Measles -- United States, 1996, and the Interruption of Indigenous Transmission

As of December 30, 1996 (week 52), local and state health departments had reported a provisional total of 488 confirmed cases of measles to CDC for 1996, and the Commonwealth of Puerto Rico had reported eight cases. In addition, indigenous transmission of measles in the United States was interrrupted for a prolonged period beginning in late 1996. This report summarizes measles surveillance data for 1996, which indicate a substantial proportion of cases were associated with continued international importations of measles and outbreaks among school-aged children who were not required to receive a second dose of measles-containing vaccine (MCV) to attend school. Case Classification

Of the 488 provisional cases, 355 (73%) were indigenous to the United States, including 332 (68%) cases acquired in the state reporting the case and 23 (5%) cases resulting from spread from another state. International importations accounted for 47 (10%) cases of measles, and an additional 86 (18%) cases were epidemiologically linked to imported cases. Importations originated from or occurred among persons who had traveled in Germany (seven cases); Greece and Japan (five each); Austria, India, and Philippines (three each); China, Italy, and Russia (two each); and England, Kenya, Liberia, Nepal, Somalia, Tahiti, and Turkey (one each). For eight 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. None of the imported cases were acquired in countries in the Americas.

Age and Vaccination Status

Of the 465 measles patients for whom age was known, 117 (25%) were aged less than 5 years, including 37 (8%) aged less than 12 months and 25 (5%) aged 12-15 months. A total of 195 (42%) measles patients were aged 5-19 years, and 153 (33%) were aged greater than or equal to 20 years.

Vaccination status was reported for 354 patients. Of the 226 (64%) who were not vaccinated, 170 (75%) were eligible to be vaccinated (i.e., aged greater than 12 months and born after 1956). Vaccination status varied by age group: all 32 patients aged less than 1 year were unvaccinated, compared with 44 (71%) of 62 patients aged 1-4 years, 65 (48%) of 136 patients aged 5-19 years, and 85 (69%) of 124 patients aged greater than or equal to 20 years. Of the 77 patients for whom dates of vaccination were available, 51 (66%) had received at least one dose of MCV after their first birthday and greater than or equal to 14 days before rash onset. Five cases of measles were reported among persons who had received two doses of MCV after their first birthday, and one case was reported in a person who had received three doses of MCV.

Outbreaks

Twenty-three outbreaks (i.e., clusters of three or more epidemiologically linked cases) were reported by 15 states, accounting for 76% of all cases. The number of cases associated with outbreaks ranged from three to 121 (median: five cases). Transmission of measles occurred in school settings in seven outbreaks, and these outbreaks accounted for 55% of all cases reported in 1996. In four outbreaks (Alaska, Texas, Utah, and Washington), cases among school-aged children occurred primarily in those who had received only one dose of MCV; in two other outbreaks (Massachusetts and Minnesota), cases occurred among school-aged children who had religious or philosophic exemptions to vaccination. In Hawaii, an outbreak occurred in a college without a prematriculation vaccination requirement.

In outbreaks related to vaccine failure among school-aged children, the age distribution of cases reflected the type of second-dose policy implemented in the state. In Utah, which had the largest outbreak in the country in 1996 (121 cases, including cases resulting from spread to other states), a requirement for a second dose of measles-mumps-rubella vaccine (MMR) at kindergarten entrance has existed since 1992; at the time of the outbreak, children aged 5-9 years should have received a second dose of MMR. In this outbreak, 75 cases occurred among persons aged 10-19 years, and two cases occurred among children aged 5-9 years. Similarly, in Texas and Washington, which both require a second dose of MMR at middle school entry, outbreak-associated cases occurred among either primary school students, or among high school juniors or seniors who entered secondary school before the policy was implemented. In Alaska, which had not implemented a requirement for a second dose of MMR at the time of the outbreak *, the 63 total cases occurred among elementary school students (17 cases), middle school students (17), and high school students (six) (1).

The source case for six outbreaks (California, Hawaii, Massachusetts, New York, Pennsylvania, and Washington) was traced to an international importation. Genomic sequences from measles virus isolates from four outbreaks without an identified source case (Alaska, Massachusetts {a different outbreak from the outbreak listed above in Massachusetts}, Minnesota, and Utah) were similar to sequences from viruses that were identified as importations from Europe and Southeast Asia, suggesting that an additional 205 (42%) of the 488 provisional cases reported for 1996 were related to international importations.

With the exception of an outbreak of measles in Hawaii (which was linked both by case investigation and molecular epidemiology to international importations of measles virus), indigenous transmission of measles in the United States appears to have been interrupted in late 1996. From October 18, 1996, to February 10, 1997 (16 weeks), only one case of measles (with rash onset on December 16) not linked to an international importation was reported in the United States. An indigenous case with rash onset in February is still under investigation.

Reported by: State and local health depts. Measles Virus Section, Respiratory and Enterovirus Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; Child Vaccine Preventable Diseases Br, Epidemiology and Surveillance Div, National Immunization Program, CDC.

Editorial Note

Editorial Note: Since the resurgence of measles in the United States during 1989-1991 (when approximately 55,000 cases of measles were reported), the annual numbers of reported cases of measles have steadily declined. However, measles among international travelers and outbreaks in schools continue to occur.

Despite coverage levels with MCV of greater than 95% among schoolchildren, most outbreaks during 1985-1988 occurred in schools among children who had been appropriately vaccinated (2,3). This prompted the Advisory Committee on Immunization Practices (ACIP) and the American Academy of Pediatrics to recommend that all children receive a second dose of MCV (preferably MMR) at either age 4-6 years or 11-12 years (4). By 1995, a total of 41 states and the District of Columbia had implemented requirements for a second dose of MMR at either kindergarten or middle school entry (CDC, unpublished data, 1996). In 1996, patterns of outbreaks in schools indicated that gaps in coverage persist and that complete second-dose coverage of all cohorts of school-aged children is necessary to eliminate outbreaks of measles among these children. In addition, further implementation of college prematriculation vaccination requirements for a second dose of MCV should reduce the risk for measles transmission in colleges and universities (5).

ACIP guidelines recommend that, during outbreaks in school settings, affected schools initiate a program of revaccination and consider revaccinating children in unaffected schools that may be at risk for transmission of measles (4). The findings of a study of revaccination of schoolchildren during a measles outbreak in Albuquerque (6) indicated that no measles cases occurred 28 days after revaccination in schools without measles cases in school districts where cases had been reported. The decision to revaccinate children in unaffected schools is difficult and should be based on the likelihood of spread to such schools and the availablity of personnel to conduct vaccination clinics. In these circumstances, CDC can provide vaccine to state health departments to prevent the spread of outbreaks. The ACIP is revising its guidelines to recommend that all school-aged children receive a second dose of MCV by the year 2001; during the interim, to limit the spread of measles transmission and to prevent future outbreaks, public health officials should consider revaccinating schoolchildren in unaffected schools in counties where measles cases have occurred.

Of the provisional measles cases reported for 1996, 69% had international sources: 133 (27%) cases were identified as international importations or were linked to international importations by routine case investigation, and 205 (42%) cases were linked to international importations by molecular epidemiology. Both surveillance and molecular epidemiologic data indicate that the sources of international importations have been predominantly European or Asian; no known cases of measles have been imported from the Americas in 1996. Recent progress by the Pan American Health Organization (PAHO) toward the goal of eliminating measles from the Western Hemisphere has resulted in decreases in the incidence of measles in the hemisphere and in the numbers of cases imported into the United States from other countries in the Americas (7,8). At an international meeting sponsored by PAHO and the World Health Organization in Atlanta in July 1996, participants agreed that global measles eradication is technically feasible with currently available vaccines and that a goal of global eradication should be established (9).

The strategy to eliminate indigenous transmission of measles in the United States includes 1) achieving high population immunity among both preschool children (with one dose of MMR) and school-aged children (with two doses of MMR), 2) improving the sensitivity of surveillance for and increasing laboratory confirmation of measles cases, 3) rapidly implementing outbreak-control measures, and 4) supporting international efforts to eliminate measles. In particular, patterns of transmission of measles cases in 1996 highlight the importance of achieving high levels of second-dose coverage in all cohorts of schoolchildren as well as college students and assisting in global efforts to control measles.

References

1. CDC. Measles outbreak among school-aged children -- Juneau, Alaska, 1996. MMWR 1996;45:777-80.

2. Markowitz LE, Preblud SR, Orenstein WA, et al. Patterns of transmission in measles outbreaks in the United States, 1985-1986. N Engl J Med 1989;320:75-81.

3. Hutchins SS, Markowitz LE, Atkinson WL, Swint E, Hadler S. Measles outbreaks in the United States, 1987 through 1990. Pediatr Infect Dis J 1996;15:31-8.

4. CDC. Measles prevention: recommendations of the Immunization Practices Advisory Committee (ACIP). MMWR 1989;38(no. S-9).

5. Baughman AL, Williams WW, Atkinson WL, Cook LG, Collins M. The impact of college prematriculation immunization requirements on risk for measles outbreaks. JAMA 1994;272:1127-32.

6. Hutchins SS, Markowitz LE, Mead P, et al. A school-based measles outbreak: the effect of a selective revaccination policy and risk factors for vaccine failure. Am J Epidemiol 1990;132:157-68.

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

8. Anonymous. International importations of measles from the Americas into the United States, 1990-1994. EPI Newsletter (February) 1995;17:1-2.

9. Anonymous. Meeting on advances in measles elimination: conclusions and recommendations. Wkly Epidemiol Rec 1996;71:305-9.

   
   

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