There was a study about March last year where they tested 2015-2018 blood samples. About half had T cells that identified at least one of two proteins on covid. It was due to people having had the common cold from other corona viruses. So those people have at least some resistance to covid. T cells id viruses and then the body reacts. T cells are how your body stays immune to chicken pox and other things decades later, even though antibodies don't last that long. No one heard shout that study, because it implied half the people don't have a big concern.