Safety of a New Intranasal Vaccine Against Equine Influenza: A Study of the Stability of the Ts Ca Phenotype of the Reassortant Strain in a Natural Host

We tested our intranasal live-attenuated vaccine from a cold-adapted strain of equine influenza (ts ca phenotype) in a natural host in order to establish 1) its safety and stability during serial horse to horse passage, and 2) the possibility of spontaneous transmission from horse to horse. To study stability, the vaccine was administered to 5 yearlings. Vaccine viruses isolated from the last group retained their cold-adaptation and temperature sensitivity phenotype, without reversion to the wild-type virus phenotype or occurrence of clinical influenza disease.The vaccine virus did not spontaneously spread in any of the unvaccinated horses when these were mixed with vaccinates in the same field. These results demonstrate that the developed influenza vaccine is safe and phenotypically stable, not transmitted spontaneously. Viruses isolated from the latter group retained the phenotype of the complete attenuation without reversing to the phenotype of the wild-type virus or causing a clinical flu disease.