Outbreak of Measles Following an Imported Case -- Florida

As of December 13, 1982, 203 confirmed cases of measles, with rash onsets from September 10 through December 3, were reported from Dade County, Florida. Three additional cases in Florida were linked to the outbreak.

The source of the outbreak was an imported case* in a 14-year-old male resident of Dade County, who lacked documentation of immunity to measles.** He had vacationed in Peru from July 31 to August 31. From August 24 to 26, he had face-to-face contact with a cousin in Peru who had fever and a generalized maculopapular rash. After returning to the United States, he attended Junior High School A from September 1 until rash onset on September 10. He transmitted measles to seven students at Junior High School A, all of whom lacked documentation of measles immunity. They had rash onsets from September 20 through September 27. This led to a county-wide outbreak concentrated primarily in school children (Figure 1).

The highest attack rate, 3.9%, occurred in Junior High School A, which accounted for 73 (36.0%) of the 203 cases. The outbreak was concentrated in Junior High School A from September 10 through October 12 (Figure 1). However, by October 28, measles cases were reported in 11 additional schools, geographically widespread across the county. Ultimately, measles cases occurred in 32 of 186 schools in south Dade County and in 25 of 270 schools in north Dade County. In addition, four day care centers, one community college, and one military school were affected. *Imported measles: measles illness in a person (U.S. citizen or foreign national) who has onset of rash within 18 days of arrival from a foreign country, and for which no indigenous source can be identified. **Immunity to measles: defined by the State of Florida as a dated record showing administration of measles vaccine on or after the first birthday and on or after January 1, 1968, or written certification of previous physician-diagnosed measles illness.

As part of the response to the outbreak, local authorities ordered a review of immunization records of all students in public and private schools in Dade County; 286,000 immunization records were reviewed to identify all students who had inadequate evidence of immunity to measles. Initially, susceptible students were excluded from those schools in which there were measles cases. On October 28, a county-wide measles emergency was declared by the County Health Director permitting non-compliant students, including those with medical or religious exemptions to vaccination, to be excluded from school attendance beginning on November 8 in south Dade County, and on November 22 in north Dade County. Specially assigned State Immunization Program staff joined county health personnel to review records and to administer vaccines. No measles cases were reported among non-compliant students more than 9 days after the school exclusion day.

After control measures were implemented, changes occurred in the age distribution and measles immunity status of reported cases. From September 10 through October 15 (the date the immunization clinic was held in Junior High School A), 54 (85.7%) of 63 measles cases occurred among students aged 10-14 years, whereas 18 (20.5%) of 88 cases occurred in that age group with rash onsets from October 28 through December 3 (Table 1). In Junior High School A, all nine cases with rash onset from September 10 through October 3 were classified as preventable.*** However, no preventable cases were reported in Junior High School A one incubation period after the control clinic.

Overall, 55 (27.1%) of 203 cases are classified as preventable; 42 persons (20.7%) lacked documentation of immunity to measles (24 of whom were vaccinated before the first birthday), and 13 (6.4%) had been vaccinated less than 2 weeks before rash onset. Of the 148 (72.9%) cases classified as not preventable, 110 persons (54.2%) had physician-signed records of immunity to measles,** 18 (8.9%) were less than 16 months old, 12 (5.9%) were born before 1957, and 8 (3.9%) had religious exemptions to vaccination. Reported by: RA Morgan, MD, MP Allison, MT Brown, MD, MB Enriquez, MD, O Fajardo, RE Foden, HO Garcia, MD, S Jones, AC Kimbler, D Kumjian, C Link, P Onley, A Marler, ZB Reece, Dade County Dept of Public Health, PW Bell, RF Adams, BJ Lisko, Dade County Public Schools, Miami, TL Cook, RW Curtiss, BJ Downs, SH King, MD, DL Roach, MB Rothman, JA Spencer, SM White, NS Windham, JL Velez, J Youngblood, JJ Sacks, MD, Acting State Epidemiologist, Florida Dept of Health and Rehabilitative Svcs; Immunization Div, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: This outbreak resulted from an imported measles case in a U.S. citizen who transmitted infection to seven other students in a junior high school. All eight cases were preventable; none of the eight had documentation of immunity to measles, although five had received vaccine before 12 months of age. An earlier imported case had occurred while school was in summer recess, but that case caused only one import-associated case. This outbreak might not have occurred if the first eight persons with measles had been appropriately vaccinated. Before these importations, Dade County, like most of the United States, was probably free of measles. Imported cases are an important source of measles throughout the country (1). Over 25% of reported cases during the first 26 weeks of 1982 were imported or import-associated (2). While such cases are generally not preventable, transmission to others is usually limited when immunization levels are high (1-3). ***Preventable case: defined by the State of Florida as measles illness in a person more than 15 months old and born after 1956 who lacks documentation of immunity to measles, and has no contraindication or religious exemption to receiving vaccine.

In Dade County schools, identification of susceptible students was difficult, because many health records merely certified vaccination without specifying dates of vaccination. Dated records allow reviewers to identify students who had been vaccinated before their first birthdays and need revaccination. Florida law now requires use of a dated form.

The high proportion of cases with documentation of adequate vaccination does not imply a low vaccine efficacy. If an outbreak occurs in a highly vaccinated population, a substantial proportion of cases would be expected among vaccinees (4). For example, if an outbreak were to occur in a population of which 90% was vaccinated with a 90%-effective vaccine, approximately half the cases would be expected among prior vaccine recipients. When highly effective vaccines, such as the current measles vaccine, have been given to very large proportions of a population, disease occurrence is infrequent and usually short lived. Measles vaccine efficacy has usually been greater than 90% when determined in settings similar to Dade County (5).

Successful control of this outbreak was facilitated by the collaboration of teachers, nurses, physicians, the news media, education and health officials, and parents and children. The mandatory policy to exclude non-compliant students from school attendance led to the vaccination of over 35,000 school children. The declining number of new measles cases among school children and the declining proportion of preventable cases suggested the policy's success.

Experience during other outbreaks has demonstrated that transmission was usually interrupted shortly after a decline in school-age preventable cases (6,7). The available data indicate that mandatory exclusion of non-compliant students, under the recently amended Florida statutes (8), was an effective strategy to interrupt measles transmission.

References

1. Amler RW, Bloch AB, Orenstein WA, Bart KJ, Turner PM, Jr, Hinman AR. Imported measles in the United States. JAMA 1982;248:2129-33.

2. Turner PM, Amler RW, Orenstein WA. United States: Imported measles, first 26 weeks, 1982. EPI Newsletter (Pan-American Health Organization) 1982; (in press).

3. Frank JA, Jr, Hoffman RE, Mann JM, Crowe JD, Hinman AR. Imported measles: a potential control problem. JAMA 1981;245:264-6.

4. CDC. Measles vaccine efficacy--United States. MMWR 1980;29:470-2.

5. CDC. Measles surveillance report No. 11, 1977-1981. Issued September 1982.

6. CDC. Measles--Florida, 1981. MMWR 1981;30:593-6.

7. CDC. Multiple measles importations--New York. MMWR 1981;30:288-90.

8. Florida statutes, s232.032, 1980 Supplement, amended 7/2/81, CS/HB 559.

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