I am a molecular biologist, and it kinda works this way. B cells are called memory cells because they hold onto that "memory" of the invader for a really long time. You probably haven't had an MMR or a Tetanus vaccine in 10+ years because the body is really good at remembering. But we have to get flu boosters every year because the flu mutates so rapidly that traditional b cells won't recognize the flu after a year of mutating. (RNA viruses can't correct their mutations so they change much faster than bacteria or DNA viruses). RNAi was still pretty new when I was in school and I haven't kept up with the research so I can't speak to it's effectiveness at long term immunity.