Antigenic comparison of H3N8 equine influenza viruses belonging to Florida sublineage clade 1 between vaccine strains and North American strains isolated in 2021-2022.

Equine influenza virus strains of Florida sublineage clade 1 (Fc1) have been circulating in North America. In this study, virus neutralization assays were performed to evaluate antigenic differences between Fc1 vaccine strains and North American Fc1 strains isolated in 2021-2022, using equine antisera against A/equine/South Africa/4/2003 (a vaccine strain recommended by the World Organisation for Animal Health) and A/equine/Ibaraki/1/2007 (a Japanese vaccine strain). Antibody titers against four North American Fc1 strains isolated in 2021-2022 were comparable to those against the homologous vaccine strains. These results suggest that current Fc1 vaccine strains are effective against North American strains from 2021-2022.