The “press” and even the public ramblings of Fauci and the CDC are useless in understanding this.
My understanding is that your immunity does not so massively weaken. The “vaccination” is very narrow, however, so you get narrow immunity, based only on the spike. The Delta spike is different, so you really have no antibodies ready for it.
It may be, though, that your system is “active” and more alert to similar solutions, so you get some benefit. I think the same goes for getting a “booster” of the same, wrong solution.
I think what Fauci et al had patented, in all this, is that a “vaccine” could be turned quickly using genetic material from new variants. This has yet to be shown to be a reality, and I would contend, may indicate that the original took longer than publicly admitted.
Even if you could generate boosters for specific variants, manufacturing and logistical timelines would take quite a while.
Then there will be a new variant…