To determine if giving live Sendai virus (in the form of a vaccine) to children through their nose is safe (causes no serious illness) 6 months after enrollment complete [clinicaltrials_resource:934ce14de4a97f86b6ef912b2476ad07]

Participants will be studied in three cohorts: Healthy seropositive children 3 years up to 6 years Healthy seropositive toddlers 12 months up to 24 months Healthy seronegative toddlers 12 months up to 24 months. Each cohort will receive Sendai virus vaccine.

To determine if giving live Sendai virus (in the form of a vaccine) to children through their nose is safe (causes no serious illness) 6 months after enrollment complete [clinicaltrials_resource:934ce14de4a97f86b6ef912b2476ad07]

Participants will be studied in three cohorts: Healthy seropositive children 3 years up to 6 years Healthy seropositive toddlers 12 months up to 24 months Healthy seronegative toddlers 12 months up to 24 months. Each cohort will receive Sendai virus vaccine.