Project Summary The COVID-19 pandemic is currently gripping the world. Aside from the health consequences, the necessary decrease in human activity has resulted in economic losses without modern precedent, especially in the developing world where health care and sanitation were not sufficient even prior to the pandemic. Little is known about the durability of the human immune responses to SARS-CoV-2. A better understanding of the evolution and persistence of anti SARS-CoV-2 immunity in individuals who recover for the infection or who are vaccinated are critically needed as they will guide future vaccine efforts. The Nussenzweig/Caskey laboratory studied the initial humoral and cellular immune responses of a cohort of COVID-19 convalescent individuals. These samples were characterized by a series of assays that measure: 1. Serum levels of binding antibodies to the SARS-CoV-2 spike protein (S) and the receptor binding domain (RBD) and 2. Neutralizing activity against HIV-1 and VSV SARS-CoV-2 pseudotyped viruses and authentic SARS-CoV-2. In addition, we cloned and characterized the antibodies produced by these individuals and developed an understanding of the neutralizing epitopes on the RBD. The results showed that the initial humoral responses to SARS-CoV-2 are highly variable but that nearly all individuals develop some level of neutralizing activity. Neutralizing activity in serum decreases after by a factor of 5 after 6.2 months. In contrast memory B cell responses continue to evolve and remain largely intact. Interestingly, there was a convergence of antibody responses to SARS-CoV-2 that could be explained by structural analysis. In addition to natural infection, a cohort of volunteers that received the Moderna and Pfizer vaccines was recruited. The B lymphocytes and the antibodies they produce will be analyzed longitudinally. The hypothesis to be tested is that humoral immunity to SARS-CoV-2 infection or mRNA vaccination will decrease in parallel, but that B cell memory responses will diverge. The overall goal of this proposal is to determine the longevity and nature of the anti-SARS-CoV-2 cellular and humoral immune responses by re- examining the same cohort of individuals who either recovered from COVID-19 or received an mRNA vaccine. The results will inform our understanding of the evolution and persistence of anti- SARS-CoV-2 immunity in recovered and vaccinated individuals and will inform ongoing and future vaccine efforts.