In vivo, experiments have demonstrated that infection with influenza causes a slight upregulation of ACE2 expression levels (2-3 folds), and that co-infection with influenza and COVID-19 strongly upregulates ACE2 expression levels (20 folds) (Swets et al., 2022), but the studies did not compare the differences between the different influenza virus types. This evidence concerns the gene ACE2 and COVID-19.