Similarly, Ng et al. indicated that intestinal microbiota might also influence the immune response to COVID-19 vaccines by modulating angiotensin-converting enzyme 2 (ACE2) expression and altering the secretion of immunoregulatory molecules such as tryptophan, SCFAs, and secondary bile acids, suggesting common microbiota-mediated mechanisms across different vaccines [46]. This evidence concerns the gene ACE2 and COVID-19.