Growth properties and immunogenicity of a virus generated by reverse genetics for an inactivated equine influenza vaccine

Background Keeping vaccine strains up to date is the key to controlling equine influenza (EI). Viruses generated by reverse genetics (RG) are likely to be effective for quickly updating a vaccine strain. Objectives To evaluate the growth properties of an RG virus in embryonated chicken eggs, and to evaluate antibody responses to a formalin-inactivated vaccine derived from the RG virus in Thoroughbred horses. Study design In vitro and in vivo experiments. Methods Wild-type (WT) viruses (A/equine/Ibaraki/1/2007) or RG viruses (consisting of haemagglutinin [HA] and neuraminidase genes derived from A/equine/Ibaraki/1/2007 and the six other genes derived from high-growth A/Puerto Rico/8/34) were inoculated into embryonated chicken eggs, and the allantoic fluids were harvested at every 24 hours after inoculation. WT and RG viruses were inactivated by formalin for vaccine use. Ten unvaccinated yearlings (five each for WT or RG vaccine) received the first two doses of a primary vaccination course 4 weeks apart followed by their third dose 12 weeks later. Twenty vaccinated adult horses (10 each for WT or RG vaccine) received a single dose of a booster vaccination. Results The RG virus had high growth properties in embryonated chicken eggs. Unvaccinated yearlings responded poorly to the first vaccination, especially those that received the RG vaccine, but mounted better responses to the second and the third vaccinations, and maintained relatively high haemagglutination inhibition (HI) titres up to 28 weeks after the first vaccination. Vaccinated adult horses did not respond remarkably to the booster vaccination, but no horses showed titres below their pre-booster values even at 12 weeks after vaccination. The RG virus elicited immunogenicity in horses adequate for vaccine use. Main limitations No virus challenge study was performed. Conclusions The RG viruses are useful as an EI vaccine strain, and quick updates of an EI vaccine strain can be achieved by using RG techniques.