Protection against equine influenza virus (EIV) relies largely on the production of circulating antibodies specific for the haemagglutinin (HA) glycoprotein. The objective of this study was to determine the antibody response of National Hunt horses in training to booster vaccination. The antibody response to the six equine influenza vaccines available in Ireland (three whole inactivated vaccines, two subunit vaccines and a canary pox recombinant vaccine), was monitored by single radial haemolysis (SRH) for six months post vaccination. There was no significant difference between antibody response induced following booster vaccination with any of the six vaccines. The antibodies peaked between two and four weeks post vaccination, decreased significantly by three months post vaccination and declined to their original levels by six months post vaccination. Peak antibody response to the canary pox recombinant vaccine was delayed in comparison to the other vaccines. Although analysis of the mean SRH levels of the horses suggested that they were clinically protected post booster vaccination, analysis of the individual responses suggested that there was potential for vaccination breakdown in a manner similar to that observed previously in racing yards in Ireland. There was a significant correlation between the SRH level at the time of vaccination and the antibody response. The findings of the study suggest that it would be advantageous to monitor SRH levels and to vaccinate strategically. The revaccination of horses with low antibody levels three months post booster vaccination may have been more effective in protecting horses in this yard than the annual vaccination of horses with high SRH levels. Eighteen of the 44 (41%) horses included in this study did not demonstrate a significant rise in SRH level to H3N8 following booster vaccination. It is presumed that annual revaccination is the minimum necessary to protect all horses against EI but this assumption needs to be systematically evaluated. It has been demonstrated that shorter intervals are required for optimum protection of young horses and it may be that longer vaccination intervals are sufficient for older horses with several years of vaccination history. Further investigations in a larger population of horses will be necessary to determine if the findings of this study are applicable to the population at large.