CD4 and COVID-19: Nevertheless, a few studies from our group and others have detected cross-reactive CD4+ and CD8+ T cells, directed toward specific sets of conserved SARS-CoV-2 epitopes, not only from unvaccinated COVID-19 patients but also from a significant proportion (~50%) of unexposed pre-pandemic healthy individuals (UPPHI) who were never exposed to SARS-CoV-2 (1, 16, 18, 20–22, 32, 33, 41–44).