Immunity gap for measles in young adults: Seroprevalence study in blood donors in southern Italy

Following the COVID-19 pandemic, measles has seen a resurgence across Europe, including Italy. Increased vulnerability among young adults has raised concerns about waning vaccine-derived immunity. This study evaluated measles immunity among adult blood donors in the Apulia region to identify sero-susceptibility patterns and inform targeted public health strategies. A retrospective seroprevalence study was conducted using sera from 1579 healthy blood donors, aged 18 to 65 y, who attended donation centers in Apulia region between November 2023 and February 2024. Anti-measles virus (MV) IgG antibodies were measured using the ELISA test. Statistical analyses to identify associations between sero-susceptibility and demographic factors were performed. Overall seroprevalence of anti-MV IgG was 85.3% (95% CI: 83.4–87.0). The prevalence of sero-susceptibility varied significantly by age (p <.0001), ranging from 39.8% among individuals aged 18–24 y to 4.0% among the 45–54 age group. No differences were observed based on sex or province of residence. Among sero-susceptible individuals with known vaccination status, 77.2% were under 35 y of age, and 69.6% had received two doses of the measles vaccine. Geometric Mean Concentration levels increased with age and were significantly higher in unvaccinated individuals, which suggests that natural infection provides more lasting immunity. However, measles immunity among adults in Apulia remains below the elimination threshold, with a substantial immunity gap concentrated among young adults, despite high vaccination coverage. These results suggest a decline in vaccine-induced immunity and underscore the importance of targeted surveillance and preventive strategies, such as evaluating booster vaccination policies, to prevent future epidemics.