Measles -- United States, 1995

As of March 20, 1996, local and state health departments had reported a provisional total of 301 confirmed measles cases to CDC for 1995. This represents the lowest number of cases ever reported in 1 year since measles first became notifiable in 1912 and a 69% decrease from the 963 cases reported for 1994. This report summarizes the epidemiologic characteristics of measles cases reported in the United States in 1995, and documents important epidemiologic trends, including a shift in age distribution and the continued occurrence of international importations.

Age. Of the 285 measles patients for whom age was known, 109 (38%) were aged less than 5 years, including 39 (36%) aged less than 12 months and 34 (31%) aged 12-15 months. A total of 64 (22%) measles patients were aged 5-19 years, and 112 (39%) were aged greater than or equal to 20 years. Of the 33 measles patients with internationally imported cases, eight (24%) were aged less than 5 years, 14 (42%) aged 5-19 years, and 11 (33%) aged greater than or equal to 20 years.

Vaccination Status. Vaccination status was reported for 219 (73%) measles patients. Among the 96 (44%) who were not vaccinated, 56 (58%) were eligible to be vaccinated (i.e., aged greater than 12 months and born after 1956). Vaccination status varied by age group: 29 (55%) patients aged 1-4 years were unvaccinated, compared with 12 (26%) aged 5-19 years and 28 (32%) aged greater than or equal to 20 years. Of 62 measles patients for whom data were available about dates of vaccination, 55 (89%) had received at least one dose of measles-containing vaccine (MCV) on or after their first birthday and greater than or equal to 14 days before onset of symptoms; seven (11%) were considered to be unvaccinated or inadequately vaccinated; three (5%) received their first dose of measles-containing vaccine (MCV) less than 14 days before onset of symptoms; and four (6%) had received one dose of MCV before their first birthday. Five (8%) cases were reported among persons who had received two doses of MCV after their first birthday.

Case Classification. Among the 301 reported cases, 268 (89%) were indigenous to the United States, including 259 cases (86%) acquired in the state reporting the case and nine (3%) resulting from spread from another state. International importations accounted for 33 cases (11%), and an additional 11 cases were epidemiologically linked to imported cases of measles. Importations originated from or occurred among persons who had traveled in Germany (10), Canada (three), Italy (three), Pakistan (three), China (two), France (two), Malaysia (two), Austria (one), Belgium (one), Costa Rica (one), Egypt (one), Japan (one), and the Philippines (one). For two of the imported cases, the exact source was unknown because the patient had traveled in more than one country outside the United States during the exposure period.

Outbreaks. Nineteen outbreaks (i.e., clusters of three or more epidemiologically linked cases) were reported by 12 states in 1995 and accounted for 74% of all reported cases. Five of these outbreaks began in late 1994. The number of cases involved in outbreaks ranged from three to 73 (median: seven cases). The largest outbreak (73 cases) occurred in a community in Ventura County, California, and primarily involved adults. Two outbreaks (25 cases in New Mexico and 17 cases in Louisiana) occurred primarily among unvaccinated children in day-care settings, and a fourth outbreak (13 cases) occurred among students in a college in Washington. The outbreak that occurred latest in the year primarily involved adult members (nine cases in 1995, 18 in 1996) of a group in Minnesota that declines vaccination because of religious reasons.

CDC performed genomic sequencing of measles viruses isolated from five different outbreaks in 1995. None of the sequences were related to genotypes of viruses circulating during the measles resurgence in the United States during 1989-1991. The isolates from 1995 are genotypically similar to viruses recently isolated in Europe and Japan.

Reported by: State and local health depts. Measles Virus Section, Respiratory and Enteric Viruses Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; National Immunization Program, CDC.

Editorial Note

Editorial Note: The number of reported measles cases in 1995 was a historic low. Since the resurgence of measles during 1989-1991, when incidence was highest among unvaccinated preschool-aged children (1), an increasing proportion of cases have been reported among older age groups. In 1995, 39% of cases occurred among persons aged greater than or equal to 20 years, compared with 24% in 1994 (2). The low number of cases and shift in age distribution highlight the effectiveness and improved implementation of the recommendations of the Advisory Committee on Immunization Practices to provide the first dose of measles-mumps-rubella vaccine (MMR) at age 12-15 months and to give a second dose of MCV (preferably MMR) at age 4-6 years or 11-12 years (3).

During April 1994-March 1995, coverage with MCV was 89% among children aged 19-35 months (4). In addition, an estimated 33%-50% of school-aged children had received a second dose of MMR; as the recommendation for the second dose is more widely implemented, the proportion of cases among school-aged children should decline further. Improved implementation of prematriculation vaccination requirements among students in college and other post-high school educational institutions will increase levels of immunity to measles among young adults.

International importations continue to contribute to the transmission of measles in the United States. Although none of the large outbreaks reported during 1995 were epidemiologically linked to importations, genomic sequencing of isolates from some outbreaks indicates that the strains currently circulating in the United States are similar to viruses recently identified in Europe and Japan. This finding is further evidence that indigenous measles transmission was interrupted in the United States in late 1993 (5). The importation of only three cases from Canada and one from Central America during 1995 is consistent with low levels of current measles activity throughout the Western Hemisphere (6).

Although indigenous transmission of measles is at a historic low, sustained efforts are necessary to further reduce the number of cases. These levels must include assuring uniformly high levels of vaccination coverage among preschool-aged children, particularly in medically underserved urban areas, and improving the sensitivity of surveillance by conducting active case detection at sentinel sites in areas at high risk for measles transmission and measles importation. Recent advances in molecular epidemiology have enabled rapid identification of the source of wild-type measles virus, underscoring the importance of collecting virus isolates from as many cases as possible to improve characterization of patterns of transmission and determine international sources for measles infections in the United States. The continued importations of cases from other countries underscore the needs to support elimination of measles in the Western Hemisphere and to improve global efforts to control measles.

References

1. Gindler JS, Atkinson WL, Markowitz LE, Hutchins SS. Epidemiology of measles in the United States in 1989 and 1990. Pediatr Infect Dis J 1992;11:841-6.

2. CDC. Measles -- United States, 1994. MMWR 1995;44:486-7,493-4.

3. CDC. Measles Prevention: recommendations of the Immunization Practices Advisory Committee (ACIP). MMWR 1989;38(no. S-9):1-13.

4. CDC. National, state, and urban vaccination coverage levels among children aged 19-35 months -- United States, April 1994-March 1995. MMWR 1996;45:145-50.

5. CDC. Absence of reported measles -- United States, November 1993. MMWR 1993;42:925-6.

6. de Quadros CA, Olive JM, Hersh BS, et al. Measles elimination in the Americas: evolving strategies. JAMA 1996;275:224-9.

   
   

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