PROJECT SUMMARY/ ABSTRACT At this point in the COVID-19 pandemic, with vaccine roll-out ongoing, one of the most urgent questions facing people living with HIV (PLWH) and their providers is whether HIV modulates the immune response to and subsequent effectiveness of the SARS-CoV-2 vaccines. Unfortunately, phase 3 trials for all three of the U.S.-authorized vaccines did not report HIV specific data and/or did not include enough PLWH to examine the impact of HIV infection on vaccine efficacy. PLWH might plausibly experience a less durable SARS-CoV-2 specific neutralizing antibody (NAb) response to a SARS-CoV-2 vaccine, as has been seen in response to vaccines for other infections. This lack of durable NAb responses may be mediated by inflammatory state of HIV infection that persists despite adequate suppressive antiretroviral therapy (ART), T cell exhaustion, and/or lower CD4/CD8 ratios. Our preliminary work in the UCSF Long-term Impact of Infection with Novel Coronavirus (LIINC) COVID-19 recovery study has demonstrated waning antibody responses but stable CD4+ and CD8+ T cell responses among HIV-negative individuals recovering from natural SARS-CoV-2 infection. However, surrogate virus neutralization titers and IgG concentrations were lower among PLWH compared to adults without HIV following mRNA vaccination, raising concerns that PLWH might have a diminished humoral response to vaccination. Whether PLWH mount less durable humoral and cell-mediated immune responses to COVID-19 vaccines than those without HIV is largely unknown, including the mechanisms of these differences, although this information could inform clinical strategies, including additional boosters or safety measures after vaccination. This proposal will answer two vital questions about the response to vaccination among PLWH. Aim 1 will provide novel, urgently needed insights into how the SARS-CoV-2 neutralizing antibody response to a cultured B.1.617.2 (delta) variant, IgG concentration, and antibody magnitude and durability could differ by HIV status over time following mRNA-based SARS-CoV-2 vaccination, including following boosters. Aim 2 will examine T cell memory responses and germinal center development generated by mRNA-based SARS-CoV-2 vaccination among PLWH compared to those without HIV out to a year following vaccination. Harnessing, the research infrastructure of the UCSF CFAR, the LIINC study, and a large, aging population of PLWH served by the Ward 86 clinic, this analysis will leverage an ongoing cohort to address whether PLWH mount attenuated immune responses to COVID-19 vaccines. Such data will inform clinical and public health responses, including the need for additional vaccine doses or safety strategies for PLWH during COVID-19.