Why can Equine Influenza persist when widespread vaccination is used?
To prevent an infectious agent from continuing to circulate and produce clinical disease within a given horse population, more than 80% of the total horse population needs to be immune effectively all the time. This means that all horses need to be on regular regimes of vaccination (using vaccines containing appropriate and up-to-date strains of virus) to ensure that the immunity in the total horse population is sufficiently high all the time to prevent excretion of Equine Influenza virus. Such a consistent level of effective vaccination in a total horse population has never been achieved and so Equine Influenza continues to circulate. With a few notable exceptions, testing for circulation of virus is never carried out in combination with vaccination programs and so in most countries where Equine Influenza is endemic; there is no knowledge of the level of infection circulating in the horse population. Regular vaccination in countries where Equine Influenza is present in fact promotes e
