Shots: Core vaccines
that all horses should have at least once a year are EEE, WEE, WNV, Tetanus and Rabies EEE, WEE, WNV should be given in the spring prior to the onset of mosquito season to allow the horse's body to build a good resistance before the mosquitos start biting. And you do not
have to give the Fort Dodge WNV vaccine twice a year....it is proven effective for a full year. It is recommended
that horses in areas where there is a long mosquito season (like in Texas, Lousiana, etc) where mosquitos are seen all year round that horses be boostered for WNV at 6 months intervals--this is no matter which WNV vaccine you use. Risk based
vaccines are those which are recommended based upon your horse's specific situation--age, use, living arrangements. These are Rhino (EHV), Influenza, Strangles, Potomac Horse Fever, etc. The use of these is based upon risk of exposure.
You really need to assess YOUR horse's situation both by looking at risk factors like age, diet, management of the areas he is kept in, etc and by assessing what kind of worm burden he tends to carry--some horses are more susceptible to parasites than others. You should start by having a fecal egg count done (be sure the Egg Reappearance Period has passed for the last drug you used) to see what kind of load he is carrying and assess his risks of continued reinfection to decide if you should deworm currently and when you should reassess or retreat.
Appropriate strategic deworming programs are the way to go to minimize the risk of heavy parasitism and to help slow the developement of chemical resistant parasites.
If you haven't already read it, I've posted a long-winded post on deworming several times. You can find it on this thread: http://www.horseforum.com/viewtopic....ight=deworming
Basically, you need to get your vet involved and discuss with him the specifics of your situation to work out a plan that fits your horse's needs.
If you can answer the questions in my long post I linked to, I can help you try to work out something to discuss with your vet.