Influenza remains a worldwide public health threat in the 21st Century. While the incidence of influenza decreased markedly during the COVID-19 pandemic, influenza is certain to re-emerge and contribute to an increased burden of respiratory viral disease together with SARS-CoV-2. The impact of infliuenza is magnified by a vaccine that affords incomplete protection, and by the episodic appearance of new pandemic strains. Influenza has a disproportionate impact in older adults, with 90% of the deaths in the United States attributed to influenza occurring in individuals over the age of 65; nursing home residents with a high prevalence of frailty are particularly vulnerable. Two vaccines are currently approved for use in adults over age 65—a high-dose (HD) unadjuvanted vaccine and a MF59-adjuvanted standard-dose (SD) vaccine. Here, we will carry out the first randomized study to compare immunologic, transcriptomic and proteomic signatures of response to these two vaccines in older adult residents of nursing homes or patients in a homebound primary care program, compared to young adults receiving the same vaccines. We are continuing our longstanding work with the Yale Section of Geriatrics, collaborating with a group of geriatricians that serve as medical directors for these faciilties and programs who are responsible for annual influenza vaccination. We will leverage our expertise in carrying out studies in human immunology, particularly those that have elucidated innate immune and gene expression signatures of vaccine response and how aging affects these signatures; for example, we carried out the largest study of gene expression signatures of response to SD vaccine in young and older adults enrolled over five consecutive vaccine seasons. Samples obtained prior to and following vaccination will undergo detailed analyses by CyTOF (Core C) for innate and adaptive immune cell composition, activation status, and intracellular cytokine production. We will also carry out unbiased studies of innate immune PRR function in PBMC populations and platelets. The breadth of antibody responses to the influenza hemagglutinin protein will be assessed using microneutralization studies with heterologous viral strains and cryo-electron microscopy-based epitope mapping. These findings will be integrated with an established CITE-seq with cell hashing platform (Core C) for single-cell transcriptomic analyses that will also be combined with B cell receptor sequencing to compare responses in young vs. nursing home older adults, and with proteomic analyses on serum samples done in collaboration with the Boston Children’s Hospital HIPC center. Finally, in a subset of participants, we will obtain skin biopsies of the vaccination site to visualize early cellular infiltrates and regional lymph node biopsies following vaccination to gain insights into immune responses in tissues and secondary lymphoid organs using an innovative spatial transcriptomics program, DBiT-Seq. Understan...