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

HPIV3 and HMPV are viruses that can cause breathing problems in children. The goal of this clinical trial is to look at the safety of 2 experimental HPIV3/HMPV vaccines in HPIV3-seropositive children ≥ 24 months to \< 60 months of age. Children will receive B/HPIV3/HMPV-PreF-A vaccine, B/HPIV3/HMPV-F-B365 vaccine, or placebo, and participants will not know which study product they have received. The main goals of the study are to find out whether these vaccines are well-tolerated and infectious in HPIV3-seropositive children. The general procedures include daily temperature measurements and daily contact with the participant for the first 28 days, giving a single dose of one of the 2 study vaccines or placebo delivered by nasal sprayer, about 9 in-person visits, a physical examination, 7 clinical assessments, 2 blood samples, 9 nasal swabs and monthly contacts with the participant between Days 29-180. Additional visits may occur if the child has a respiratory illness, fever, or ear infections. The illness visit will include a nasal swab and a clinical assessment.

Inclusion Criteria: * ≥24 months of age and \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: * \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 \