[quote] The antibodies don’t last
That's...not how completely measuring immunity to a pathogen works. The levels of circulating antibodies in the bloodstream may get lower as the infection clears from the body and remain at a "low-level" throughout, but the true strength of one's immune system lies within the adaptive immune system; specifically with memory B and T cells.
Think of antibodies as the foot-soldiers of the immune system. And in the immune system "chain of command," the B cells are their immediate superiors, and the T cells (specifically CD4/T helper cells) are the immediate superiors of the B cells. Then you have other cells such as the dendritic cells also known as antigen presenting cells (APC) which act as a sort of "military police" of the immune system, constantly patrolling for foreign invaders and "arresting them" by popping them (or portions of them) into their MHC (major histocompatibility complex) "squad cars," and then delivering them to the T cell "Battalion commanders" who then order their B cell "Company commanders" to rally the appropriate troops (read: antibodies) back into the bloodstream. If it were a scene from a film like, say, Osmosis Jones, it'd go something like this:
Dendritic Cells: (into radio) APC to cellular command, we've got a bogey approaching the system, and are in pursuit. [italic]hops into MHC squad cars and give chase to mysterious foreign invader[/italic]
T-cell Helper Officer: (over radio) Copy that. Ident intruder and report back.
Dendritic cell: (into radio) Sir, we have apprehended the intruder and are now forwarding you the ID.
T-cell Helper Officer: (over radio) 10-4. [italic]receives ID confirmation from APC unit and immediately recognizes intruder ID[/italic] (to T-Cell killer officer) Shit. It SARS-CoV-2.
T-cell Killer officer: Again?? You sure? These guy's helmets look kinda different.
T-cell Helper Officer: Positive. I'd recognize those epitope markings anywhere.
T-cell Killer Officer: Oh shit! You're right. Wow, why wouldn't they change those? We literally have a poster of those exact markings in the office.
T-cell Helper Officer: I don't know. They're retarded. Mobilize your guys, I'm gonna call down to humoral, and tell them they need to dispatch their SCV2 unit.
T-cell Killer Officer: (salutes) Aye, sir! [italic]leaves to go fuck shit up[/italic]
T Helper Cell Officer: (to other T Helper Cell officer) Get cytokines to the bronchiole epithelium. Tell them hold the line until humoral sends in their team!
T Helper Cell Officer 2: (salutes) Aye, sir!
T-cell Helper Officer: (into phone) Cellular command to humoral this is CD4-O1 reporting invasion of SARS-CoV-2 currently making their way to the bronchiole epithelium. Be advised, bogies have modified helmets but have stupidly kept the following epitope markers in tact and readily identifiable for your pursuit: S864-882. We need antigen-specific antibodies circulating yesterday!
B-cell Officer: (over phone) Copy! (to their antibody subordinates) Alright guys, those bastards we fought in February 2020 are back wearing shitty disguises on their helmets. Look for these epitope markers. Move! Move! Move!
Antibodies: (salute) Aye, sir! [italic]antibodies immediately flood the bloodstream subsequently increasing the number of circulating antibodies until virus is defeated and cleared from the system before the host experiences severe disease. Thanks to the intel they received from the memory T and B cell units, they were at an advantage and able to clear the virus quicker than before. They continue to "occupy" the bloodstream for roughly 4-6 months until B-cell commanders slowly "withdraw most of them" in segments from their post leaving only a few behind to support the dendritic MPs in their continued sentry duties of the host's immune system[/italic]
The End.
Obviously, there are far more intricacies involved, but you get the idea.
TL;DR: Being vaccinated and having prior infection confers strong memory T and B cell immunity.