Low Measles Seropositivity Rate among Thai Adolescents in the Thai National Immunization Program.

To achieve the goal of measles elimination, herd immunity with 95% seroprotection in the community is required. This study aimed to describe the measles seropositivity rate among Thai children and adolescents. A cross-sectional study was conducted among children aged 3-18 years in Bangkok and its suburbs. Measles IgG antibodies were measured using a EUROIMMUN enzyme-linked immunosorbent assay kit. Seropositivity is defined as a measles IgG titer of ≥200 IU/L, due to a correlation with a >85% positive rate with a plaque reduction neutralizing titer of >120. Factors associated with seropositivity were analyzed using logistic regression analysis. From May to July 2020, 570 children with a median (IQR) age of 11.7 (9.4-14.8) years were enrolled. The geometric mean titer (GMT) of anti-measles IgG was 281 IU/L (95% CI; 257-306). The proportion of children with seropositivity was inversely correlated with age; 3-5 years 85.3%, 6-9 years 72.5%, 10-14 years 50.7%, and 15-18 years 56.3%. Adolescents aged 10-18 years had a lower measles seropositivity rate compared with young children; aOR 0.29 (95% CI 0.17-0.48). Only half of the adolescents who received two doses of measles-containing vaccine maintained measles IgG above the seropositive level. A measles booster dose for young adults may be needed to achieve the measles elimination goal.