I have been able to help horses and humans with these seasonal allergies... Most of the products can be ordered online or bought from your local health food stores. Pm me for more info.
To me the best thing is boost the immune system and don't vaccinate. Vaccinations will lead to all kinds of issues especially allergies and asthma etc. Thats my opinion based on research I have done. I really boost my immune system this time of year especially with silver. But you can try giving some french's mustard 30cc for 3 days straight sometimes that will blow a lot of that out.
I'm having several horses with allergy issues this year. Two have been on meds for 2 weeks now and are improving. I highly recommend you take your horse to the vet. This can be the beginning of heaves and you want to get the jump on it asap!
The best thing is to find out exactly what is causing the allergic reaction, then you can better control it from there. Our vet did a blood test and we found hay to be what he was allergic to.
I also highly recommend you go to a vet and have your horse scoped. My horse had white discharge and they diagnosed him with IAD (Inflammatory Airway Disease). The vet said more than 50% of performance horses have this. It is a milder form of heaves, but heaves is progressive and so is IAD. It will continue to get worse without treatment. The vet's also said you usually can only really help IAD or heaves with a steriod therapy. You definitely don't want to just let it go as it can continue to get worse and potentially end performance careers. Wheezing, coughing, white nasal discharge tells me it more than just a mild case of seasonal allergies.