@sinbaje:
This "tender knot" is a localized reaction to the vaccine. It should go away on its own but can take a while; upwards of 8 weeks or more. Those of us in the vet field consider this "normal" but one has to wonder. I would certainly let your vet know for future references that he had a reaction to the vaccine as the next reaction (if he were to have one) could be more serious (ie hives, swelling, etc).
Benadryl would no longer be effective. That said - you could ask your vet about pretreating with benadryl prior to injection to help minimize any possible future reactions. You could give orally an hour or two prior to exam. Talk to your doctor re: proper dosaging and exact timing.
Best to just leave it alone and let his body deal with it.
To add:
Vaccines are given in different areas of the body. The most commonly known is that Rabies is given in the Right Rear. So that if a reaction such as a lump is formed due to the vaccine, the doctor will know which vaccine caused the reaction. It would be safe to give benadryl before each vaccine as to not take a chance but it should be known which vaccine will give a reaction to that animal.
