Measles in an Immunized School-Aged Population -- New Mexico

From February 10, to April 15, 1984, 76 cases of measles were reported in Hobbs, New Mexico. Sixty-two cases (82%) were serologically confirmed. The outbreak began in one junior high school and spread through the community (Figure 1). Forty-seven cases (62%) occurred among students attending Hobbs Municipal Schools. Cases occurred in the first, second, and fifth to 12th grades and spread through seven schools. Twenty-nine (62%) of the patients attended the seventh through ninth grades, and nine (19%) attended the 10th through 12th grades. The attack rate was 0.6% for the entire school system and 3.8% for the index junior high school. The school system reported that 98% of students were vaccinated against measles before the outbreak began. The outbreak was eventually controlled by excluding unvaccinated students from school and by aggressive case follow-up to identify susceptible contacts. A case-control study was conducted to determine risk factors for measles in this highly vaccinated school-aged population.

One control matched for sex, grade, and school was randomly selected for each of the 47 students with measles. Parents were interviewed to determine the vaccination history of each child. Controls were confirmed not to have had a rash illness during the outbreak. Vaccine providers named by the parents were contacted to verify the immunization histories furnished by the parents.

All but one of the 47 patients and all the controls had histories of measles vaccination. Among the 43 patients and 39 controls who had received one dose of measles vaccine, no association was found between measles vaccine failure and time since vaccination, vaccination before 15 months of age, or type of vaccine administered. Measles patients, however, were significantly less likely to have a measles vaccination record that could be documented by a provider (Table 3). Both patients and controls vaccinated in New Mexico were significantly more likely to have provider verification than were patients or controls vaccinated outside New Mexico.

Comparison of the 20 measles cases (19 serologically confirmed) and 30 controls with provider-verified immunization records demonstrated that measles vaccine failure was not associated with time elapsed since immunization. However, vaccine failure in this population was significantly associated with vaccination at 12-14 months of age (Table 4). The risk of measles for children vaccinated at 12-14 months of age was 4.7 times higher than for persons vaccinated at 15 months of age or older. However, when lack of provider verification is considered as a risk factor along with age at vaccination, more cases were associated with the former risk factor (odds ratio 6.4) than with vaccination at 12-14 months (odds ratio 4.7). Reported by J Kernan, MD, EB Burke, MD, TJ Gerend, MD, TG McCormick, MD, Hobbs, D Tabor, Hobbs Municipal School District, R Lucero, D Esquibel, T Salazar, Immunization Program, M Powell, R Steece, Scientific Laboratory Div, J Montes, P Hayes, Epidemiology Office, H Hull, MD, State Epidemiologist, New Mexico Health and Environment Dept; Div of Field Svcs, Epidemiology Program Office, Div of Immunization, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: Several important issues related to measles control are raised by this outbreak. The first concerns the age distribution of patients--62% of school-aged patients were in grades 7-9. This age-specific distribution is consistent with 1984 national surveillance data and represents a change from 1983 (1). The reason for this shift is unknown.

The New Mexico data suggest that some of the problems may relate to inaccurate school records. Vaccination histories could only be verified with providers for 49% of the 47 patients studied. This outbreak demonstrates that provider-verified records had the highest correlation with protection. If provider verification had been required, 23 (49%) of the 47 cases might have been preventable, since patients would have had to be vaccinated or revaccinated to remain in school. Further studies are needed during similar outbreaks to evaluate the validity of school records. The current measles elimination strategy, which emphasizes measles immunity requirements for school entry, has been successful in keeping measles occurrence near record low levels, suggesting that records in most communities are accurate.

When all patients and controls are analyzed, no differences in risk for developing measles between groups could be found for age at vaccination. However, when only patients and controls with provider-verified records were analyzed, there appears to be an increased risk for children vaccinated at 12-14 months, compared with children vaccinated at 15 months of age or older, a finding that has been described previously (2). The Immunization Practices Advisory Committee (ACIP) does not routinely recommend revaccination of children initially vaccinated between 12 months and 14 months of age, because protection of this group appears to be high (80%-95% or higher).

While children vaccinated at 12-14 months of age appear to be at greater risk of developing measles than those vaccinated at 15 months of age or older, no evidence currently exists to suggest that they are capable of sustaining transmission in the absence of other risk factors. Consequently, routine revaccination of children vaccinated between 12 and 14 months is not warranted. However, such revaccination might be considered in outbreak situations where measles is sustained and other risk factors cannot be identified. The present measles elimination strategy has been successful in eliminating measles from most of the country.

References

1. CDC. Measles--United States, first 26 weeks, 1984. MMWR 1984;33:495-6, 501-4.

2. CDC. Measles surveillance. Report no. 11, 1977-1981. September 1982.

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