Equine influenza is one of the most important infectious diseases in horses and outbreaks cause considerable economical losses for the horse industry. Prophylactic vaccination is the most effective measure to prevent clinical disease and to limit the spread of infection. It is therefore imperative that vaccinations are performed in a manner to ensure optimal protective efficacy. Regimes with shorter intervals between re-vaccinations than those recommended by the manufacturers are sometimes used e.g. for Swedish competition horses. In this project we intend to monitor influenza specific antibody levels in trotters in training that were vaccinated with 6 months interval. In this cohort monthly serum samples were collected from 66 horses during 13 months. The results will provide important novel knowledge on the real outcome of influenza prophylactics in performance horses. Based on this information we will be able to provide relevant advice to practitioners, trainers and horse owners.