The waning of maternal measles antibodies: A multi-country maternal-infant seroprevalence study.

OBJECTIVES: To assess geographical variation in maternal measles antibody levels from birth to nine months of age, to inform recommendations for the timing of the first measles vaccine dose. METHODS: Stored infant serum samples from 11 countries taken at delivery and/or follow-up time points prior to measles vaccination (N=2845) were tested for measles plaque reduction neutralisation (PRNT) and measles, mumps, and rubella immunoglobulin G at a central laboratory. Antibody decay in infants was modelled using linear mixed effects models with participant-level random intercepts and random slopes. Proportions of infants with antibody concentrations above the clinical protection threshold (0.12 IU/mL) were estimated at each age. RESULTS: At birth, most (94%, 519/552) infants had PRNT ≥0.12 IU/mL, but geometric mean concentrations ranged from 0.32 IU/mL (Guatemala) to 1.60 IU/mL (Pakistan). There was no geographical variation in the decay rate of PRNT nor immunoglobulin G. Geometric mean PRNT fell below 0.12 IU/mL between ages 2.5 months (Guatemala) and 6.2 months (Pakistan). At age 6 months, <50% of infants had PRNT ≥0.12 IU/mL in all countries except Pakistan. CONCLUSIONS: Reliance on maternal antibodies for protection until age 9 months or later leaves most infants with insufficient direct protection against measles infection between ages 6-9 months.