Research Project 3 Summary There are insufficient data regarding the long-term humoral immune responses induced after SARS-CoV-2 infection. Our preliminary data indicate that there is variation in the magnitude and duration of antibody responses following SARS-CoV-2 infection. While IgG and IgA antibodies against spike (S) and the receptor binding domain of S (S-RBD) appear to remain constant over time, neutralizing antibody (nAb) titers wane and are not detected in up to 25% of infected individuals who have detectable anti-S and anti-S-RBD antibodies. We have also observed that during the convalescent phase of SARS-CoV-2 infection, individuals with more severe COVID-19 (i.e., hospitalized, older, and male patients) have significantly greater serological responses to SARS-CoV-2. The antibody responses mediating protection from re-infection are not defined, and neither are responses that may mediate greater pathology. From studies of other viruses, it is clear that a variety of antibody functions contribute to protection from re-infection and modulate disease severity. Both nAbs and non-nAbs can mediate a number of different activities, which include complement activation and antibody- dependent cellular cytotoxicity (ADCC), which may contribute to pathogenesis as well as protections from SARS-CoV-2. The overarching goal of JH-EPICS Research Project 3 is to analyze the magnitude and duration of the total as well as functional antibody responses after SARS-CoV-2 infection. We have developed a core set of serological assays to be applied to a prospective, demographically diverse cohort of hospitalized patients presenting with mild, moderate, and severe COVID-19 disease. Plasma samples have and will continue to be collected at multiple timepoints from enrollment through one year post-enrollment. Aim 1 will systematically evaluate antibody isotype switching and the subclasses and quality of the immunoglobulins (IgG, IgM, and IgA [monomeric and dimeric]) that recognize the SARS-CoV-2 S and S-RBD. Aim 2 will characterize the kinetics and duration of the neutralizing antibody response against SARS-CoV-2 and the ability of viruses to escape from nAbs. Finally, Aim 3 will analyze the function of non-neutralizing SARS-CoV-2-specific serological response by assessing ADCC, complement-mediated cytotoxicity, and complement fixation activity toward SARS-CoV-2 virus particles and virus-infected cells. Using linear regression analyses and modeling of these data in the context of clinical and demographic information, we are uniquely positioned to determine the modifiers that drive a protective antibody response following SARS-CoV-2 infection or, eventually, vaccination.