ACE2 and influenza: Although this mechanism may be generally relevant for respiratory viruses, the ability of influenza to up-regulate ACE2 (Smith et al., 2020; Ziegler et al., 2020)—a feature not included in the within-host model (Pinky & Dobrovolny, 2020)—could counteract this mechanism and explain the increased infectivity of SARS-CoV-2 found experimentally (Bai et al., 2021).