Immunosenescence and CMV-Associated Immune Signatures on SARS-CoV-2 Booster Responses

Abstract Ageing remodels antiviral immunity, yet its influence on responses to repeated mRNA vaccination is not fully defined. We evaluated humoral and SARS-CoV-2 spike–specific T-cell responses in 41 adults—stratified by age (50 vs. ≥60 years), sex, prior SARS-CoV-2 infection and cytomegalovirus (CMV) serostatus—before and after a fourth dose of the bivalent BNT162b2 vaccine. Anti-RBD IgG titres increased in nearly all participants, with no measurable impact of age, sex, infection history or CMV status, and baseline titres predicted post-booster antibody levels. In contrast, cellular immunity showed clear heterogeneity across ageing-related variables. Although the booster enhanced IFN-γ production and reduced TNF-α-associated inflammatory activity at the cohort level, older adults and males exhibited significantly lower post-boost frequencies of IFN-γ–producing CD4+ T cells. Prior SARS-CoV-2 infection was associated with attenuated CD4+ recall responses, whereas infection-naïve and female participants showed the strongest functional gains. Immunosenescence markers were associated with reduced cellular responsiveness. CMV-related immune remodelling—including higher anti-CMV IgG levels and expansions of differentiated CD8+ subsets—correlated with diminished IFN-γ responses in CD4+ and CD8+ T cells after boosting, suggesting that chronic CMV imprinting constrains heterologous antiviral immunity even in mid-adult life. Humoral and cellular changes were largely uncoupled, supporting the need to evaluate both arms of adaptive immunity. These findings indicate that while a fourth bivalent BNT162b2 dose reliably reinforces humoral immunity across ages, the magnitude and quality of cellular responses are shaped by age, sex, infection history and CMV-associated immunosenescence. Incorporating immune-ageing markers into vaccination strategies may improve booster efficacy in older populations.