Final answer:
Secondary vaccine failure occurs when booster immunizations are not administered, leading to a decrease in immunity due to waning immune memory which boosters are intended to reinforce. The option (D) is correct.
Step-by-step explanation:
Secondary vaccine failure occurs When booster immunizations are not administered. Vaccinations rely on stimulating a primary immune response through exposure to noninfectious antigens from pathogens, which provides immune memory.
Over time, the immune memory can wane and the number of memory cells can decrease. Booster immunizations are administered to essentially re-expose the immune system to the antigen, thereby reinforcing the immune memory and maintaining a sufficient level of protection against the pathogen. Certain vaccines, like the tetanus vaccine, require boosters every ten years to mimic these repeat exposures and maintain a high level of immunity. Therefore, option (D) is correct.