
This two-dose approach appears effective. In 1990 a peak of 27,000 measles cases were reported in the United States; in 2006, only 45 cases were reported. Of the vaccine eligible subjects (ie, born after 1957 and older than 12 months) who developed measles, 69 percent either did not know whether they had been vaccinated or had not received two doses of the vaccine [5].
While indigenous measles is rare, continued protection of adults and children remains important, particularly since imported cases still have the potential to serve as a major source of future exposures. Adults with measles are at increased risk of mortality compared with older children, and measles in pregnancy is associated with premature labor and spontaneous abortion [6].
Like measles, the incidence of mumps in the United States fell dramatically after the introduction of the live virus vaccine in 1967. There was a resurgence of mumps in 1987 to 13,000 cases, probably because mumps immunization was not recommended by the American College of Pediatrics until 1982, leaving a cohort of young adults, born after 1956 but before 1982, at risk. In 2006, 6339 cases of mumps were reported in the US. Eighty-four percent of these cases occurred in six states - Iowa, Kansas, Wisconsin, Illinois, Nebraska and South Dakota [7]. The median age of cases with mumps was 22 and almost all cases occurred despite receipt of 2 doses of MMR vaccine. Factors such as close contact in college dormitories have been suggested as a reason for the 2006 outbreak of mumps.
As a result of the outbreak, ACIP recommendations for prevention and control of mumps have been updated (www.cdc.gov/mmwr/preview/mmwrhtml/mm5522a4.htm). Evidence of immunity through documentation of vaccination is now defined as one dose of MMR for preschool-aged children and adults not at a high risk for exposure and infection and two doses of live mumps vaccine for school-aged children (ie, grades kindergarten through 12), and adults at high risk for exposure and infection (ie, healthcare workers, international travelers, and students at post-high school education institutions). Additional recommendations for outbreak control include administering a second dose of MMR for preschool children and adults not at high risk for exposure and infection if these persons are part of a group that is experiencing an outbreak.
Continued protection of adults remains important because the most serious complications of mumps arise more frequently in adults than in children, including neurologic complications, orchitis leading to sterility, and fetal death.
While adults born before 1957 are considered immune to measles and mumps, rubella immunity is not assured for adults. Rubella immunity is established by a positive serologic test or documented evidence of rubella or MMR immunization on or after one year of age. A clinical diagnosis of rubella is not reliable. The rubella vaccine was effective in reducing cases from 57,600 in 1969 to 213 cases in 1996 [6]. There was a brief resurgence in rubella in 1990 to 1991, with 40 to 45 percent of cases occurring in adults and teenagers aged 15 and older. Disturbingly in 1991, there was also an increased incidence of congenital rubella syndrome, representing a failure of the immunization campaign. In 1992 to 1994, eight percent of young adults were estimated to be susceptible to rubella [6]. Data from NHANES III indicated that persons born from 1970 to 1974 had the lowest rate of protection against rubella (78 percent), further highlighting the need for continued vigilance in immunization of children and adults against this disease [8]. Even though rubella is rare (only 8 cases were reported in 2006), continued protection of adults (particularly women of childbearing age who could become pregnant) and children is essential if the most important consequences of rubella (congenital rubella syndrome, miscarriages, and fetal deaths) are to be eliminated.