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

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.

Eligibility Criteria:
Key Inclusion Criteria:

* Age 24 months to \< 48 months of age
* Healthy by medical history and physical examination or presence of stable underlying chronic medical condition for which hospitalization has not been required in the previous year

Key Exclusion Criteria:

* History of allergic disease or reactions likely to be exacerbated by any component of the investigational product
* Acute illness or evidence of significant active infection (including fever \>= 100.4 degrees Fahrenheit (38.0 degrees Celsius) at randomization
* History of asthma or history of recurrent wheezing
* Any known immunosuppressive condition or immune deficiency disease
* Current or expected receipt of immunosuppressive medications within a 28 day window around vaccination
* Use of aspirin or salicylate-containing medications within 28 days prior to randomization or expected receipt thru 28 days after vaccination
* Use of antiviral agents with activity against influenza viruses within 48 hours prior to first dose of investigational product or anticipated use of such agents through the end of the study follow-up period
* Receipt of any non-study seasonal influenza vaccine within 90 days of Dose 1 or planned receipt of non-study seasonal influenza vaccine prior to 28 days after last vaccination
* Receipt of immunoglobulin or blood products within 90 days before randomization into the study or expected receipt during study participation
* Known or suspected mitochondrial encephalomyopathy
* History of Guillian-Barre syndrome
* Administration of intranasal medications within 10 days prior to randomization, for expected receipt through 10 days after administration of each dose of investigational product