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Measles immunization of children at 1 year of age with a single dose of the current vaccine has successfully reduced measles incidence in Argentina. However, the optimal schedule of measles vaccination of young infants would balance the risk of early loss of maternal antibody in the majority of infants with the risk of primary vaccine failure because of passive measles immunity. This study is the first to document a significant association between loss of passive measles antibody and age among infants born in 1995 and 1996 in Córdoba City, Argentina.This is a seroprevalence study of 340 infants to investigate the duration of transplacentally derived measles antibody, assayed by a neutralization test, during the first 8 months of age in Córdoba City, Argentina.The proportion with detectable neutralizing measles antibodies decreased from 85% at 1 month of age to 8% at 8 months of age. The simple logistic model with age (in weeks) as the only variable showed that the decline in the proportion of infants with a positive antibody titer was sharpest during the second and fifth months of age (6.6 and 6.8% per week during a 4-week period, respectively).These findings suggest that 80% of infants are susceptible to measles infection for at least 3 months before routine immunization at 12 months of age.