This study is a Phase 1, randomized, double-blind, placebo-controlled, dose-escalation clinical trial to evaluate the safety of and immune response to CodaVax-RSV in healthy children. They will be vaccinated in spring to early autumn 2023 and followed through the 2023-24 RSV season. 18 children aged 2 to 5 years who are RSV-seropositive (have antibodies to RSV) and 33 children aged 6 months to < 2 years who are RSV-seronegative (do not have antibodies to RSV) will be enrolled in escalating-dose cohorts. A safety committee will review the safety profile of each dosing group before the next dose-escalation. Children will receive 2 doses of the vaccine at one of several dose levels or placebo (saline solution with no active ingredient) as nose drops; doses will be 28 days apart.

A parent/guardian will record temperature and other conditions in a diary daily for 7 days after each dose. The parent/guardian will be contacted by telephone on the day after Dose 1 for safety assessment and review of the diary data. Children will return to the clinic 3, 7, 14, and 28 days after each dose. The parent/guardian will then be contacted by telephone monthly until 1 year after the second dose.

Study procedures include physical examinations, vital signs, and collections of blood and nose/throat swab samples to look at safety of the vaccine and to analyze body's immune response.

Inclusion Criteria:

1. Age at the time of informed consent:

   • Part A: 2 to 5 years, inclusive
   • Part B: 6 months to < 2 years

2. RSV Status at Screening:

   • Part A: RSV-seropositive
   • Part B: RSV-seronegative

3. Good general health status

4. Product of normal full-term pregnancy (36 to 42 weeks gestation)

Exclusion Criteria:

1. Household contact with any of the following groups of individuals for the period up to 14 days after each dose:

   • Pregnant women

   • Infants < 6 months of age

   • With hospitalization for asthma or other chronic respiratory disease in the past 5 years

   • Immunocompromised individuals, which includes, but is not limited to, those with the following conditions:

     • AIDS
     • Receipt of chemotherapy within the past 6 months
     • Current receipt of immunosuppressive agents
     • Solid organ or bone marrow transplant

2. Enrolled in the same classroom at full-time day care with infants < 6 months of age for 14 days after each dose

3. Household contact of another child enrolled into the study

4. Inadequate venous access for repeated phlebotomy

5. Height and weight ≤ 5th percentile for age and sex (according to CDC growth charts for children in Part A, according to World Health Organization Child Growth Standards for children in Part B)