Perforin expression of CD56bright NK cells positively correlated with IL-6, Sequential Organ Failure Assessment (SOFA) scores and neutrophil count and inversely correlated with partial pressure of oxygen (PaO2) and fraction of inspired oxygen (FiO2) in COVID-19 patients [46], indicating hyperinflammed state of NK cells in severe COVID-19. Here, IL6 is linked to COVID-19.