Influenza occurs on an annual basis, producing significant mortality in most years, mainly in those with underlying conditions and those at extremes of age. This study is a placebo-controlled trial to examine efficacies of the live attenuated and inactivated influenza vaccines, and immune correlates of protection; the study will take place in Michigan over a three year period, and will target adults aged 18-49 years. Both vaccine preparations are licensed for this age group. In the first year, participants will be randomly assigned to receive one of the two licensed vaccines or placebo. In the second year, participant receipt of vaccine (updated according to national recommendations) or placebo will continue as assigned in the first year for returning participants, and new enrollees will be randomly assigned to intervention. In the third year, when duration of immunity will be evaluated, no interventions will be given. In all study years, blood specimens for serologic studies and specimens for virus identification will be collected and a number of laboratory studies will be performed so that it will be possible to achieve the study's objectives. The primary annual study outcome for efficacy is the proportion of subjects with laboratory-confirmed symptomatic influenza A or B illness, defined as influenza virus isolated in tissue culture from a throat swab obtained from a participant during an episode of illness meeting the influenza case definition that is confirmed as influenza by fluorescent antibody or polymerase chain reaction (PCR) techniques; or serologic confirmation of influenza infection in a participant with an episode of illness meeting the influenza case definition, specifically a 4-fold rise in hemagglutinin inhibition assay (HAI) antibody titer to a circulating influenza strain between post-vaccination (pre-season) and post-season paired sera. The influenza case definition is illness with at least one respiratory symptom (cough or nasal congestion) and at least one constitutional symptom (fever or feverishness, chills or body aches). Subjects will be evaluated for serious adverse events throughout each annual study period. Annually, the proportion of participants with local reactions during the first seven days or any adverse reactions during the first thirty days after vaccine receipt will be compared with recipients of placebo.