The purpose of this study is to compare 2 licensed flu vaccines to each other and to placebo (inactive substance). The study will be conducted among healthy adult participants aged 18-49 years and is expected to last 3 years. During year 1, participants will be assigned to receive 1 of the 2 licensed flu vaccines or placebo, given as either nasal spray (live-attenuated vaccine or placebo) or injection (inactivated vaccine or placebo). Participants will receive the same assigned vaccine or placebo during year 2. During year 3, participants will be followed, but will not receive flu vaccine. Each year blood samples will be collected before and 1 month after each vaccination and at the end of each flu season in order to measure how the body responds to the vaccine and how well participants were protected from the flu. During the flu season, participants with flu-like illness will provide information on symptoms and provide a throat swab to test for virus identification.