Immunity is long-lasting because some of the many kinds of cells that make up the normal human immune system are naturally able to retain a “memory” of the defense efforts aroused by certain infections or vaccines.
There are many kinds of immune cells, including T cells and different types of white cells. Their interaction with disease is complex and involves long chains of interactions and responses.
A vaccine fools the body into thinking it is being infected and creating a defense against that organism that it keeps in its immune memory. Later, if exposed to the infection, the body fights it quickly.
For example, after an actual infection with a disease like measles, which people rarely get more than once, immune memory is created, and certain cells are ready to fight again upon reexposure. The memory remains for years.
For some vaccines, protection may last a long time, even a lifetime. But for others, vaccines may protect for only a short time.
First, if vaccines produce “long-lasting” immunity, why the necessity of boosters? Second, it is so rare as to be almost-unheard-of for anyone to have a second case of naturally-acquired measles, as natural immunity is lifelong. Third, if vaccines are so effective at protecting against “vaccine-prevantable disease”, why did 77% of those in the recent mumps epidemic show records of being fully vaccinated against mumps?
While the concept of immune function is fascinating, there is a difference between artificially acquired and naturally acquired immunity.