Equine influenza is a contagious diseases caused by equine influenza viruses which belong to the orthomyxovirus family. Outbreaks of equine influenza cause severe economic loses to the horse industry and consequently competition horses are required to be regularly vaccinated against equine influenza. Currently available inactivated vaccines are only able to induce protection against clinical disease and virus excretion after a primary vaccination course consisting of three vaccine applications at 4-6 and 22-26 weeks apart, respectively. It has been suggested that these vaccines induce no adequate protection in horses at 22-26 weeks (5 months) in the primary vaccination course (immediately prior to the last booster), despite various alternative vaccination regimens proposed. In this paper we describe the efficacy and safety profile, tested in an experimental setting according to European legislation of a novel inactivated equine influenza vaccine formulation (Prequenza). This formulation consists besides influenza antigen, of second generation ISCOM-Matrix as an adjuvant. The vaccine aims at the induction of protection from the onset of immunity, i.e. after the first two vaccine applications, until the first booster given 5 months later, against challenge with a virulent equine influenza strain. The protection against A/equine/Kentucky/95 (H3N8) was evidenced by a reduction of clinical signs of influenza, a reduction of virus excretion and a reduction of fever. The vaccine was shown to be safe in pregnant mares, foals and is used safely since 2 years as a commercial vaccine in Europe.