 Innovation: The study will help in the validation of clinical tools and biomarkers as prognostic indicators of influenza illness severity in vaccinated older adults, point-of-care diagnostics that would direct other preventive strategies to reduce the impact of influenza illness in vaccinated older adults, and correlates of protection to evaluate the potential of new influenza vaccines to enhance protection against the serious complications of influenza illness. The investigators have established that GrzB activity and the IFNg: IL-10 ratio in influenza-stimulated PBMC correlate with protection against influenza, and preliminary data to show that low GrzB activity in influenza-stimulated PBMC correlates with more severe disease and higher levels of frailty. The innovations of this project are the established methods for developing correlates of protection, the clinical insights into how frailty affects immune-mediated protection against influenza, and the direct translation of this research to provide a reasonable method for primary care clinicians to estimate vaccine effectiveness in an individual older person. These results will translate to the practical design of clinical trials to evaluate the potential for new influenza vaccines to better protect against the serious complications of influenza and complement those based on antibody titers andor clinical outcomes alone. 