LOSS OF MATERNALLY DERIVED MEASLES IMMUNITY IN ARGENTINEAN INFANTS

Background. Measles immunization of children at 1 year of age with a s ingle dose of the current vaccine has successfully reduced measles inc idence in Argentina. However, the optimal schedule of measles vaccinat ion of young infants would balance the risk of early loss of maternal antibody in the majority of infants with the risk of primary vaccine f ailure because of passive measles immunity. This study is the first to document a significant association between loss of passive measles an tibody and age among infants born in 1995 and 1996 in Cordoba City, Ar gentina. Methods. This is a seroprevalence study of 340 infants to inv estigate the duration of transplacentally derived measles antibody, as sayed by a neutralization test, during the first 8 months of age in Co rdoba City, Argentina. Results. The proportion with detectable neutral izing 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 wi th 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, respectiv ely). Conclusions. These findings suggest that 80% of infants are susc eptible to measles infection for at least 3 months before routine immu nization at 12 months of age.