The goal of this clinical trial is to assess the safety, reactogenicity and immunogenicity of an mRNA-based vaccine, JCXH-108, the prevention of Respiratory Syncytial Virus (RSV) infection and diseases.

Participants will be randomized to receive either JCXH-108 or placebo.

This Phase 1 study plans to enroll a total of 75 participants.

Three cohorts with two different dose levels will be explored and each cohort will enroll 25 participants. Participants in each cohort will be randomized (4:1) to receive either JCXH-108 (n=20) or placebo (normal saline, n=5). A low dose of JCXH-108 will be explored vs placebo in the 18-45 years of age group first. A high dose will be explored vs placebo in the 18-45 years and ≥ 60 years of age groups once safety data is reviewed for the Sentinel subjects in the low dose cohort. The dose level of JCXH-108 will depend on the time the participant joins the study. Each participant will receive a single dose of JCXH-108 administered intramuscularly (IM) on day 1.

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).

Double-blinded study