Efficacy of a canarypox-vectored recombinant vaccine expressing the hemagglutinin gene of equine influenza H3N8 virus in the protection of ponies from viral challenge

Objective-To determine onset and duration of immunity provided by a 2- or 3-dose series of a new canarypox-vectored recombinant vaccine for equine influenza virus (rCP-EIV vaccine) expressing the hemagglutinin genes of influenza H3N8 virus strains A/eq/Kentucky/94 and A/eq/Newmarket/2/93 in ponies. Animals-Forty-nine 1- to 3-year-old male Welsh Mountain Ponies that were seronegative for equine influenza virus. Procedures-Vaccinated and control ponies were challenged with aerosolized influenza virus A/eq/Sussex/89 (H3H8), representative of the Eurasian lineage of circulating influenza viruses. In trial 1, control ponies and ponies that received rCP-EIV vaccine were challenged 2 weeks after completion of the 2-dose primary vaccination program. In trial 2, ponies were challenged 5 months after 2 doses of rCP-EIV vaccine or 1 year after the first boosting dose of rCP-EIV vaccine, administered 5 months after completion of the primary vaccination program. After challenge, ponies were observed daily for clinical signs of influenza and nasal swab specimens were taken to monitor virus excretion. Results-The challenge reliably produced severe clinical signs consistent with influenza infection in the control ponies, and virus was shed for up to 7 days. The vaccination protocol provided clinical and virologic protection to vaccinates at 2 weeks and 5 months after completion of the primary vaccination program and at 12 months after the first booster. Conclusion and Clinical Relevance-The rCP-EIV vaccine provided protection of ponies to viral challenge. Of particular importance was the protection at 5 months after the second dose, indicating that this vaccine closes an immunity gap between the second and third vaccination.