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Measles vaccine is less effective when given before 12 months of age due to interference with transplacentally acquired maternal antibody. Levels of maternal antibodies decay in the first year of life, but at what point they drop below protective levels is uncertain.
Researchers measured measles-neutralizing antibody levels in 196 serum samples collected during the first year of life from term infants admitted to a single hospital in Canada. Infants with immunologic abnormalities were excluded. Estimates of protection based on logistic regression modeling include the following:
As maternal age increased, infant susceptibility to measles increased across all infant age groups.
With each additional month of infant age, the predicted mean antibod…