In both our A/H7N9 (Fig. 8a) and COVID-19 (Fig. 8b) datasets, IL18R1 expression inversely correlated with expression of GZMA, GZMB, PRF1 (Perforin 1), EOMES, KLRG1 and CX3CR1. As IL18R1 expression in humans (Fig. 1c-e, Fig. 2, Fig. 3) and mice (Fig. 4b) is associated with disease severity, it was not surprising that GZMA, GZMB, PRF1, EOMES, KLRG1 and CX3CR1 gene expression decreased with disease severity (Fig. 8a, b). Here, PRF1 is linked to COVID-19.