Antigenic differences between equine influenza virus vaccine strains and Florida sublineage clade 1 strains isolated in Europe in 2019.

From late 2018 to 2019, equine influenza virus (EIV) strains of Florida sublineage clade 1 (Fc1), which had until then been circulating mainly in the United States, suddenly spread across Europe causing many outbreaks, and Florida sublineage clade 2 (Fc2) strains, which had been circulating mainly in Europe, have not been detected in Europe since 2018. Since 2010, the World Organisation for Animal Health (OIE) has recommended that EIV vaccines contain an Fc1 strain that is like A/equine/South Africa/4/2003 or A/equine/Ohio/2003. Accordingly, Japanese vaccines contain A/equine/Ibaraki/1/2007 as the Fc1 strain. To evaluate the effectiveness of these vaccines against the Fc1 strains detected in Europe in 2019, we performed virus neutralization tests using horse antisera. Challenge viruses used were Irish strain A/equine/Tipperary/1/2019 and two recombinant viruses generated by reverse genetics. Recombinant viruses possessing hemagglutinin (HA) and neuraminidase (NA) derived from A/equine/Tipperary/1/2019 (rA/equine/Tipperary/1/2019) or British strain A/equine/Essex/1/2019 (rA/equine/Essex/1/2019) were generated. Equine antisera against A/equine/South Africa/2003 and A/equine/Ibaraki/2007 were produced by experimental infection. Antibody titers against A/equine/Tipperary/1/2019, rA/equine/Tipperary/1/2019, and rA/equine/Essex/1/2019 were 2.5- to 6.3-fold lower than those against the homologous vaccine strains A/equine/South Africa/4/2003 or A/equine/Ibaraki/2007. These results suggest that the ongoing evolution of the Fc1 viruses may impact on antigenicity and although antibodies against current vaccine strains neutralize the 2019 strains, ongoing surveillance is essential for optimum choice of candidate vaccine strains.