"Equine encephalitis comes in three forms: Eastern, Western and Venezuelan. All three are combined with tetanus toxoid in one vaccine. This is commonly referred to as a VEW-T or 4-way vaccine". The 5-way adds the Flu/Rhino. (This is information retrieved from the internet as I am not a veterinarian nor do I claim to be one). My veterinarian recommends vaccinations for our horses at 4 mos if dams were not vaccinated before foaling or as late as 6mo if the dam was vaccinated.