PRF1 and COVID-19: These CD8+ TTE cells are mostly cytotoxic T cells expressing cytotoxic effector genes, such as GZMA, GZMB, PRF1, and NKG7. Indeed, we found that the COVID-19 patients possessed a small CD4+ TTE cell population (mean: ~5.6%), which was barely detected in the healthy individuals (mean: ~1.3%, P = 0.0035, Wilcoxon rank sum test).