Measles immunity in a large urban population in Northeast Brazil: a population-based assessment using PRNT and ELISA.

OBJECTIVES: Waning measles immunity in post-elimination settings may increase susceptibility and facilitate viral reintroduction. This study assessed measles immunity in an urban population in Northeast Brazil using neutralizing antibody testing and IgG serology. METHODS: A population-based seroprevalence study was conducted in 2024 among residents aged ≥2 years in 30 sentinel areas of Salvador, Brazil. Measles-specific IgG antibodies were measured by ELISA in all participants, and neutralizing antibodies were assessed by plaque reduction neutralization test (PRNT) in a stratified subsample (n=671). Seroprevalence was estimated by age group. Agreement between ELISA and PRNT was evaluated using Cohen's kappa, and factors associated with PRNT positivity were examined using logistic regression adjusted for age and sex. RESULTS: Among participants, 60.8% were female and 38.3% were aged ≥50 years. Overall seroprevalence was 91.4% by PRNT and 85.4% by ELISA, with 86.3% agreement between assays (kappa = 0.34, p < 0.001). The highest PRNT seropositivity was observed among individuals aged 10-19 years and ≥50 years, whereas the highest PRNT seronegativity occurred among young adults aged 20-29 years (23.4%). In adjusted analyses, children and adolescents aged 5-19 years had lower odds of PRNT positivity compared with those aged 2-4 years. No significant differences were observed by sex. CONCLUSIONS: Despite high overall seroprevalence, relevant immunity gaps were identified, particularly among young adults, indicating increased susceptibility to measles in this urban population. The combined use of PRNT and ELISA highlights the need for targeted vaccination strategies in post-elimination urban settings.