A Trial Evaluating the Safety and Effects of an RNA Vaccine ARCT-021 in Healthy Adults

This is a multiregional, multicenter, Phase 2, randomized, observer-blind study designed to evaluate the safety, reactogenicity, and immunogenicity of the study vaccine in younger and older adult participants. Enrolled participants will be randomly assigned to receive either study vaccine ARCT-021 or placebo (sterile saline). Approximately 600 participants (300 each in younger \[18 to \<56 years of age in United States or 21 to \<56 years of age in Singapore\] and older \[≥56 years of age\] participants) will be enrolled (including at least 50% of participants in the older cohort ≥65 years of age). Participants will be stratified by age and then randomly assigned (3 ARCT-021:1 placebo) to receive 2 doses of study vaccine separated by 28 days. At 180 days after second study vaccination (Day 208), participants in Study Groups 1, 2, and 3 will be randomly assigned again to receive a single booster dose of study vaccine (randomly assigned as 1 ARCT-021:1 placebo). Study Group 4 will not be randomized but will receive 1 dose of placebo at Day 208. Study Groups are summarized in Table 1. Study vaccine will be administered in an observer-blind fashion. Participants will be followed for safety and immunogenicity through 180 days after booster vaccination (Day 388). At a subset of clinical sites, all enrolled participants will also undergo blood sampling for evaluation of CMI responses. Vaccine doses will be assigned as follows: Younger Age Cohort: Study Group 1: n= 75 participants, ARCT-021 7.5 µg (first dose), Placebo (second dose Study Group 2: n= 75 participants, ARCT-021 5.0 µg (first dose), 5.0 µg (second dose) Study Group 3: n= 75 participants, ARCT-021 7.5 µg (first dose), 7.5 µg (second dose) Study Group 4: n= 75 participants, Placebo (first dose), Placebo (second dose) Booster Vaccine: Study Groups 1, 2, 3: 113 participants, ARCT-021 5.0 µg or 7.5 µg, 112 participants, Placebo Study Group 4: n= 75 participants, Placebo Older Age Cohort: Study Group 1: n= 75 participants, ARCT-021 7.5 µg (first dose), Placebo (second dose Study Group 2: n= 75 participants, ARCT-021 5.0 µg (first dose), 5.0 µg (second dose) Study Group 3: n= 75 participants, ARCT-021 7.5 µg (first dose), 7.5 µg (second dose) Study Group 4: n= 75 participants, Placebo (first dose), Placebo (second dose) Booster Vaccine: Study Groups 1, 2, 3: 113 participants, ARCT-021 5.0 µg or 7.5 µg, 112 participants, Placebo Study Group 4: n= 75 participants, Placebo A DSMB will be in place to independently review the safety data of participants. Pausing Rules are also utilized in this study to reduce risk to study participants. The expected duration of participation for an individual participant is approximately 14 months, inclusive of the Screening period.