The primary objective of this Phase 3 study is to compare the immunogenicity of two novel vaccination strategies, consisting of vaccination with an HD influenza vaccine or an MF59a influenza vaccine, to the standard-dose non-adjuvanted vaccination (standard of care) in a population of SOT recipients.

The main secondary objectives are to evaluate the efficacy of the novel vaccination strategies in reducing the incidence of influenza, to correlate the humoral responses to vaccination with protection from influenza, and to assess the influence of immunosuppression on influenza vaccine responses.

Safety objectives include the assessment of the reactogenicity of the different vaccines and to describe the incidence of acute rejection and the development of anti-Human Leucocyte Antigens (HLA) antibodies after vaccination.