CTSG and influenza: To select variables that predict severe influenza, patient characteristics (influenza type, age, duration of illness, sex, ethnicity, and bacterial status), and DEGs (MPO, PRTN3, ELANE, AZU1, BPI, CEACAM6, DEFA4, LCN2, CEACAM8, CTSG, RETN, OLFM4, RNASE3, DEFA3, MMP8, HP, DEFA1B, and LTF) were included in the LASSO regression analysis.