Aim 2: Determine LAIV efficacy in preventing shedding of viral strains upon challenge: Using the same 75 participants with CF and 45 healthy siblings from aim 1, the investigators will compare the proportion of children shedding vaccine strains among the LAIV naïve and LAIV experienced. The clinical efficacy will be extrapolated from the efficacy in preventing shedding and derived as 1 - (% shedding in LAIV experienced/% shedding in LAIV naïve).