Study of the Infectivity, Safety and Immunogenicity of Two Recombinant, Live-Attenuated, B/HPIV3 Vectored Vaccines Expressing the Fusion Glycoprotein of HMPV Delivered by Nasal Spray to HPIV3-Seropositive Children 24 to <60 Months of Age

Inclusion Criteria: * ≥24 months of age and \<60 months of age at the time of inoculation * HAI Screening for HPIV3-neutralizing antibody is obtained within the calendar year of inoculation * Seropositive for HPIV3 antibody, defined as serum HPIV3 HAI titer \>1:8 * Pre-inoculation serum sample for HPIV3-neutralizing antibody specimen is obtained no more than 42 days prior to inoculation * In good health based on review of the medical record, history, and physical examination at the time of inoculation * Received routine immunizations appropriate for age based on the 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 * Expected to be available for the duration of the study * Parent/guardian is willing and able to provide written informed consent Exclusion Criteria: * \<24 months of age and \>60 months of age at the time of inoculation. * Born at less than 34 weeks gestation * Maternal history of a positive HIV test before or during pregnancy. * Evidence of chronic disease * Known or suspected infection or impairment of immunological functions * Bone marrow/solid organ transplant recipient * Major congenital malformations, including congenital cleft palate or cytogenetic abnormalities * Suspected or documented developmental disorder, delay, or other developmental problem * Cardiac abnormality requiring treatment * Lung disease or reactive airway disease * More than one episode of medically diagnosed wheezing in the first year of life * Wheezing episode or received bronchodilator therapy or racemic epinephrine within the past 12 months * Wheezing episode or received bronchodilator therapy after the age of 12 months * Previous receipt of supplemental oxygen therapy in a home setting * Previous receipt of an investigational HPIV3 or HMPV vaccine * Previous receipt or planned administration of human immunoglobulin within the past 6 months. Receipt of licensed monoclonal antibody products for passive prophylaxis against RSV (e.g., nirsevimab) within the past 6 months is not a cause for exclusion * Previous receipt of any blood products within the past 6 months. * Previous anaphylactic reaction * Previous vaccine-associated adverse reaction that was Grade 3 or above. * Known hypersensitivity to any study product component * Member of a household that contains an infant who is less than 12 months of age at the date of inoculation through the 10th day after inoculation * Member of a household that, at the date of inoculation through the 10th day after inoculation, contains an immunocompromised individual including but not limited to: * a person who is HIV-infected * a person who has cancer and has received chemotherapy within the 12 months prior to enrollment * a person living with a solid organ or bone marrow transplant * Attends a daycare facility that does not separate children by age and contains a child \<12 months of age at the date of inoculation through the 10th day after inoculation * Receipt of any of the following prior to enrollment: * inactivated influenza vaccine within 3 days prior, or * any other inactivated vaccine, messenger RNA (mRNA) vaccine, or live-attenuated rotavirus vaccine within the 14 days prior, or * any live vaccine, other than rotavirus vaccine, within the 28 days prior, or * another investigational vaccine or investigational drug within 28 days prior, or * salicylate (aspirin) or salicylate-containing products within the past 28 days * Scheduled administration of any of the following after planned inoculation: * inactivated vaccine or live-attenuated rotavirus vaccine within the 14 days after, or * any live vaccine other than rotavirus within the 28 days after, or * any salicylate or salicylate-containing products within the 28 days after, or * another investigational vaccine or investigational drug in the 56 days after * Receipt of any of the following medications within 3 days prior to study enrollment: * systemic antibacterial, antiviral, antifungal, anti-parasitic, or antituberculous agents, whether for treatment or prophylaxis, or * intranasal medications, or * other prescription medications except the permitted concomitant medications listed below * Permitted concomitant medications (prescription or non-prescription) include nutritional supplements, medications for gastroesophageal reflux, eye drops, and topical medications, including (but not limited to) cutaneous (topical) steroids, topical antibiotics, and topical antifungal agents * Any of the following events at the time of enrollment: * fever (temporal or rectal temperature of ≥100.4°F), or * upper respiratory signs or symptoms (rhinorrhea, cough, or pharyngitis), or * nasal congestion significant enough to interfere with successful inoculation, or * otitis media, or * contact with a person diagnosed with COVID-19 disease or active SARS-CoV-2 infection within the preceding 10 days