There's also the potential that a greater degree of ambient exposure (due to the virus being now basically endemic in any public shared space, used by a reasonably large public demographic) has led to exposure levels insufficient to induce illness, but sufficient to generate immune responses, leading to less naive immune reactions when people DO get subjected to disease inducing levels of virus. (You know, like from that wedding that they just *HAD* to attend)
"Herd Immunity" is not supported by current documentation-- that is not really what I am suggesting here. More "Less naive immune response". That is by no means the same thing.
Compare: You have had the flu many times in your life. If you get exposed to the flu, you will get the flu, but your body knows how to deal with flu, because it has had it before many times. With some good OTC meds, you probably are not going to die from it. / You are a native pacific islander who has never encountered flu before, and some jackass breathed H1N1 all over you. You have a very high fever, are experiencing extreme dehydration from vomiting and diarrhea, are experiencing delirium, and quite possibly could in fact die from it.
In the former, you have "Less naive immune response", and in the latter, you have "Completely naive immune response."
1st wave infection was very much "Completely naive immune response". (While common cold is ubiquitous, AND a coronavirus, COVID-19 is a vastly different beast, and the majority of the population had never been exposed to a pathogen of that family before.)