voluntary Hold because of COVID-19, all participants were off study prior to the hold.

Croup is an illness of young children that is caused by a virus. With this illness, the child has fever, cough, and hoarseness. Although the illness usually gets better in 2 to 4 days, some children may be admitted to the hospital and a few infants may require an intervention to help their breathing. This illness is most often caused by a virus called parainfluenza virus type 1, but it can be caused by other viruses. The experimental vaccine that is being investigated in this study is intended to try to prevent croup caused by parainfluenza virus type 1. Currently, there are no vaccines to prevent this virus, or medications available to treat the illness once infection has occurred.

This research study is testing a new experimental live-virus vaccine that is given by placing liquid drops in the nose. The Sendai virus is very similar to the virus that causes croup, but it has never been found to cause illness in people. Previous studies in animals have shown that the vaccine provided protection against the croup virus, and did not cause illness. Many people have been exposed to the Sendai virus, but no one has been known to develop illness. Several healthy adults have been given the Sendai virus vaccine being studied, and they did not experience any serious side effects or illness.

The main aim of the study is to assess the tolerance and safety of escalating doses of intranasal Sendai virus in children and toddlers.

The secondary objective of the study is to assess the magnitude and duration of the immune response elicited by intranasal Sendai virus. Responses between seropositive and seronegative children will be compared.

This study also tests the safety and immunogenicity of a booster vaccination at the highest dose tolerated during primary immunization dose-escalation.

Inclusion Criteria;

• Parent or legal guardian willing and able to give informed consent and comply with study requirements
• Child is greater than or equal to one year of age and less than six years of age
• Adequate blood, liver and kidney function
• Has not or will not receive other vaccinations within 30 days of receiving study vaccine

Exclusion Criteria:

• History of allergy to eggs or gentamicin
• History of lung disease, asthma, and hospitalization for respiratory illness, immunodeficiency, sickle cell disease, or any other serious underlying condition
• Children who have a family history of primary immunodeficiency or lack history of maternal or paternal immune status
• Height or weight less than 5th percentile
• Upper respiratory tract infection (URI) or household member with URI
• Evidence of developmental delay or evolving neurological disorders
• Household member or daycare contact (i.e. stay in the same room in daycare) less than 24 months of age if same room contact is expected within 14 days of vaccination or 7 days after booster, or prior to evidence of vaccine clearance
• Immunodeficient household members or other close contacts if same room contact is expected within 14 days of vaccination or 7 days after booster, or prior to evidence of vaccine clearance
• Current use of investigational or immunosuppressive drugs (e.g., steroids)
• Current use of antibiotics or antivirals