That’s what I assumed was behind it; this idea that immunity means perfect, lifetime ability to not be infected by a disease. But that notion is just a misunderstanding of what “immunity” means from a biological standpoint. The immune system is a collection of biological cells and functions that our bodies have developed to resist bacteria, viruses and other agents that cause disease. Vaccines basically provide instructions to the immune system so that it can prepare for better defense when it encounters an invader that it doesn’t have information about. The protection is never perfect because our individual immune systems vary in their effectiveness and health, and because some viruses and bacteria are better than others at mutating and avoiding the defensive information that the vaccines provide. Immunity provided by vaccines against most, if not all, diseases fade over time. Even vaccines against polio and measles aren’t perfect in their efficacy. If the CDC changed its definition of vaccines, it was to make it more accurate.