Earlier quoted context omitted.
This is just something to think about. After I was called back in to the office and started seeing people more often I came down with what I would have sworn was covid three times. Absolutely the worst flu-like symptoms I'd ever had that I could recall, each affecting me in a novel way, but negative on covid each time. They could have been false negatives, of course, but I suspect I just hadn't been training my immun…
There is no scientific evidence for being able to "train your immune system". There is plenty of evidence the opposite happens. Covid especially can cause permanent immune damage.
So being exposed to certain pathogens can trigger a lasting effect, the length of which depends on the pathogen. If that effect has gone away or lessened severely over many months then it makes sense that only the innate immune response would be available against those pathogens, and the adaptive response would need to be learned again.