PRF1 and COVID-19: Interestingly, a comparison of effector CD8+ T cells from age-matched healthy controls and COVID-19 patients revealed that uninfected individuals had two dominating effector cell populations, GzmA and GzmB double-positive cells and GzmA, GzmB, and perforin triple-positive cells, whereas in COVID-19 patients, the triple-positive cells completely dominated the response (Fig. S3).