It seems plausible that the immune system requires stressors to function well, and the absence of stressors may reduce immune system "strength". Selye's general adaptation syndrome seems applicable here as a first approximation. Muscles atrophy from disuse. Why not also immune systems?

"Immunology Is Where Intuition Goes to Die"

https://www.theatlantic.com/health/archive/2020/08/covid-19-...

The immune system is never dormant. It is constantly engaged. We don't live in a sterile environment, don't eat perfect sterile food, don't breathe perfect air, etc. There are multiple aspects to our immune system, and a lot of them are being stressed daily.

It's not "immunity debt" but rather that immunity for seasonal diseases is not permanent - you lose it if you don't use it, basically, with "refresh period" about the same length as the disease cycle.

I don't have citations handy, it's just something I've seen mentioned here and there some months ago, as a possible explanation why after 2 years of lockdowns, many more people seem to catch the common seasonal diseases.