Effect of differing immunization policies on circulating measles antibody levels in US and Canadian children

Objective: To examine the effect of differing age-at-immunization policies on measles antibody levels in US children immunized at the age of 15 months compared with Canadian children immunized at the age of 12 months. Subjects and Methods: Healthy, volunteer US and Canadian children aged 6 to 11 years who had been immunized with a single dose of the measles vaccine were enrolled from 1991 to 1993. Measles antibody was measured with a whole-virus, measles-specific IgG enzyme-linked immunoassay. Age, race, sex, country of residence, general health status, age at time of measles immunization, and time from immunization to sampling were recorded. Results: Of the 1052 children enrolled in the study, US children (n=719) had a significantly higher measles seroprevalence (87%) compared with Canadian children (n=333) (76%; P<.001). After adjustment for time from immunization and age at immunization, the differences in seropositive rates were no longer significant (odds ratio [OR], 1.53; 95% confidence interval, 0.87-2.69; P=.15). We found a significant dose-response relationship between age at the time of immunization and the odds of being seropositive after immunization, with ORs varying from 1 with immunization at 13 months or younger to 2.30 with immunization at 16 months of age (P=.01). Conclusions: The current US policy of immunizing with the first dose of measles at the age of 12 months may be less effective than a policy of immunizing at 12 to 15 months of age. These findings may be highly significant as we move toward an era in which measles exposure may be rare and policies are developed to eradicate measles.