[TeMPOraL]

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.