Yes exactly.Interesting, that had never occurred to me. Do you mean an antihistamine so he doesn't react as much to the shot?
The treatment for a vaccine reaction, depending on severity, is an injection of diphenhydramine, with the potential addition of a steroid (dexamethasone) injection, and possible recommendation of oral diphenhydramine (Benadryl) every 8-12 hours.
We always recommend for the owners to give one or two doses (every 12 hours) leading up to the next time that vaccine is given. A note is added to the file, so we can ask if the diphen has been given. If the owner forgot (or can't) we will give the injectable form and then wait 15 min before vaccinating. Another (oral) dose can be given at home in 8-12 hours but it's rarely necessary.
Of course it's important to weigh pros and cons of each vaccine for each individual pet, but for important vaccines I can't think of any time that the pre-treating regimen has not worked.