PROJECT SUMMARY Vaccines save lives, and the rapid development of novel vaccines against severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) was a triumph for the medical community. While the rapid deployment of these vaccines has undoubtedly attenuated the severe morbidity and mortality induced by the virus, their efficacy is decreased against variant strains, in children, and with increasing time post-administration. These clinical findings highlight our lack of understanding of how durable, protective immunity is induced by vaccination and how this varies across the population. Studying vaccine induced memory in humans has two major challenges. First, viral exposures over life confound the identification of vaccine-specific memory; second, the stores of memory lymphocytes reside in the tissues, which makes monitoring the vaccine response in healthy individuals challenging. The SARS-CoV-2 pandemic affords us the unique opportunity to study the response to a novel vaccine formulation without confounding natural antigenic exposure and the ability to distinguish infection from vaccination by serology. Additionally, our unique organ donor tissue resource provides a validated model to investigate tissue-localized vaccine-specific immunity. The goal of this proposal is to understand how vaccine- induced immune memory is distributed across tissue and affected by host factors such as age. The central hypothesis of this proposal is that induction and maintenance of vaccine-specific memory is controlled in lymph nodes, and specific early induction events directly impact immune memory development and vary with age. I will address this hypothesis and meet the goals of the study by using flow cytometry and high- dimensional sequencing to evaluate the relationship between circulating and tissue-localized vaccine memory and how they differ in phenotype, function, and across age. I will also investigate how the initial, inflammatory host-specific response to the mRNA-1273 vaccine differs across age and correlates to the quantity of immune memory induced. The results of this study will elucidate the importance of lymph nodes in the vaccine response and highlight the benefits and downfalls of mRNA vaccines across various host factors. These results will have implications for future vaccine design and may play a role in managing both the SARS-CoV-2 pandemic and any future ones.