As in its effectiveness wanes in time. I’m not saying vaccines are useless.
Smallpox vaccine (and some others) has a dual effect: preventing infection (back in the day, get it every ten years) and preventing death. The latter effect seems very durable, at least with Dryvax.
Of course CDC has discontinued Dryvax and replaced it with a supposedly safer product. Since smallpox is gone, we will hopefully never test the new stuff in the real world.