Background: A diverse repertoire of naive T cells is thought to be essential for a robust response to new infections. However, a key aspect of aging of the T cell compartment is a decline in numbers and diversity of peripheral naive T cells. We have hypothesized that the age-related decline in naive T cells forces the immune system to respond to new infections using cross-reactive memory T cells generated to previous infections that dominate the aged peripheral T cell repertoire.Results: Here we confirm that the CD8 T cell response of aged, influenza-naive mice to primary infection with influenza virus is dominated by T cells that derive from the memory T cell pool. These cells exhibit the phenotypic characteristics of virtual memory cells rather than true memory cells. Furthermore, we find that the repertoire of responding CD8 T cells is constrained compared with that of young mice, and differs significantly between individual aged mice. After infection, these virtual memory CD8 T cells effectively develop into granzyme-producing effector cells, and clear virus with kinetics comparable to naive CD8 T cells from young mice.Conclusions: The response of aged, influenza-naive mice to a new influenza infection is mediated largely by memory CD8 T cells. However, unexpectedly, they have the phenotype of VM cells. In response to de novo influenza virus infection, the VM cells develop into granzyme-producing effector cells and clear virus with comparable kinetics to young CD8 T cells.