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

Objective: To assess geographical variation in maternal measles antibody levels from birth to nine months of age, to inform recommendations for the timing of 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=2,845) 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 0.32 IU/mL (Guatemala) to 1.60 IU/mL (Pakistan). There was no geographical variation in 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.