Measles Among Children of Migrant Workers -- Florida

A total of 93 clinical measles cases,* with rash onsets from January 1 through April 27, 1983, were reported from Dade County, Florida (Figure 1). Eighty-seven (93.5%) of these occurred among migrant workers and their dependents; 21 of these 87 resided in migrant-worker camps. The 93 cases occurred after an outbreak of over 200 cases in another part of Dade County (rash onsets September 10-December 3, 1982) (1).

The 1982 outbreak was concentrated in schoolchildren; the 1983 outbreak occurred principally among preschoolers--71 (76.3%) children under 5 years old. Complications from measles occurred principally in children under 5 years old. During the first 8 weeks of the outbreak, four of 36 children (11.1%) under 5 years old had otitis media and three (8.3%) had pneumonia. The highest complication rates occurred in infants under 15 months of age. Of these 22 infants, three (13.6%) developed otitis, and two (9.1%) had pneumonia. One child was hospitalized (a 2-week-old with pneumonia); no deaths occurred.

Two major chains of transmission were defined--one at a medical clinic and one in migrant-worker camps. Twelve (22.6%) of the 53 cases occurring in the first 8 weeks of the outbreak were believed to have been acquired at the medical clinic.

A door-to-door survey was conducted in all three affected migrant-worker camps in the county by bilingual teams from the Dade County Department of Public Health. In one camp, rosters from 244 of the 387 housing units were studied. Ninety-three percent of these units housed individuals under 26 years of age. Of 985 occupants identified, 649 (65.9%) had documented histories of vaccination or were over 20 years old.** Nineteen of the remaining 336 were under 6 months old--the minimum age of vaccination recommended for outbreak control (2)--leaving 317 persons at risk of disease, an average of 1.3 persons (317/244) per household.** Assuming the same average occupancy for homes that were not surveyed, an estimated 186 additional susceptibles were present. Special on-site immunization clinics delivered 264 vaccinations, providing vaccine to 52.4% of the estimated population at risk. Despite these clinics, transmission persisted (Figure 1). Door-to-door vaccinations were provided on April 14; transmission ended shortly afterward. Reported by M Enriquez, MD, H Garcia, MD, A Kimbler, RA Morgan MD, Dade County Dept of Public Health, Miami; HT Janowski, MB Rothman, JL Velez, JJ Witte, MD, JJ Sacks, MD, Acting State Epidemiologist, Florida State Dept of Health and Rehabilitative Services; Office of Migrant Health, Bureau of Health Care Delivery and Assistance, Health Resources and Svcs Administration; Div of Field Svcs, Epidemiology Program Office; Div of Immunization, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: Although the source of this outbreak could not be identified, transmission may have occurred from the 1982 outbreak among schoolchildren in Dade County (1). Physician- and school-based reporting systems could have failed to detect the link between the two outbreaks because many of the cases in the 1983 outbreak occurred in preschoolers who often did not seek medical care. As in other outbreaks among preschoolers, transmission in the medical clinic played an important role in sustaining this outbreak (2-4).

To minimize the risk of transmission, it is important to separate potentially infectious children from others in waiting rooms (2). In this outbreak, the clinic staff established a triage system at the facility. It was recommended that children who arrived with symptoms compatible with the prodrome of measles be isolated from other patients. Staff also reviewed the immunity status of other clinic patients and vaccinated susceptible patients from 6 months to 25 years old.

The Immunization Practices Advisory Committee (ACIP) recommends routine measles vaccination at 15 months of age (5). However, in this outbreak, a substantial proportion of cases (47.3%) occurred among infants under 15 months of age. Because the risk of complications is high among such infants, the ACIP also recommends that, during outbreaks, infants as young as 6 months of age may be vaccinated when exposure to measles is likely. Infants under 12 months old should receive single-antigen measles vaccine rather than measles-rubella (MR) or measles-mumps-rubella (MMR) vaccines, because rubella and mumps vaccines are not recommended for that group. To ensure protection against measles, such infants should be revaccinated when they are about 15 months old.

The recommendation to vaccinate at a younger age is made in consideration of the risk of measles complications in such infants and the benefit of vaccination, as well as the possible risk that some persons vaccinated before 11 months of age may have less predictable immune responses to measles vaccine when revaccinated on or after their first birthdays. After revaccination, approximately half the infants who fail to seroconvert initially will develop persistent hemagglutination inhibition (HI) antibody; the remaining half will not develop sustained levels of HI antibody. However, all such children, whether HI-antibody negative or positive, have antibody detectable by a sensitive plaque neutralization test (6). There is no evidence to suggest that such children are susceptible to measles.

Immune globulin (IG) may also be used to prevent or modify measles in infants (5). However, IG should not be used in an attempt to control outbreaks. IG may be especially indicated for susceptible household contacts of measles patients (particularly if under 1 year of age). The recommended dose is 0.25 ml/kg (0.11 ml/lb) of body weight (maximum dose, 15 ml), intramuscular, within 6 days of exposure. Measles vaccine should be given about 3 months later when the passive measles antibodies should have disappeared (if the child is then about 15 months old).

Because a majority of the cases in this outbreak occurred among migrant workers and their dependents, control was difficult. Special clinics in the camps reached an estimated 52% of the targeted population, but transmission persisted. Higher immunization levels were probably needed to interrupt transmission. In future outbreaks in the migrant population, it may be necessary to consider door-to-door immunizations early in the outbreak.

Rapid case-reporting systems are necessary in areas with migrant populations because migrant workers and their dependents may reside only transiently in any one location. Prompt follow-up and control measures should be instituted within 48 hours of a case report. Following this outbreak, no transmission occurred to other migrant populations in the United States. CDC and the Office of Migrant Health, Bureau of Health Care Delivery and Assistance, Health Resources and Services Administration, recommend that all migrant workers and their dependents have documentation of immunity to measles.*** Standard immunization records of the type issued by state health departments should be filled out and given to patients at the time of vaccination to avoid repeated doses at subsequent locations.

References

1. CDC. Outbreak of measles following an imported case--Florida. MMWR 1982;31:657-9.

2. CDC. Imported measles with subsequent airborne transmission in a pediatrician's office--Michigan. MMWR 1983; 32:401-3.

3. CDC. Measles--Texas. MMWR 1981;30:209-11.

4. CDC. Measles in medical settings--United States. MMWR 1981;30:125-6.

5. Immunization Practices Advisory Committee. Measles prevention. MMWR 1982;31:217-24, 229-31.

6. Wilkins J, Wehrle PF. Additional evidence against measles vaccine administration to infants less than 12 months of age: altered immune response following active/passive immunization. J Pediatr 1979;94:865-9.

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