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Aims

A 6-week interval is currently recommended during the primary course of the equine influenza (EI) vaccines Equip® FT or Equip® F. This study aimed to evaluate immunogenicity and safety of a 3-week accelerated interval in order to improve flexibility and to reduce the period of susceptibility to EI virus (EIV) infection.

Methods

Three groups of 10 ponies were respectively vaccinated with the vaccine Equip® FT according to the recommended 6-week V1–V2 interval (T02), Equip® F or Equip® FT according to an experimental accelerated 3-week interval (T03 and T04, respectively). These groups were boosted (V3) with Equip® F 5 months after V2. Ten ponies in a control group were injected with saline (T01). The antibody response to the vaccine antigens (tetanus toxoid and EIV) were measured 2 weeks after V2 and V3 with the toxin binding inhibition test (ToBI) and single radial haemolysis (SRH), respectively. All animal work received ethical approval.

Results

The accelerated schedule of immunisation was well tolerated. At both time points, in all vaccinated groups, SRH least squares means for A/eq/Borlange/91 and A/eq/Kentucky/98 antigens were well above 85 mm2, a level described previously to provide clinical protection against homologous EIV strains. Non inferiority of SRH least square means were demonstrated at 2 weeks post V2 for both antigens for T03 and T04, and for A/eq/Kentucky/98 antigen in group T04 after V3. For tetanus,ToBI least squares means were 86.3 iu/mI for T02 and 45.9 iu/ml for T04 after V2 and 2.6 iu/ml for T02 and 2.2 iu/ml for T04 after V3. A level of 0.02 iu/ml has been described as being protective in various publications.

Conclusion and practical relevance

A 3-week accelerated primary course interval was well tolerated and serology results suggested good immunogenicity.

Ethical animal research

The study was conducted under the Animal Health Trust Home Office Project License and with approval of the Zaventem Ethics Review Assessment team (Zoetis). Sources of funding: The study was sponsored by Pfizer/Zoetis Animal Health. Competing interests: Equip® FT is owned by Zoetis, who sponsored this study. Romain Paillot reports no conflict of interest. All other authors are employed by the study sponsor.