This study will be the first assessment of the efficacy of MEDI7510 for the prevention of RSV disease. It will also provide estimates of vaccine efficacy and of endpoint incidence in the placebo arm. It will also assess the safety and immunogenicity of concurrent dosing of MEDI7510 and IIV to expand on the observations made in the Phase 1b study of MEDI7510. It will also expand the safety database of subjects dosed with MEDI7510. The study will also assess the immune response to MEDI7510 in Season 1 and Season 2.