MEASLES VACCINATION OF INFANTS IN A WELL-VACCINATED POPULATION

During outbreaks of measles, measles vaccine is recommended for infants considered to be at risk who are 6 months of age and older, In a prospective trial the serologic response to early measles immunization has been evaluated in 125 infants given monovalent measles vaccine at 6 to 8.5 months of age and measles-mumps rubella at 15 months, The response to vaccination was measured by plaque reduction neutralization (PRN) assay and enzyme immunoassay, Infants were grouped by the mother's immunization history: natural immunity (n = 60, Group 1); killed followed by live, further attenuated vaccine (n = 22, Group 2); and live, further attenuated vaccine only (n = 43, Group 3). The prevaccination geometric mean titer (GR IT) by PRN for Group 1 (GMT = 69) was significantly higher than that of Group 2 (GMT = 18) or 3 (GMT = 13), Seroconversion (4-fold increase in PRN titer) rates after monovalent vaccine were 31, 71 and 76% for Groups 1, 2 and 3, respectively, Seroconversion percentages were higher when measured 6 to 8 weeks after vaccination compared with 4 to 5 weeks, After measles-mumps-rubella greater than or equal to 97% of all infants had PRN titers >120 and were measles IgG-positive by enzyme immunoassay, These data show that as demographics shift to a well-vaccinated maternal population and susceptibility in younger infants, measles vaccination before the currently recommended age will be effective.