I had a horse that was great for shots.
She had a WN vaccine, then the vet came out to do the booster. I was not there, but the barn manager was. She was fine, 30 minutes later, after everyone had left because of a snow storm, my best friend got there, walked past her stall and say she had a reaction. She was in shock, trouble breathing, lips looked like a draft, eyes swelling shut. My friend and her boyfriend at the time, made a mix of all meds they could find- dex, benedryl, etc, since clearly she was in distress, and they got it in her, before her throat closed up.
She was ok after that, however the next time A vet came out, she panicked. Over time, she was much, much better. But somehow, I do think she knew vet= feeling like crap.