Waning Measles Immunity and Sustained Rubella Seroprotection Among Adults in Istanbul, Türkiye.

Background and Objectives: Despite long-standing MMR vaccination programmes, outbreaks of vaccine-preventable diseases continue to occur worldwide, raising concerns regarding waning immunity and susceptibility gaps in adult populations. Between 2017 and 2019 alone, WHO reported over 880,000 measles cases globally-a fourfold increase over the preceding two years-with outbreaks in both low- and high-income countries despite high childhood vaccination coverage. This study evaluated the seroepidemiology of measles, mumps, and rubella among adults in Türkiye over a nine-year period. Materials and Methods: A retrospective laboratory-based study was conducted using ELISA IgM and IgG results obtained between 2014 and 2022 at a tertiary-care university hospital. Seropositivity rates were analysed by year, age group, and sex. Temporal trends were assessed using the Cochran-Armitage trend test; factors associated with IgG seropositivity were assessed using multivariable logistic regression. Results: Among 10,407 adult test records, overall IgG seropositivity was 75.8% for measles, 94.9% for rubella, and 86.3% for mumps; IgM positivity remained low throughout (0.9%, 0.4%, and 1.9%, respectively). Measles IgG declined significantly from 78.4% in 2014-2019 to 62.6% in 2022 (p < 0.001), whereas Rubella IgG remained stable (93.2-95.7%) and Mumps IgG showed no significant temporal change. Seropositivity increased markedly with age for measles (18-29 years: 70.2% vs. ≥50 years: 93.4%; p < 0.001) and mumps (p = 0.030). Multivariable analysis confirmed that Measles IgG seropositivity was substantially lower in 2022 than in the pooled 2014-2019 period (OR = 0.432, 95% CI: 0.319-0.584, p < 0.001). Rubella seroprotection among women of reproductive age was 94.7% (95% CI: 93.9-95.5%), close to but not statistically distinguishable from the 95% population immunity benchmark used in rubella elimination frameworks. Conclusions: A significant measles immunity gap exists among younger adults in Türkiye, with Measles IgG seropositivity declining from 78.4% in 2014-2019 to 62.6% in 2022, consistent with a cohort effect whereby vaccine-era cohorts exhibit lower protection than older adults with naturally acquired immunity. Rubella immunity, by contrast, remained close to the 95% elimination benchmark throughout. These findings underscore the importance of targeted serological surveillance and evidence-based booster vaccination strategies.