I think, you have explained herd immunity developed naturally by pandemic and manipulated by vaccination very well in detail.Once people are vaccinated they will not be trying as hard to avoid infection. So more people will become infected at some level of infection. As long as we have enough people vaccinated, that will not result in another wave of infection - because once you are vaccinated, your odds of developing enough of an infection to become infectious are much lower.
However, that does NOT mean that there's no infection at all - it just means that your body is much better at fighting it off now, using a plan of attack that often involves antibodies.
So you get vaccinated. You have antibodies for months to years. You have the ABILITY to produce those antibodies for years to decades. (Probably; no one has data going for more than 16 months so far for SARS-CoV-2, but that's the usual progression.) At first you get moderate amounts of reinfection since newly vaccinated people will be out and about in a population that still has significant infection rates. Those will produce more antibodies. As time goes on and the disease dies back, that will happen less and less often. But as it is likely to remain endemic in the world for a few years, we will still see people with antibody responses to infection challenges.
Once again - memory B cells.
Yes it does happen. Here,my concern still is, whether such immunity is acquired by developing B Cell immunological memory
or by actual presence of antibodies? Some big path lab people told me " Generally, IgG antibodies remain present in the body for several months to years after an infection for long-term protection against the offending microorganism. Pre-formed, circulating IgG antibodies are ready to attack when re-exposed to the microorganism resulting in infection prevention or a quicker immune response with less severe symptoms."
However I have a say in it:-
that long term IgG antibodies persistence in the body is meant for a purpose to deal with some chronic or latent state
of Covid Virus persisting in the body post acute infection treatment(may also be some traces of injected vaccine)?
I am not sure presence of these antibodies for prolonged time in anticipation of any future offending microorganism is
physiological or not because if any such offending microbe will infect in future Memory B Cells can instantly secrete desired
antibodies to handle that infection.Then what is the need of unnecessary maintaining antibodies by body for 6 months or so unless some infection or its traces still exist post infection for long term against which consistent antibody meadiated immune response is necessary. It made me to think about possibility of its chronic or latent stage.