Abstract: With the global COVID-19 pandemic, we recognize a significant need for vaccines that modify COVID-19 in SARS-CoV-2 infected individuals. Addressing this gap, the National Institutes of Health (NIH) led rapid constitution of the CoVPN, partnering 5 NIH supported clinical trial networks, to create an enhanced network of physician-scientists at 145 United States (US) and 71 international clinical trial sites in 17 countries dedicated to developing globally effective vaccines for SARS-CoV-2. Due to its extensive experience implementing global HIV vaccine trials over the last 20 years, the HIV Vaccine Trials Network (HVTN) LOC was selected as the LOC for CoVPN vaccine trials. This Phase 3, observer-blinded, placebo-controlled, crossover-vaccination study will test the efficacy of the COVID-19 ancestral strain mRNA vaccine in preventing COVID-19 disease regardless of severity, and preventing COVID-19 severe disease in people who are at risk for severe COVID-19 in areas of the world where the prevalence of the SARS-CoV-2 1.351 strain is substantial. The trial has been designed to provide as rapid assessment of efficacy as possible in regions of the world where local access to effective vaccines are limited. Subjects will be randomized in a 1:1 ratio to receive immediate or deferred vaccine. The blinded cross-over is expected to take place when vaccine benefit is declared based on at least 50 primary COVID-19 endpoints, anticipated to be accrued approximately 3 months post-trial start. All trial endpoint assays will be done using qualified and validated assays for diagnosis and immune monitoring. Specific aims of this study are to demonstrate efficacy of COVID-19 mRNA (Moderna mRNA-1273) vaccine to prevent virologically-confirmed symptomatic COVID-19 starting 14 days after dose 2 in adults who are at risk of severe COVID-19; to assess vaccine efficacy of COVID-19 mRNA vaccine to prevent severe COVID-19 starting 14 days after dose 2 in adults who are at risk of severe COVID-19; and to assess safety and tolerability of COVID-19 mRNA vaccine in adults who are at risk of severe COVID-19. This efficacy trial will tell us much about ability of an mRNA vaccine against the ancestral Wuhan strain of SARS-CoV-2 to protect individuals exposed to divergent strains, and in particular 1.351 first predominant in South Africa. In addition, it will improve our understanding of the dynamics and duration of these immune responses and will inform rational design and testing of preventive and therapeutic monoclonal antibody interventions in parts of the world where alternative strains continue to proliferate. Lastly, the results of this trial will be used to assess registration of this vaccine product as well as to modify future COVID-19 vaccine trials planned over the next 12 months.