Equine influenza vaccine efficacy: the significance of antigenic variation.

To investigate the level of cross-protection induced by equine influenza H3N8 vaccines derived from different lineages, two studies have been carried out with ponies vaccinated with 'American-like' and 'European-like' vaccines and experimentally challenged with a European-like strain. The results demonstrated that equine influenza vaccines clearly protect against challenge with homologous virus if serum antibody titres are sufficiently high. On the other hand, protection is incomplete even when animals vaccinated with heterologous strains have comparative antibody levels. Nevertheless, the protection afforded by heterologous viruses can be improved by stimulating high levels of antibody. It would be advisable to update equine influenza vaccine strains regularly so that they contain similar strains to variants that are circulating in the field.