Establishing an acceptability threshold for equine influenza vaccines.

Shortcomings in the original methods (based on haemagglutination of erythrocytes) used to measure potency of equine influenza vaccines and antibody responses stimulated by vaccines, coupled with the lack of a reliable challenge system in the target species, has hindered progress in identifying the antigenic content required to provide protection. Reliable methods are now available for measuring the haemagglutinin (HA) content of vaccines and the antibody responses they elicit. The development of challenge systems in the target species has allowed antibody levels consistent with protection to be identified under experimental conditions. Comparison of vaccine performance under experimental conditions and in the field suggests that exposure to a nebulised virus aerosol provides a realistic challenge for the assessment of equine influenza vaccines.