Is it possible that ncov only went to herd immunity in a subpopulation? Maybe the masks et al did work and essentially made one cohort immune, but there was a second (almost certainly lower-class) cohort which used them wrong or not at all?
Naturally the first cohort are the ones coming in for vaccines - and thus getting iatrogenically exposed to the virus. Hospitals are not biolabs and there's 0% chance they're using proper containment.
Anyway, for some reason deaths are now wholly unassociated with infections, and instead constant, similar to a factory output or hospital capacity.