CD8A and influenza: SCFAs and DAT produced by Bacteroidetes and/or Clostridium can enhance influenza-specific CD8+ T-cell function and type I IFN signaling in macrophages, thereby enhancing protection against influenza infection [54] As mentioned, microbiota in the lung–gut axis plays an essential role in mediating viral infections; therefore, microbiota dysbiosis may play an important role in COVID-19 infection.