Measles -- United States, First 26 Weeks, 1994

As of July 2, 1994 (week 26), local and state health departments in 31 states had reported a provisional total of 730 measles cases * to CDC for 1994 (1) (Figure_1). This represents a greater than fourfold increase over the historic low of 167 cases reported by 18 states during the same period in 1993. In addition, 250 cases were reported in 1994 for the U.S. territories of Guam (211) and the commonwealths of the Northern Mariana Islands (26) and Puerto Rico (13). This report summarizes the epidemiologic characteristics of measles cases reported in the United States for the first 26 weeks of 1994. Characteristics

Case classification. Of the 730 reported cases, most (696 {95%}) were indigenous to the United States, including 588 (80%) acquired in the state reporting the case and 108 (15%) that resulted from spread from another state **. Fifteen states reported a total of 30 (4%) internationally acquired cases -- one of which initiated a college outbreak in New Jersey resulting in approximately 100 cases. The 30 international importations originated from or occurred among persons who had traveled in Asia (Hong Kong, Indonesia, Japan, Korea, and Vietnam), Europe (England, France, Germany, Spain, and Switzerland), Latin America (Dominican Republic, Ecuador, and Mexico), Canada, Iran, and Israel. Of the 30 persons with internationally acquired measles, 11 were aged less than 5 years; 10, aged 5-19 years; and nine, aged greater than or equal to 20 years. Six of the 20 persons for whom data were available were U.S. citizens.

Age. Of the 725 persons with cases for whom age was known, 172 (24%) occurred among persons aged less than 5 years, 368 (51%) among persons aged 5-19 years, and 185 (26%) among persons aged greater than or equal to 20 years. Of the 172 cases among persons aged less than 5 years, 49 (28%) occurred among persons aged less than 12 months. Of the 71 cases for whom serologic testing for measles was reported, 70 were serologically confirmed. Vaccination Status

Of 274 reported patients for whom vaccination data were available, 44 (16%) had received at least one dose of measles-containing vaccine (MCV) on or after their first birthday and greater than 14 days before the onset of symptoms. A total of 81 (30%) patients considered to be unvaccinated received a first dose of MCV less than or equal to 14 days before the onset of symptoms; most vaccinations were administered during an outbreak involving previously unvaccinated persons (2). Five cases were reported among persons who had received two doses of MCV; for two of these five persons, the second dose was administered less than or equal to 14 days before symptom onset.

Of the 230 patients who were either unvaccinated or vaccinated less than or equal to 14 days before illness onset, 166 (72%) had a religious or philosophic exemption to vaccination. Forty-three (19%) patients were unvaccinated but vaccine-eligible (i.e., U.S. citizen aged greater than or equal to 16 months with no medical, religious, or philosophic exemption to vaccination), and 21 (9%) were younger than the recommended age for vaccination. Vaccination status varied by age group. Of measles patients aged 5-19 years, 14% had received at least one dose of MCV at an appropriate age, compared with 23% of patients aged 1-4 years. Outbreaks

Fifteen measles outbreaks (clusters of five or more epidemiologically linked cases) were reported by 10 states during the first 26 weeks of 1994 and accounted for 82% of all cases reported for this period. Six outbreaks (range: 25-148 cases) occurred in high schools or colleges, five (range: 5-32 cases) among preschool-aged children, and four (range: 5-126 cases) in other settings. All high school and college outbreaks occurred in institutions with no vaccination requirements (two institutions) or a requirement for only one dose of MCV (four institutions). Three of the largest outbreaks occurred among persons who do not routinely accept vaccination in St. Louis County, Missouri (148 cases, high school); Jersey County, Illinois (52 cases, college); and Salt Lake County, Utah (126 cases, community). In addition to these outbreaks, a large outbreak (approximately 200 cases) occurred predominantly among preschool-aged children in Guam.

CDC performed genomic sequencing of measles viruses isolated from seven outbreaks in the continental United States during 1993- 1994. Preliminary analysis indicates that all of the viruses from these recent outbreaks (most from 1994) are genotypically different from viruses isolated during the 1989-1991 measles resurgence. All viruses obtained during 1989-1991 were closely related by sequence analysis, even though they were obtained from cases in different geographic regions. In contrast, isolates from recent U.S. outbreaks were genotypically similar to viruses from European or Japanese sources.

Reported by: State and local health depts. L Espaldon, MD, Guam Dept of Public Health and Social Svcs. BJ Francis, MD, State Epidemiologist, Illinois Dept of Public Health. HD Donnell, Jr, MD, State Epidemiologist, Missouri Dept of Health. CR Nichols, MPA, State Epidemiologist, Utah Dept of Health. National Immunization Program; Measles Virus Section, Respiratory and Enteric Viruses Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Although measles incidence has increased since the historic low reported in 1993, measles incidence during the first 26 weeks of 1994 remains substantially lower than in previous years. In addition, epidemiologic characteristics of cases reported in 1994 are consistent with patterns observed since the end of the measles resurgence during 1989-1991. These patterns include 1) a shift in age incidence from preschool-aged children to older age groups, 2) the importance of international importations in the spread of measles, and 3) the spread in groups whose members do not routinely accept vaccination -- in particular, cases among groups with religious or philosophic exemption to vaccination accounted for 45% of all cases reported during the first 26 weeks of 1994. Maintaining communication with these groups permits rapid detection of cases and prompt implementation of outbreak-control measures when cases occur and may encourage some members to accept vaccination.

During 1994, measles cases have occurred predominantly among high school- and college-aged persons, many of whom previously had received one dose of measles vaccine. In contrast, during the 1989- 1991 measles resurgence, cases occurred predominantly among preschool-aged children. Since 1991, the proportion of cases among persons aged less than 5 years has decreased substantially -- from 49%-50% during 1991-1992 to 24% during the first 26 weeks of 1994. This decline may have resulted from systematic efforts to increase measles vaccination coverage (approximately 85% in 1993) among preschool-aged children at 24 months of age (3).

The outbreaks among previously vaccinated high school- and college-aged persons emphasize the importance of implementing and enforcing vaccination with a second dose of MCV among persons in these age groups. Findings of a recent assessment indicated that the risk for measles outbreaks is lower among colleges that enforce prematriculation requirements for measles vaccination when compared with those that do not have or do not enforce such policies (4).

The laboratory findings during 1994 are consistent with other epidemiologic data suggesting that measles transmission may have been interrupted in the United States in late 1993 (5) and indicate that international importations account for a substantial proportion of disease attributable to measles in 1994. Although only one large outbreak has been epidemiologically linked to a known importation, genomic sequencing of measles viruses suggests that cases in 1994 resulted from reintroduction of measles by international importations.

Although indigenous measles transmission in the United States may have been transiently interrupted, the continued occurrence of measles among U.S. residents demonstrates that additional efforts are required to attain the Childhood Immunization Initiative goal of sustained elimination of indigenous measles in the United States by 1996. These efforts should include 1) rapid detection of cases and implementation of appropriate outbreak-control measures, 2) achievement and maintenance of high levels of vaccination coverage among preschool-aged children in all geographic regions, and 3) greater implementation and enforcement of the two-dose recommendation among high school and college students. In addition, the source of measles infection should be established for all cases to define better the chains of disease transmission and to help develop more effective control measures.

State and local health departments are encouraged to investigate thoroughly all cases to identify the source of measles infection and to obtain specimens for virus isolation. Specimens should be obtained from all sporadic cases and from selected outbreak-associated cases. Specimens may be collected from nasal washings within 1-3 days of rash onset or from urine samples within 2 weeks of rash onset. Additional guidelines for specimen collection and handling can be obtained from CDC's Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, telephone (404) 639-3512, or from CDC's National Immunization Program, telephone (404) 639-8226.

References

1. CDC. Table II. Cases of selected notifiable diseases, United States, weeks ending July 2, 1994, and July 3, 1993 (26th week). MMWR 1994;43:478.

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

3. CDC. Vaccination coverage of 2-year-old children -- United States, third quarter, 1993. MMWR 1994;43:556-9.

4. Baughman AL, Williams WW, Atkinson WL, Cook LG, Collins M. The impact of college prematriculation immunization requirements on risk for measles outbreaks. JAMA 1994 (in press).

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

* Comprises cases reported to CDC's National Notifiable Diseases Surveillance System through July 2, 1994 (week 26), and cases reported subsequently that occurred during this period. 

** Acquired in another state or linked within two generations to an out-of-state importation.

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