Studies with inactivated equine influenza vaccine. 2. Protection against experimental infection with influenza virus A/equine/Newmarket/79 (H3N8).

Forty ponies immunized with inactivated virus vaccine containing A/equine/Miami/63 (H3N8) virus and six unvaccinated, seronegative ponies were experimentally challenged with a representative of recent equine H3N8 virus isolates, A/equine/Newmarket/79. All unvaccinated ponies became infected as judged by virus excretion, febrile responses and antibody responses, but only two of the vaccinated ponies were fully protected. Pre-challenge antibody levels to A/Newmarket/79 virus detected by single radial haemolysis (SRH) correlated well with the degree of clinical protection but the levels required for complete protection (SRH zones greater than 65 mm2) were high. The importance of these results in relation to conventional vaccination procedures against equine influenza is discussed.