Update of inactivated equine influenza vaccine strain in Japan

Japan established a vaccine selection system, in which a committee evaluates veterinary influenza vaccines to determine if the vaccine should be updated. In 2013, it was concluded that the present equine influenza vaccine strains did not have to be updated, but clade 2 (Fc2) viruses of the Florida sublineage should be included. We collected three Fc2 viruses as candidates and conducted comparative tests. Results indicated that A/equine/Carlow/2011 (H3N8) is not suitable, because of its unstable antigenic characteristics. A comparison between A/equine/Richmond/1/2007 (H3N8) (Richmond/07) and A/equine/Yokohama/aq13/2010 (H3N8) (Yokohama/10) in eggs showed that they shared equal growth properties. Immunogenicity test in mice showed that Yokohama/10 induced higher HI antibody titers than Richmond/07. Therefore, we concluded that Yokohama/10 was the most suitable strain.