PRF1 and COVID-19: Compared to NK cells from healthy donors, NK cells in both patient groups had increased expression of the cytotoxic molecules perforin and granzyme B. However, NK cells from severe COVID-19 patients had significantly impaired cytotoxic activity and produced less IFN-γ and tumor necrosis factor-alpha (TNF-α) when compared to cells from ambulant COVID-19 or FLI patients.