A dog that recovers from parvovirus should remain protected for life, unless a new strain of parvovirus mutates. The parvovirus vaccine protects for a maximum of three years. A dog that was vaccinated over three years ago must have a new course of vaccines. Likewise, at times of high challenge, such as being in the house with a parvo pup, there is an argument for getting a top up dose of vaccine if the adult dog last had a booster more than a year (but less than three years) ago.