Using the area under the curve (AUC) concept as is commonly used in pharmaceutical bioequivalence studies, the bioequivalence of three equine influenza vaccines was demonstrated. A retrospective analysis was performed using this technique on data generated in three trials in which each of the three vaccines had been used. In total, data from 63 pony and horse foals were used. The AUC of the single radial hemolysis (SRH) titres against Influenza A/equi-1/Prague/56 (Pr/56), A/equi-2/Newmarket-1/93, and A/equi-2/Suffolk/89 (Suf/89) were calculated for each horse. It was concluded that calculation of the AUC from four time-points permitted a suitable estimate for vaccine potency. Using pooled data, it appeared that the AUC permitted better evaluation of vaccine potency than simply considering the highest post vaccinal titre (Titremax). In two studies, a minimal value for the AUC was associated with protection against Influenza (H3N8) challenge 50-153 days later.