Phil, Right,
disease and vaccines have similar effects on the immune system – they both create antibodies that decrease with time and variants, and they also create B and T cells that do not attack virus immediately upon entering the body but provide some protection against severe disease. There might be differences – which one is waning faster and how much antibodies are created, but differences are smaller than similarities. Protection from B and T-cells are hard to measure, so less is understood about them.
One difference is that disease may do invisible internal damage and thus a second disease may _sometimes_ be really bad. I know people who sneezed first time and ended up in ICU second time.
So, bottom line – if 6-8 months passed after your last immune stimulation by either of them, you should at least measure antibodies and possibly get a vaccine – and behave more responsibly.
Timing is not surprising. Overall, most similar viruses, both disease and vaccines, give protection on the order of a year. People have colds annually … I do have a feeling though how COVID differs from normal colds: neither my wife or me went to big schools or attended tishes. And kids were at home until school. When our oldest kids went to school one year, we evidently encountered “new to us” viruses – and the whole family was sick for several months, a much harder hit than an occasional cold.