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 orpostmenopausal.

• 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 inthis study.

• Subjects must agree to not be vaccinated with any RNA-based vaccines 30 days beforeD1 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 Investigatorwithin the past month OR hospitalization >24 hours for any reason within the pastmonth 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 expectedAEs 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 orgranulocyte-macrophage colony-stimulating factor, and (4) Three or more consecutivedays of systemic corticosteroids. Note: subjects on stable dose of steroidreplacement (for chronic disease such as iatrogenic deficiency) of prednisone ≤10mg/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-mediatedconditions (e.g., Hashimoto thyroiditis) that are well controlled and stable areallowed.

• 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 orplacebo).