Something I always try to tell my clients is that vaccines can cause soreness and lethargy. Unfortunately, some people seem to get very upset if this happens. I occasionally have clients who bring their dogs back to me the day after being given vaccines because their pet is just laying around and not eating. In probably better than 99% of the cases, there is nothing to worry about, and they never needed to come back in the first place. So I thought I would spend a little time telling everyone about this so maybe you won't worry about your own pets and won't bug your vets.
When we give a vaccine we are stimulating the immune system. The goal is to create antibodies so that if the pet (or person) is exposed to that disease, their body will be prepared to fight it off. Vaccines stimulate the immune system in a controlled way that should cause antibody formation without causing the actual disease. However, being infected with a disease will also stimulate the immune system. When it becomes active, more happens than just antibody production. The cells that produce antibodies also release biochemicals that cause a cascade of events throughout the body. Some of these chemicals can cause inflammation (locally or systemically), raise the body temperature, and similar effects. Know how you feel sore and drowsy and just "blah" when you have a cold? That's the effect of these biochemicals. The same ones can be released by vaccines, and all it means is that the immune system is working properly.
Each individual's immune system reacts differently with illnesses and vaccines. When getting a tetanus or flu shot some people will be just a little sore at the injection site. Some people will be really sore in their whole arm. And some will run a low-grade fever and feel achy. The same thing happens with dogs and cats. Many have no side-effects whatsoever. But some can feel various degrees of lethargy and soreness for 24-48 hours.
This is not a vaccine "reaction". Instead, it is a vaccine "response", and is normal. True adverse reactions usually cause excessive vomiting, facial swelling, or hives along the body. In a true anaphylactic reaction, the pet (or person) will suffer a very quick reaction and will generally collapse as the blood pressure drops. In 12 years of practice, I have seen this happen 3-4 times out of the thousands and thousands of pets I've vaccinated.
So if your pet is sore or lethargic for a day or two after vaccines, don't worry. This is nothing to be concerned about, and if you just wait another day or so, everything should be back to normal. But if you have any doubts or questions, please contact your vet.