Persistence of measles antibodies after 2 doses of measles vaccine in a postelimination environment

Objective: To evaluate the persistence of measles antibodies after 2 doses of measles vaccine in a setting where exposure to wild- type measles was unlikely. Measles was declared eliminated from the United States in 2000, an achievement attributed to effective implementation of a routine 2- dose vaccination policy. Some have questioned whether measles transmission could resume if immunity wanes in the absence of boosting from wild- type measles. Design: Prospective, observational, volunteer cohort study. Setting: Rural Wisconsin health maintenance organization. Participants: Children who received the second measles vaccine dose at kindergarten ( aged 4- 6 years) or middle school ( aged 10- 12 years) in 1994 or 1995. Serum samples were collected periodically during a 10- year period for the kindergarten group and a 5- year period for the middle school group. Intervention: Second dose of measles vaccine. Main Outcome Measure: Measles antibody levels were assessed by plaque- reduction neutralization: titers less than 8 mIU/ mL were considered seronegative and suggestive of susceptibility to measles, and titers of 120 mIU/ mL or less were considered low and suggestive of potential susceptibility. Results: During the study period, no measles was reported in the study area. Voluntary attrition reduced the study population from 621 at enrollment to 364 ( 58.6%) by study end. Before the second dose, 3.1% ( 19/ 621) had low titers, of whom 74% ( 14/ 19) were antibodynegative, with geometric mean titers being significantly higher in kindergarteners ( 1559 mIU/ mL) than in middle schoolers ( 757 mIU/ mL) and rates of negativity significantly lower ( 1.0% [ 3/ 312] vs 3.6% [ 11/ 309]). One month after the second dose, 0.2% ( 1/ 612) had low titers and none was seronegative, with geometric mean titers being significantly higher in kindergarteners ( 2814 mIU/ mL) than in middle schoolers ( 1672 mIU/ mL). By study end, 4.9% ( 18/ 364) had low titers and none was seronegative, with no significant difference in geometric mean titers between kindergarteners ( 641 mIU/ mL) and middle schoolers ( 737 mIU/ mL) when both groups were aged 15 years. Projections suggest that the proportion of persons with low antibody levels may increase over time. Conclusions: Measles antibody persisted in all vaccinees available for follow- up 10 years after a second dose of vaccine, with no seronegative results detected. Declining titers suggest the need for vigilance in ensuring disease protection for the vaccinated population.