AVMA (the American Veterinary Medical Association) wrote this helpful post about what to expect after your pet gets vaccinated. Vaccinations are not completely benign: there is a small risk associated with every medical procedure, including vaccinations. These include minor side effects, including mild fever, decreased energy level and appetite, and localized swelling. More rare include the serious and potentially life threatening side effects from an allergic reaction, including vomiting and diarrhea, swollen muzzle, or collapse. The risks of not vaccinating typically far outweigh the possible side effects of vaccination, but it is always good to be aware of the risks, however small.