A Phase 1 Study to Evaluate JCXH-108, an mRNA-based Vaccine Against RSV

Main Inclusion Criteria: * Sex: Male or female; female subjects may be of childbearing potential or postmenopausal. * Age: 18-45 years of age or ≥ 60 years of age at screening * Status: Healthy subjects. * Subjects must agree to not be vaccinated with any RSV vaccine while participating in this study. * Subjects must agree to not be vaccinated with any RNA-based vaccines 30 days before D1 through 30 days after D1. Main Exclusion Criteria: * Subjects with current diagnosis of RSV infection or diseases. * Previous vaccination against RSV. * Subjects with any respiratory illness deemed clinically relevant by the Investigator within the past month OR hospitalization \>24 hours for any reason within the past month prior to Day 1. * Subjects with history of myocarditis or pericarditis, or with adverse events (AEs) after mRNA vaccination that are in nature and severity beyond the common expected AEs necessitating medical intervention. * Subjects who received any non-live vaccine within 14 days prior to Day 1. * Subjects who received within 28 days prior to Day 1: (1) Any live vaccine, (2) Immunomodulators or immune-suppressive medication, (3) Granulocyte or granulocyte-macrophage colony-stimulating factor, and (4) Three or more consecutive days of systemic corticosteroids. Note: subjects on stable dose of steroid replacement (for chronic disease such as iatrogenic deficiency) of prednisone ≤10 mg/day or equivalent are allowed. * Subjects who currently receive other investigational agents or devices. * Subjects with active or suspected immunosuppression, immunodeficiency, asplenia, recurrent severe infections or autoimmune diseases. Certain immune-mediated conditions (e.g., Hashimoto thyroiditis) that are well controlled and stable are allowed. * Subjects receiving systemic antiviral therapy. * Subjects with a positive screening test for hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) antibodies, or anti-human HIV-1 and 2 antibodies, syphilis. * Subjects with a positive test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at Screening or on Day 1 prior to vaccine administration (JCXH-108 or placebo).