Interestingly, we found that IL-9 enhanced infection of Omicron variant of SARS-CoV-2 in ACE2.Tg mice, as indicated by loss of body weight, increased lung lesions and viral load (Fig. 1m, n; Supplementary Fig. 4d) with increased infiltration of immune cells and lung tissue damage (Supplementary Fig. 4e). This evidence concerns the gene ACE2 and infection.