RecruitingBiological interventionGEneRating Mucosal Immunity After INfluenzA Infection and Vaccination in Lung and Lymphoid TissuEThis experimental medicine study aims to compare immune responses in healthy adult volunteers aged 18-55 years against influenza vaccination and infection in the upper and lower respiratory tract, following administration of a live-attenuated influenza vaccine delivered by nasal spray versus influenza A (H3N2) viral challenge.