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During a measles immunization campaign 203 children were enrolled in an antibody response study. Of this group, follow-up clinical data and sera samples were available from 125 children three weeks after immunization and from 90 children ten months later. Seventy-six of the children had been previously vaccinated, ten had a history of measles and 39 denied vaccination or illness. Twenty-six of the children had prevaccination hemagglutination inhibiting antibody titers of less than 5. Of this group 12 had a primary immune response (IgM measles antibody) with geometric mean titers (GMT) of 90 and 40 three weeks and ten months respectively after vaccination. In contrast, the other 14 children with initial titers of less than 5 had secondary immune responses (only IgG measles antibody) with GMTs of 28 and 9 three weeks and ten months after vaccination. Since the antibody responses in these children who had previously been stimulated by measles antigen were modest and transient, it is suggested that booster immunization may not be effective in preventing future secondary vaccine failures. Also noted in this study was a poor correlation between historical data and actual measles antibody.