Methods: The cohort of children with CF will be vaccinated with LAIV and followed for 56 days by phone for the development of severe adverse events following immunization (AEFI). Both periods (4 weeks) will be monitored using the same methodology: a diary for monitoring of symptoms and hospital charts. The cohort of children without CF will be vaccinated with LAIV and followed for 8 days by phone (one phone call on day 8) for the development of severe AEFIs.