Within a 3-month countywide epidemic of measles in Jacksonville, Florida, 28 cases occurring among a kindergarten enrollment of 145 were carefully studied since 25 of these children had been previously immunized with a live, attenuated measles virus vaccine and immune globulin. Nineteen children had been vaccinated prior to their first birthday. Six children were vaccinated at 13 to 20 months of age. The median measles (rubeola) hemagglutination-inhibition (HI) antibody titer in sera of five convalescent patients immunized before their first birthday was 1:320 and for complement fixation it was 1:128. Sera from nine classmates who did not contract the disease showed a median HI antibody of 1:40 and a median complement fixing antibody (CF) of 1:16. Six had been vaccinated before their first birthday.

In a control group of five other children, three had been immunized prior to 12 months of age, one at 13 months, and one at 18 months. Sera from the three earliest contained no antibody; however, sera from the other two had detectable antibody.

The analysis of serologic data supported the contention that the outbreak was causally related to defective protection associated with the use of vaccine plus globulin in infants. It also demonstrated persistence of CF antibody many years after immunization and suggested the presence of a booster phenomenon.

A review of the clinical illness of the 25 children who had been given the vaccine and 22 who had not revealed little difference in the severity of the disease.

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