Vaccines are biological products that are created in order to prepare a dog’s immune system to withstand infections. Certain bacteria and viruses are put into the vaccines, but they are modified in a way that does not cause active infection when it enters your dog’s body. Instead, the vaccine trains your dog’s immune system to recognize and destroy the real disease if your dog is ever exposed to it.
Vaccines are very effective, and while some may not prevent 100% of a disease, they can significantly reduce the severity of the disease’s clinical signs, even preventing death in some cases. Vaccines can prevent highly contagious diseases that are transmissible to other animals and people. Dogs that are very young or immunocompromised may not be able to receive vaccines, but if the adult dogs in their neighborhood are all vaccinated, then this will dramatically reduce the risk of illness for un-vaccinated dogs. This is called herd immunity.