Assess the Safety and Immunogenicity of One or Two Doses of Sing2016 M2SR H3N2 Influenza Vaccine

This is a Phase 1b, randomized, double-blind, dose-escalating, age de-escalating, placebo-controlled study of 200 children, ages 6 months to 17 years. This clinical trial is designed to assess the safety, tolerability/reactogenicity, and immunogenicity of one and two doses of Sing2016 M2SR H3N2 influenza vaccine (manufactured by FluGen) administered intranasally in seven cohorts of children. The study design includes pre-planned Safety Review Committee( SRC) reviews. The first two groups to be vaccinated will be Cohorts 1 and 2. Cohort 1 consists of 45 children 9-17 years old. Thirty of them will receive one dose of the vaccine at a dose of 10\^9 TCID50, and 15 will receive one dose of placebo. Cohort 2 comprises 45 children 2-8 years old. Thirty of them will receive one dose of the vaccine at a dose of 10\^8 TCID50 and 15 will receive one dose of placebo. Sites will enroll participants into Cohorts 1 and 2 simultaneously. Once 25 or more participants in each of the first 2 cohorts (Cohorts 1 and 2) have completed Day 8, SRC will evaluate if any halting rules are met and if it is deemed safe enrollment in Cohort 3 may open. Cohort 2 must fully enroll before enrollment in Cohort 3 may begin. Cohort 3 consists of 25 children 2-8 years old. 15 of them will receive one dose of vaccine at 10\^9 TCID50 and 10 will receive one dose of placebo. Once all 25 participants in Cohort 3 have completed Day 8 of follow-up, similar to Cohorts 1 and 2, the SRC will review to ensure no halting rules are met and if no rules are met, and the SRC determines it is safe to proceed, simultaneous enrollment into Cohorts 4 and 5 can begin. If any halting rules are met or any concerns are raised by the SRC, an external SMC may meet to discuss the data for recommendations on either progression or clinical trial modification before progression to the next cohort. Cohort 4 consists of 25 children 2-8 years old; 15 of them will receive two doses of vaccine at 10\^9 TCID50 and 10 will receive two doses of placebo, with a 28-day interval between the first and second doses. Due to the limited availability of product, and funding to support additional years of enrollment into Cohorts 5, 6, and 7, the decision was made to stop enrollment after the final participant was enrolled into Cohort 4. The primary study objective is to assess the safety and tolerability of one and two administrations of the Sing2016 M2SR H3N2 influenza vaccine at 10\^8 or 10\^9 TCID50 delivered intranasally to healthy participants, 2 to 17 years of age. The secondary study objective is to assess the humoral immunogenicity (serum antibody and mucosal antibody responses) directed against homologous viral strains after one and two administrations of Sing2016 M2SR H3N2 influenza vaccine at 10\^8 or 10\^9 TCID50 delivered intranasally to healthy participants, 2 to 17 years of age.