True. Puppies receive maternal antibodies through nursing. That is why they receive so many boosters - we are not sure when exactly those maternal antibodies become ineffective, and it varies with every puppy, so the puppy shots are done in a series so that our bases are covered. When you vaccinate a puppy that still has those maternal antibodies in effect, they kill the vaccine as if the body was actually being infected by the virus. Therefore, the vaccine was useless.