Reduced measles immunity in infants in a well-vaccinated population.

The recommended age for measles vaccination is based in part on information gathered when most mothers had natural measles. Nowadays many mothers have received measles vaccine. To assess this change measles antibody neutralization titers (NT) were determined for 278 mother-infant pairs. One hundred sixty-four mothers, born before 1958, likely had had natural measles (Group 1). Sixty mothers received one to three killed plus one attenuated measles vaccination (Group 2) and 54 received 1 attenuated measles vaccination only (Group 3). NT were determined for the mother and for the infant at birth and in the infant during the fourth and sixth months. Group 1 mothers and infants at every age had higher geometric mean NT than those in Groups 2 or 3 (P less than 0.05). By 7 months 65% of Group 1 infants and greater than 90% of Group 2 and 3 infants had an NT less than 1:10. The rate of antibody decay was significantly faster for Group 1 infants (P less than 0.05). Earlier vaccination in the infant should be considered.