Currently the recommendation for vaccination with the Rhino/Flu vaccine (which contains EHV1 and EHV4 strains) is recommended every six months. Some veterinarians recommend this vaccine more frequently for horses that may have a greater risk of being exposed. The vaccine will either prevent the disease or make the symptoms less severe. It may not protect against the neurological form of EHV.
This brings me to the most important part of disease prevention. That is biosecurity. Biosecurity simply means that you are protecting you and your horse from exposure by doing a few simple things to reduce exposure. Most communicable diseases are spread by nose to nose contact by horses. By keeping your horse from having nose to nose contact with other horses when you are at events or shows you will help reduce risk. Also don’t let your horse drink out of community water troughs. Do not share tack, especially bits and bridles. Don’t pet other horses and then go and pet your horse. You can be a means of transmission of communicable disease also. Following these few simple rules will help to prevent disease spread to your horses. The following article explains Equine Herpes Virus and answers many of the common questions horse owners have about the disease.
Below is a good article with other helpful information on Equine Herpes Virus.