Measles -- United States, 1994

As of June 13, 1995, local and state health departments in 39 states had reported 958 measles cases to CDC for 1994. This represents the second lowest number of cases ever reported, after the historic low of 312 cases in 1993 (1). In addition, 303 cases were reported for the U.S. territory of Guam (228) and the commonwealths of the Northern Mariana Islands (29) and Puerto Rico (46). This report summarizes the epidemiologic characteristics of measles cases and outbreaks reported in the United States during 1994.

Age distribution, complications, and hospitalizations. Of the 954 measles patients for whom age was known, 247 (26%) were aged less than 5 years, including 73 (8%) who were aged less than 12 months and 69 (7%) who were aged 12-15 months. Nearly one half (475) of all measles patients were aged 5-19 years, and 232 (24%) were aged greater than or equal to 20 years. Among the 537 measles patients for whom information was available, 45 (8%) were reported to have been hospitalized; the median duration of hospitalization was 4 days (range: 1-22 days). Among 338 (35%) measles cases for which information on laboratory testing was provided, 229 (68%) were serologically confirmed. Vaccination status. Vaccination status was reported for 848 (89%) measles patients. Among 762 vaccine-eligible persons, * 171 (22%) were reported to have documented receipt of at least one dose of measles-containing vaccine, and 539 (71%) were unvaccinated. Fifty-two (7%) persons with reported unknown vaccination status were considered to be unvaccinated. Four cases occurred among persons with documentation of two appropriately spaced doses of measles vaccine greater than 14 days before onset of symptoms. Among 301 unvaccinated measles patients who were eligible for vaccination and for whom a reason for nonvaccination was reported, 294 (98%) cited a religious (154 {51%}) or philosophic (140 {47%}) exemption to vaccination. Almost all (92% {277}) of these cases occurred in outbreaks in Illinois, Missouri (2), Nevada, and Utah. Cases among persons claiming religious or philosophic exemption to vaccination accounted for 36% of all reported cases in 1994. Case classification. Among 949 reported cases for which the epidemiologic classification is known, 874 (92%) were indigenous to the United States, including 719 (76%) acquired in the state reporting the case and 155 (16%) resulting from spread from known importation from another state. International importations and cases occurring within two generations of these importations accounted for 75 (8%) measles cases in 1994. These cases were reported from 24 states and, for those for whom the country of origin was reported, occurred most frequently among persons arriving from Europe (26 cases) and East Asia (18). Cases resulted from importations from the Americas (eight), the Middle East (six), and Africa (two). Among the 75 persons with internationally imported measles, 23 (31%) were aged less than 5 years; 32 (43%), 5-19 years; and 20 (27%), greater than or equal to 20 years. Outbreaks. Twenty-two outbreaks (clusters of five or more epidemiologically linked cases) were reported by 15 states during 1994 and accounted for 74% (705) of all reported cases. Two of these outbreaks began in 1994 and continued into 1995 (only cases that occurred during 1994 are reported here). Eight outbreaks, which included 12-156 cases, occurred in schools (six outbreaks) or colleges (two), five outbreaks (range: five-22 cases) involved predominantly preschool-aged children, and nine (range: six-134 cases) occurred in other settings and primarily involved young adults. The largest college outbreak (94 cases) resulted from spread from an importation, and two other outbreaks followed known importations. A total of 176 cases (18% of all reported cases) were related to international importations in 1994.

A single chain of transmission that was first recognized in a Colorado ski resort (3) extended into nine additional states and resulted in the largest outbreak of 1994 (247 cases); this outbreak involved students who were unvaccinated because of religious exemptions and who attended a college in Illinois or a school in Missouri (2). Two other outbreaks involving persons with philosophic exemption to vaccination occurred in Salt Lake City, Utah (134 cases), and White Pine County, Nevada (12 cases). In outbreaks among persons with religious or philosophic exemption to vaccination, school-aged children accounted for 73% of all cases, and represented 56% of all measles cases among 5-19-year-olds in 1994.

Intensive surveillance and case investigation resulted in identification of three large multistate outbreaks during 1994. Epidemiologic linkages were established among 247 cases in 10 states from the outbreak that began in Colorado, among 57 cases in six states resulting from exposures in Las Vegas, and among 146 cases from an outbreak that began in Utah and spread to Nevada.

The genomic sequences of viruses isolated from the outbreak in Illinois and Missouri was similar to that of a virus isolated from an earlier outbreak in Memphis, Tennessee. These viruses probably were recently imported into the United States because they were closely related to measles virus strains that had previously circulated in Europe. Four distinct genotypes were identified by genomic sequencing among 10 isolates from four outbreaks and three single measles cases in the United States in 1994. None of these was related to the genotype circulating during the resurgence of 1989-1991, suggesting that all of these viruses were introduced into the United States as a result of importation. 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: Although measles incidence has increased since the historic low reported in 1993, the number of cases reported during 1994 is the second fewest in the United States since measles reporting began in 1912. Important characteristics of current epidemiologic trends are the shift in age distribution of cases to older persons, the large proportion of cases in groups whose members do not routinely accept vaccination, and the increasing numbers of cases linked to international importations.

Since the measles resurgence of 1989-1991, increasing proportions of cases have occurred among school-aged children and adults, and proportionately fewer in preschool-aged children -- a substantial change from 1989-1991, when incidence was highest among preschool-aged children, of whom as many as 80% were unvaccinated (4,5). The shift in age distribution probably resulted from record-high measles vaccination coverage levels among preschool-aged children, which reached 90% in the first quarter of 1994 (6). More than half of the cases in persons aged 5-19 years were associated with outbreaks among persons with a religious or philosophic exemption to vaccination. Additional efforts will be necessary to reduce transmission among persons with objections to vaccination.

Laboratory and epidemiologic data suggest that measles transmission was interrupted in the United States during late 1993 (7). Because of the effective implementation of a strategy of mass vaccination of children in all countries in Central and South America, importations from the Americas have decreased substantially since 1991 and now represent a small percentage of all importations. However, the continued risk for international importations and spread from importations from other locations represent a challenge to the goal of measles elimination in the United States; known international importations or spread from international importations accounted for almost one fifth of reported measles cases in 1994.

The strategy for achieving the Childhood Immunization Initiative goal of eliminating indigenous measles transmission in the United States (8) is based on four components: 1) maintaining high coverage with a single dose of measles-mumps- rubella vaccine (MMR) among preschool-aged children, 2) achieving coverage with two doses of MMR for all school and college attendees, 3) enhancing surveillance and outbreak response, and 4) increasing efforts to develop and implement strategies for global measles elimination. CDC will continue to work with state and local health departments to implement recommendations to achieve high levels of population immunity, rapidly report and investigate all suspected measles cases, and enhance surveillance to facilitate rapid identification and confirmation of cases and implementation of appropriate control measures.

References

1. CDC. Table II. Cases of selected notifiable diseases, United States, weeks ending December 31, 1994 and January 1, 1994 (52nd week). MMWR 1995;43:969.

2. CDC. Outbreak of measles among Christian Science students -- Missouri and Illinois, 1994. MMWR 1994;43:463-5.

3. CDC. Interstate measles transmission from a ski resort -- Colorado, 1994. MMWR 1994;43:627-9.

4. 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.

5. CDC. Measles surveillance -- United States, 1991. MMWR 1992;41(no. SS-6):1-12.

6. CDC. Vaccination coverage of 2-year-old children -- United States, January-March 1994. MMWR 1995;44:142-3,149-50.

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

8. CDC. Reported vaccine-preventable diseases -- United States, 1993, and the Childhood Immunization Initiative. MMWR 1994;43:57-60.

• Persons aged greater than or equal to 12 months who were born after 1957. Persons born in or before 1957 are considered to be immune based on the likelihood of their having had measles before licensure of measles vaccine in 1963.

  
  

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