CD28 and influenza: T cell memory was described with naïve (CD45RA + CCR7+), central memory (CD45RA− CCR7+), effector memory (CD45RA− CCR7−), and terminally-differentiated effector memory TEMRA (CD45RA + CCR7−) populations,42 and the lack of CD28 co-receptor was used as a marker of T cell aging.78 Based on prior evidence of impaired influenza vaccine responses in older adults,44,45 CD8 + naive cells and CD8 + CD28− TEMRA cells were selected for association analyses with age and frailty.