Evaluate the Shedding and Immunogencity of Different Formulations of FluMist in Children 24 to <48 Months of Age - FluMist

This study is being conducted to compare the immunogenicity, safety, and viral shedding of a new A/H1N1 strain that will be incorporated into the FluMist quadrivalent formulation for the 2017-2018 influenza season with the previous A/H1N1 strain that was included in the vaccine in the 2015-2016 influenza season.

This randomized, double-blind, multi-center study will enroll approximately 200 children 24 to less than (<) 48 months of age. Participants will be randomized in a 1:1:1 ratio to receive two doses of either FluMist quadrivalent 2017-2018, FluMist quadrivalent 2015-2016 formulation, or FluMist trivalent 2015-2016 formulation. Participants will be screened within 30 days prior to randomization. Randomization will be stratified according to whether the participant ever received prior influenza vaccination. Approximately 50% of the participants will not have been previously vaccinated. All participants will receive two doses of investigational product on Study Days 1 and 28, and followed for a 28-day follow-up period after each dose. Blood and nasal samples will be collected and safety evaluations perfomed. The duration of participants participation is approximately 2 to 3 months. The study will be conducted during the influenza "off-season" in the US. After completion of the study all participants will be offered and strongly encouraged to receive an inactivated influenza vaccine approved for use in the US for the 2017-2018 influenza season.