Upon stimulation with IL-12/IL-18, the percentage of Granzyme B (p = 0.007), IFN-γ (p = 0.008), TNF-α (p < 0.001) and Perforin (p = 0.03) expressing MAIT cells was significantly lower in COVID-19 patients compared to control uninfected individuals (Fig. 2A–D). Here, PRF1 is linked to COVID-19.