PRF1 and COVID-19: A recent study showed that infection and exposure to SARS-CoV-2 induced persistent memory T cells, which may be helpful in the development of a vaccine against COVID-19 [9] Activated CD8 + T cells, also called cytotoxic T cells (CTL), act by producing cytokines TNF-α and IFN-γ [10], or by releasing cytotoxic molecules such as perforin and granzyme to kill target cells [11].