PROJECT SUMMARY This supplement proposal outlines the scientific agenda for the COVID-19 Prevention Network (CoVPN) research to be carried out at the NYU VTEU. We will participate in the implementation of in DMID protocol 21- 0012, “A Phase 1/2 Study of Delayed Heterologous SARS-CoV-2 Vaccine Dosing (Boost) after Receipt of EUA Vaccines.” This Phase 1/2 study will evaluate the safety, tolerability, immunogenicity of different SARS-CoV-2 vaccine delayed boost at >12 weeks. This study will be composed of two different cohorts: 1. A cohort of persons previously vaccinated with an EUA vaccine who will be boosted with a homologous or heterologous vaccine strain on a homologous or a heterologous platform; and 2. A cohort of persons who are prospectively vaccinated with EUA standard dosing and who will be available for rapid assessment of a heterologous boost at some point in the future.