That's true for every vaccine against any disease. There is always a chance that the vaccine could fail or wear off for a variety of reasons. Historically this

That's true for every vaccine against any disease. There is always a chance that the vaccine could fail or wear off for a variety of reasons. Historically this usually happens at a very low rate, but occasionally the probability of failure becomes large enough to recommend booster shots. A major source of problems in vaccine-relate discussions is using binary language. Immunity is not a binary true/false state! Vaccines do not change a Boolean state, they greatly reduce the probability of getting the disease.