Cytomegalovirus (CMV) is associated with immunosenescence and reduced vaccine responses in the elderly (>70 years). However, the impact of CMV in young adults is less clear. In this study, healthy UK and Senegalese adults aged 18-50 years (average 29 years) were vaccinated with the Ebola vaccine candidate chimpanzee adenovirus type 3 vectored Ebola Zaire vaccine (ChAd3-EBO-Z) and boosted with modified vaccinia Ankara Ebola Zaire vectored (MVA-EBO-Z) vaccine. CMV carriage was associated with an expansion of phenotypically senescent CD4+ and CD8+ T cells expressing CD57 and killer cell lectin-like receptor G1 (KLRG1), which was negatively associated with vaccine responses in both cohorts (P