• Healthy children at least 18 months but less than 60 months of age whose legally-acceptable representative (LAR) understands and signs the trial informed consent and agrees to vaccine administration following a detailed explanation of the trial.

• Determined by medical history, targeted physical exam, and clinical judgement of the investigator to be in a good state of health. Screening laboratory values slightly outside lab normal ranges may be acceptable if the site investigator determines that they are not clinically significant. Permitted concomitant medications include nutritional supplements, medications for gastroesophageal reflux, eye drops, and topical medications, including topical steroids, topical antibiotics, and topical antifungal agents.

• Sero+ for RSV as defined by serum RSV antibody titer assay

• Participant is expected to be available for the duration of the trial.

• The LAR confirms that the subject has received routine immunizations appropriate for age based on the current Advisory Committee on Immunization Practices (ACIP) Recommended Immunization Schedule for Children and Adolescents Aged 18 Years or Younger.

• Growing normally for age as demonstrated on a World Health Organization (WHO) growth chart, AND has a current height and weight above the 3rd percentile for age.

• Healthy children at least 8 months but less than 25 months of age whose LAR understands and signs the trial informed consent and agrees to vaccine administration following a detailed explanation of the trial.

• Determined by medical history, targeted physical exam, and clinical judgement of the investigator to be in a good state of health. Screening laboratory values slightly outside lab normal ranges may be acceptable if the site investigator determines that they are not clinically significant. Permitted concomitant medications include nutritional supplements, medications for gastroesophageal reflux, eye drops, and topical medications, including topical steroids, topical antibiotics, and topical antifungal agents.

• Sero- OR sero+ for RSV antibody, defined by serum RSV antibody titer assay not more than 30 days prior to vaccination.

• Participant is expected to be available for the duration of the trial.

• The LAR confirms that subject has received routine immunizations appropriate for age based on the current Advisory Committee on Immunization Practices (ACIP) Recommended Immunization Schedule for Children and Adolescents Aged 18 Years or Younger.

• Growing normally for age as demonstrated on a World Health Organization (WHO) growth chart, AND

• If \<1 year of age: has a current height and weight above the 5th percentile for age.

• If ≥1 year of age: has a current height and weight above the 3rd percentile for age.