A booster vaccination against influenza and rhino should be given to extend immunity for these diseases. That way the flu-rhino could be given every few months for horses under competitive stress or frequent disease exposure. Other owners may regard your horse as the dreaded leper of the barn. In areas of the country prone to sleeping sickness, some veterinarians are recommending repeat vaccinations in the fall. All components of this vaccine will elicit longer immunity than flu-rhino vaccine.
Equine influenza is a determined survivor.
The virus continually changes in order to escape the diligent pursuit of the horse's immune system. The same is true of influenza virus in humans. As the virus evolves, the immune protection from a previous version of the virus may not protect against this year's mutant form. While cross-protection is essential for disease prevention, the weapons of immunity must recognize the virus as the intruder. The greater the variance between the disease virus and the one for which immunity is summoned, the greater the chance that the weapons of immunity will rush past the virus, searching for the real intruder.