MEASLES-VACCINE IMMUNOGENICITY IN 6-MONTH-OLD VERSUS 15-MONTH-OLD INFANTS BORN TO MOTHERS IN THE MEASLES-VACCINE ERA

Hypothesis. The low titer of measles antibody in infants of mothers wi th vaccine-induced immunity may allow immunization against measles bef ore 15 months of age. Methods. Six- and 15-month-old infants born to m others less than or equal to 30 years of age with no history of measle s were recruited. Infants enrolled at 6 months of age were immunized w ith monovalent measles vaccine (Attenuvax), and maternal serum and inf ant pre- and postvaccination sera were obtained. Those enrolled for pr imary vaccination at 15 months of age received either Attenuvax (N = 1 2) or M-M-R(II) (N = 3). Six-month-old infants were revaccinated with M-M-R(II) at 15 months of age; pre- and postrevaccination sera were ag ain obtained. Three antibody assays were used: a measles neutralizing assay (NT) and two enzyme immunoassays (EIA) for measles IgG and measl es IgM. Results. Among primary vaccinees, 14 of 19 infants aged 6 mont hs (74%) developed NT antibody, as did 15 of 15 infants aged 15 months (100%). The reciprocal geometric mean titer of 6-month-old serorespon ders was 23.3, significantly lower than that of the 15-month-old prima ry vaccinees (87.7, P < .001). primary seroconversion rates by EIA wer e 53% for 6-month-old infants and 100% for those aged 15 months. Revac cination of infants who had received Attenuvax at 6 months of age resu lted in 100% NT positivity; the geometric mean titer rose to equal tha t of the group given primary immunization at 15 months of age. Measles IgM antibody was detected in 10 of 14 infants tested 1 month after pr imary vaccination at 15 months, but was not detected in any of the rev accinated infants after the second dose at 15 months of age (P < .001) . Conclusions. 1) Immunization with measles vaccine in infants born to vaccine-immune mothers at 6 months of age induced NT antibody in 74% of infants. 2) Revaccination of prior 6-month-old vaccinees at 15 mont hs resulted in antibody titers equivalent to 15-month-old vaccinees. 3 ) Lack of an IgM response following revaccination suggests that even s eronegative infants may be primed to respond on re-exposure to measles .