Assessment of Measles Immunity in the Croatian Population: A Retrospective Seroprevalence Study, 2015–2025

Background/Objectives: Measles is a highly contagious infectious disease. Maintaining a high level of measles immunity in the population is essential due to the extremely high transmissibility of the measles virus (MV). We analyzed the MV seroprevalence in the Croatian population. Methods: A total of 1998 individuals tested consecutively for MV antibodies from 2015 to 2025 inclusive were included. MV IgG antibodies were detected using a commercial ELISA. Results: The overall seroprevalence rate was 75.1%, with significant yearly differences, ranging from 50.0 to 86.5%, and a declining trend since 2023. No differences were observed between sexes (males 71.6%, females 71.9%). The seroprevalence increased with age, from 60.9% in the 1–10 age group to 91.1% in the 61+ age group. Significant geographic differences were found, with higher seropositivity rates in coastal regions compared to continental regions (81.3 and 71.0%, respectively). Settlement type (urban, suburban/rural) was not associated with the MV seroprevalence. The results of the multivariable logistic regression analysis showed that year, age, and region were significantly associated with IgG seropositivity. Each additional calendar year was associated with lower odds of IgG positivity (OR = 0.82), while each additional year of age was associated with higher odds (OR = 1.04). Region was also a significant predictor (OR = 1.65), while settlement was not significantly associated with seropositivity after adjustment for other variables. Conclusions: Croatia has historically maintained high measles coverage, but lower uptake in some regions and age groups may be creating immunity gaps. The lower post-2022 seropositivity underscores significant immunity gaps, particularly among highly susceptible groups that were exposed to outbreaks and tested during that period.