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.