This new vaccine has been given to > 2,000 healthy children and to >2,000 children with asthma and well tolerated. The investigators want to know if Flumist® is well tolerated in children with CF and does not cause worsening of respiratory symptoms. The investigators will conduct a study where all participants will receive Flumist® in the nose. This study is particularly important because its results will provide safety information on a vaccine that is more efficacious for a population who needs safe and easy to administer protection against the flu.