The ARCT-154 Self-Amplifying RNA Vaccine Efficacy Study (ARCT-154-01)

Phase 1 will enroll 100 healthy participants that are randomly assigned 3:1 to receive ARCT-154 or placebo (75:25) for the initial series of vaccinations. In Phase 2, 300 participants will be randomly assigned 3:1 to receive ARCT-154 or placebo for the initial series of vaccinations. Participants that received ARCT-154 in the initial series will be rerandomized 3:1 to receive ARCT or placebo on Day 92 followed by placebo on Day 120. In Phase 3a, 600 participants will be randomly assigned 3:1 to receive ARCT-154 or placebo for the initial series of vaccinations. Participants that received ARCT-154 in the initial series will be rerandomized 3:1 to receive ARCT or placebo on Day 92 followed by placebo on Day 120. In Phase 3b, \~16,000 participants will be randomly assigned 1:1 to receive ARCT-154 or placebo for the initial series of vaccinations. In Phase 3c, \~2,400 participants will be randomly assigned 1:1 to receive ARCT-154 or Astra Zeneca COVID-19 vaccine. Blood samples will be collected and reserved for Immunogenicity evaluation for the first 1500 participants (3c-1) and assays for immunogenicity evaluation will be performed for the first 800 participants. Phase 1 participants must be \<60 years of age and healthy. Phase 2, 3a, and 3b and 3c participants will include elderly (≥60 years) and those with comorbidities. For Phase 2, 3a, 3b and 3c, prior to randomization, participants will be stratified by age (\< 60 or ≥ 60 years of age) and for participants \< 60 years of age by risk of severe COVID 19. Participants will be followed up for approximately 1 year after completion of the initial vaccination series. An independent Data and Safety Monitoring Board (DSMB) will perform ongoing review of blinded and unblinded data. An independent blinded adjudication committee will adjudicate all suspected COVID-19 cases to determine if they meet the primary endpoint requirements.