A new modified live equine influenza virus vaccine: phenotypic stability, restricted spread and efficacy against heterologous virus challenge

Flu Avert IN vaccine is a new, live attenuated virus vaccine for equine inf luenza. We tested this vaccine ill Vivo to ascertain 1) its safety and stab ility when subjected to serial horse to horse passage, 2) whether it spread spontaneously from horse to horse and 3) its ability to protect against he terologous equine influenza challenge viruses of epidemiological relevance. For the stability study, the vaccine was administered to 5 ponies. Nasal s wabs were collected and pooled fluids administered directly to 4 successive groups of naive ponies by intranasal inoculation. Viruses isolated from th e last group retained the vaccine's full attenuation phenotype, with no rev ersion to the wild-type virus phenotype or production of clinical influenza disease. The vaccine virus spread spontaneously to only 1 of 13 nonvaccina ted horses/ponies when these were comingled with 39 vaccinates in the same field. For the heterologous protection study, a challenge model system was utilised in which vaccinated or naive control horses and ponies were expose d to the challenge virus by inhalation of virus-containing aerosols. Challe nge viruses included influenza A/equine-2/Kentucky/98, a recent representat ive of the 'American' lineage of equine-2 influenza viruses; and A/equine-2 /Saskatoon/90, representative of the 'Eurasian' lineage. Clinical signs amo ng challenged animals were recorded daily using a standardised scoring prot ocol. With both challenge viruses, control animals reliably contracted clin ical signs of influenza, whereas vaccinated animals were reliably protected from clinical disease. These results demonstrate that Flu Avert IN vaccine is safe and phenotypically stable, has low spontaneous transmissibility an d is effective in protecting horses against challenge viruses representativ e of those in circulation worldwide.