HPIV1 infection can result in severe respiratory illness that often leads to the hospitalization of infants and young children. HPIV1 is responsible for approximately 6% of all pediatric hospitalizations for respiratory tract diseases and is a main cause of severe lower respiratory tract diseases, including pneumonia and bronchiolitis in children less than 6 years old. The purpose of this study is to determine the safety of and immune response to a HPIV1 vaccine, rHPIVI1 84/del170/942A, in 5 main volunteer group: first in adults (group 1 and 2), and then in seropositive children (group 3). The last 2 groups will be seronegative infants and children that are split into 2 dosing groups; one group of seronegative infants and children will receive a lower dose of vaccine (group 4) and a second group of seronegative infants and children (group 5) will receive a higher dose of vaccine. This study will last approximately 8 weeks per group. Additional information about each of the groups is as follows:

Groups 1 and 2 will consist of 35 adults total, 18 to 49 years of age. These groups will receive one immunization with rHPIV1 84/del170/942A at study entry. Immunization will be given as nose drops. Study visits will occur on Days 3, 4, 5, 6, 7, 10, and 28. Medical history, vital signs, clinical assessment, and nasal wash will occur at most visits. Blood and urine tests will occur at selected visits. A urine pregnancy test will occur for females at study entry. Follow-up phone reports will also be required on Day 56.

Enrollment into Group 3 will begin only after safety data from Groups 1 and 2 are evaluated. Group 3 will consist of 15 seropositive children, 15 to 59 months of age. This group will receive either one immunization of rHPIV1 84/del170/942A or placebo at study entry. Immunization will be given as nose drops. Study visits will occur on Days 3, 4, 5, 6, 7, 10, and 28. Medical history, vital signs, clinical assessment, and nasal wash will occur at most visits. Blood and urine tests will occur at selected visits. Parents will be required to submit reports over the phone throughout the study. These reports will include the temporal temperature from the previous day.

Enrollment into Group 4 will begin only after safety data from Group 3 are evaluated. Group 4 will consist of 21-30 seronegative infants and children, 6 to 59 months of age. This group will receive either one immunization of rHPIV1 84/del170/942A or placebo at study entry. Immunization will be given as nose drops. Study visits will occur on Days 3, 5, 7, 10, 12, 14, 17, 19, 21, 28, and 56. Medical history, vital signs, clinical assessment, and nasal wash will occur at most visits. Blood tests will occur at selected visits. Parents will be required to submit reports over the phone throughout the study. These reports will include the temporal temperature from the previous day.

Enrollment into Group 5 will begin only after safety data from Group 4 are evaluated. Group 5 will consist of 21-30 seronegative infants and children 6 to 59 months of age. This group will receive either one immunization of rHPIV1 84/del170/942A or placebo at study entry. The immunization will be given as nose drops. The vaccine dosage that participants receive in Group 5 will be greater than the dose in Group 4. Study visits will occur on Days 3, 5, 7, 10, 12, 14, 17, 19, 21, 28, and 56. Medical history, vital signs, clinical assessment, and nasal wash will occur at most visits. Blood tests will occur at selected visits. Parents will be required to submit reports over the phone throughout the study. These reports will include the temporal temperature from the previous day.