W/R/T your second point, one of the things I touched on was whether or not those with natural immunity are more likely to experience repeated exposure after acquiring immunity. For instance, is a healthcare worker more likely to experience "micro-doses" of virus exposure that helps consistently refresh antibody levels? This is a potential reason as some sources have stated that the third booster shot is generating 5-10x the antibody levels as observed than after the second shot, and a higher number of free antibodies should be associated with a smaller chance of re-infection.

The other potential that jumps out to me is that only targeting the spike protein is causing the drop in protection, as in theory that should be the only type of antibody a only-vaccinated person would have. The potential conclusion to draw there is that antibodies developed for other antigens on the virus may not be as effective as the spike protein, but when combined help retain that higher level of effectiveness when the spike protein mutates. If that's the case, I wonder if having a inactivated/attenuated vaccine as the "third-dose" may end up being more effective, or if the immune system would ignore the additional antigens in favor of the one it already has.

[1]: https://www.wsj.com/articles/covid-19-third-shot-booster-116...