Vaccines are tested to see if they work via actually giving them to horses and then exposing them to the disease causing organism...not through taking titers.
As for all the "bad reactions" to vaccines in small animals, yes they do occur but after working in small animal medicine for 10 years I can count the number of times I saw them on 1 hand---and I worked at a very large practice on the outskirts of Dallas. But we do have lots more studies that occur on dogs and cat because there are more small animal owners who provide more extensive medical care for their dogs and cat so there is more money in it for the research companies to actually do more studies. It's also more cost-effective to manage a large number of small animals than large animals---just having the room for them. We are years ahead on small animal medicine compared to equine medicine in many areas just because of feasability.
We are also vaccinating against different types of diseases in horses than in dogs and are dealing with a different species which means that their responses to diseases and vaccines aren't the same. Tetanus for example---in humans we get vaccinated every 10 years, but in horses yearly vaccination is recommended because they are more susceptible than humans are, they tend to get lots of nicks and cuts (including insect bites) which can serve as a route of infection and because they shed the bacteria that causes tetanus in their feces which means that they live in a constantly contaminated area. EEE, WEE, WNV are all transmitted by biting insects---how many bitng insects do you see on your horses daily during the spring and summer? Rabies--the big reason for recommending this yearly in horses is because of the risk of transfer to humans. More horses than most people realize contract rabies every year and for each horse they expose at least 2 people---the owner and the vet---then factor in every person who trooped through and checked on the sick horse at the barn, the person who feeds and/or clean's stalls, the vet's assistants, anyone who helped load the horse in the trailer to go to the vet's office, etc.
I would recommend that you check out the 12 part series on vaccination that was printed in The Horse magazine. It's available free on their website. It's a great place to get good information. These two in particular would be of interest to you: http://www.thehorse.com/pdf/Vaccinat...ations_pt1.pdf http://www.thehorse.com/pdf/Vaccinat...ations_pt8.pdf