A comparison of antibody responses to commercial equine influenza vaccines following primary vaccination of Thoroughbred weanlings--a randomised blind study.

Many racing authorities, sales companies and equestrian bodies have mandatory vaccination policies for equine influenza (EI). The consequences of lack of vaccine efficacy include clinical disease, disruption to training programmes, the cancellation of equestrian events and the introduction of virus to susceptible populations. The correlation between antibody against the virus haemagglutinin and protection against influenza has been well established. The objective of this study was to compare the antibody responses of 66 unvaccinated Thoroughbred weanlings on four different stud farms, following primary vaccination (V1, V2 and V3) with the five EI vaccines commercially available in Ireland (Duvaxyn IET Plus, Equilis Resequin, Equip FT, Equilis Prequenza Te, ProteqFlu Te). Antibody responses were monitored for 6 months post V3 by single radial haemolysis. The pattern of antibody response was similar for all vaccines and for all antigens tested. A rapid decline of antibody level was observed by 3 months post V2 for all vaccines. The antibody response of the horses vaccinated with the whole virus vaccine Duvaxyn IET Plus was significantly higher than that of the horses vaccinated with the other four products. Five weanlings had maternally derived antibodies (MDA) at the time of V1. The canary pox recombinant vaccine, subunit vaccine and whole virus inactivated vaccines administered to these weanlings did not induce a detectable antibody response against the background of MDA but effectively primed the animals as revaccination resulted in a strong antibody response. In this study 43% of the weanlings failed to seroconvert after V1. This high incidence of poor responders has not been reported in previous experimental studies relating to these products. The poor responders were observed in all vaccine groups except those vaccinated with Duvaxyn IET Plus. Post V2 the incidence of poor responders was reduced to 7% and all horses responded to V3. The study demonstrates that independent evaluation of influenza vaccine performance in the field is critical to add to the body of knowledge gained from experimental challenge experiments carried out for regulatory or marketing purposes. Copyright Â