Good news, and perhaps why so many of us experience COVID-19 without symptoms: Cross reactive immunity
From Cell, scientists looked at blood samples collected between 2015 and 2018- pre SARS-CoV-2- and detected CD4+ T cell responses in about half of the samples. They suspect that exposure to other, common cold-causing coronaviruses may be the cause. They state:
A final key issue to consider in the study of SARS-CoV-2 immunity is whether some degree of cross-reactive coronavirus immunity exists in a fraction of the human population, and whether this might influence susceptibility to COVID-19 disease.
From The Scientist, researchers at Charité University Hospital in Berlin and reported in a preprint last month, that found CD4+ T cells that recognized the spike protein in blood from 83 percent of COVID-19 patients and 34 percent of healthy people tested.