Decreasing Antibody Titers and the Slow Decay of Measles Immunity in Mexico’s Current Epidemiological Landscape

<b>Background:</b> Measles remains a global public health threat despite the availability of an effective vaccine and substantial progress toward elimination in many countries. Outbreaks in highly vaccinated settings suggest that waning vaccine-induced immunity, particularly among adults, may create silent susceptibility gaps capable of sustaining viral transmission. <b>Aim:</b> To evaluate age- and cohort-specific patterns of measles-specific antibody levels in Mexican adults and to examine evidence suggesting lower vaccine-induced antibody levels in younger vaccinated cohorts. <b>Methods:</b> A cross-sectional seroepidemiological study was conducted among 302 voluntary blood donors aged 18-70 years. Demographic, clinical, and vaccination data were collected through structured interviews. Serum anti-measles-virus (MV) IgG levels were quantified using a commercial ELISA. Antibody concentrations were analyzed according to age, sex, vaccination history, self-reported measles infection, and historical vaccination strategies. Multivariate linear models were applied to identify factors associated with IgG levels. <b>Results:</b> Anti-MV IgG seropositivity (>200 mIU/mL) was 67.2%, with a geometric mean concentration (GMC) of 270.43 mIU/mL. A positive correlation was observed between age and antibody levels (r_s = 0.161, <i>p</i> = 0.005). Individuals born before the introduction of the measles vaccine (pre-1970) had significantly higher GMCs (1096.63 mIU/mL) than younger cohorts. A history of natural infection tended to be associated with higher antibody levels (GMC: 428.38 vs. 257.24 mIU/mL; <i>p</i> = 0.051). In multivariate analysis, historical vaccination strategy emerged as the primary factor associated with antibody levels, whereas age alone was not significant. <b>Conclusions:</b> Cohort-specific differences in measles IgG levels suggest generational patterns of immunity and are consistent with diminished vaccine-induced antibody levels in younger adults in the absence of natural boosting. These findings highlight the importance of ongoing serological surveillance in post-elimination settings and underscore the need for targeted public health interventions.