ACE2 and influenza: Although we observed no evidence of a significant association between influenza vaccination and the lower risk of clinical outcomes among COVID-19 patients, Hui et al. [49] found that exposure of influenza A virus may upregulate the angiotensin-converting enzyme 2 (ACE2) receptors in alveolar epithelial cells, which facilitates SARS-CoV-2 virus to enter into alveolar epithelial cells and worsen clinical outcomes of SARS-CoV-2 infection.