ABSTRACT
Despite a high coverage with measles vaccine, outbreaks of measles occur with an increasing proportion in older children and adults who have been vaccinated. The aim of this study was to determine the anti- measles seroepidemiology in adolescents and young adults, and the responses of subjects to revaccination. Five hundred and ninty adolescents and young adults 15 to 25 years old previously received one or two doses of measles vaccine [15-19 years old subjects; 2 doses at 9 and 15 months of age and 20-25 years old subjects; one dose after 12 months of age] with no history of physician diagnosed measles were redcruited to determine the seroprevalence of IgG measles antibody. Two to six weeks after revaccination, anti measles IgM and IgG antibodies titers were assessed by ELISA methods, to differentiate between the primary and secondary vaccine failure. The results were compared and analyzed. From 590 subjects, 263 were serologically negative or susceptible. Of 105 susceptible revaccinated subjects 75 showed anamnestic IgG and 11 of them also showed IgM responses [probably primary vaccine failure]. No significant responses were observed in 39 seroprotected subjects to revaccination except one subject. The results showed that 44.6% of vaccinated adolescents and young adults were serologically susceptible to measles. The rates of primary and secondary vaccine failure, herd immunity with 95% coverage were 4.7%, 27.1% and 83%, respectively. The study showed that vaccination- induced anti-measles antibody decline overtime especially in the absence of natural boosting. Catch-up vaccination or third dose of measles vaccination is recommended