Additional correlates of vaccine responsiveness remain to be elucidated: Salinas et al. demonstrated patients with CVID have a relative paucity of receptor-binding domain-specific, CD19+ CD24+ CD27+ B cells compared to healthy controls [18] and Hagin et al. were unable to demonstrate a common T-cell immunophenotype in vaccine non-responders beyond an inverted CD4/CD8 ratio [17]. Here, CD4 is linked to common variable immunodeficiency.