Two hundred fifty-four infants who had received measles vaccine at <10 months of age were revaccinated at ≥15 months of age, and their immune responses were compared with 129 control infants who received their first doses of measles vaccine at ≥15 months of age. Sera were collected at the time of revaccination (study infants) or primary vaccination (control infants), 3 weeks, and 8 months later and tested for antibody by hemagglutination inhibition (HI), enzymelinked immunosorbent assay (ELISA), and cytopathic effect neutralization (CPEN). Of the 121 study infants who were initially HI negative, 116 (95.9%) made HI antibody 3 weeks postrevaccination compared with 126 (99.2%) of 127 control infants (P = 0.19). Of the 63 study infants with no initial detectable antibody by any of the three tests, 14 (22.2%) had a measles-specific IgM response 3 weeks postrevaccination compared with 37 of 50 (74.0%) randomly chosen control infants. By 8 months after revaccination, the 121 initially HI-negative study infants were significantly less likely to have detectable HI antibodies than control infants (52.1% v 97.6%) (P < .001). However, 96.7% of these 121 study infants had detectable neutralizing antibody 8 months postrevaccination, an antibody thought to correlate best with protection. This study confirms the altered immune response to revaccination in infants first vaccinated prior to 10 months of age; however, the data suggest that most of these infants were successfully primed and are probably protected after revaccination.

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