SCFAs can prime the immune system to respond to infection.13 To examine the impact of SCFA treatment on the immune response to SARS-CoV-2 infection independently of its effect on ACE2 expression, we used mice expressing both mouse and human ACE2,23 designated hACE2 mice, in which human ACE2 is constitutively expressed in epithelial cells under the K18 promoter and is thus unaffected by SCFA treatment (Figure S3A). This evidence concerns the gene ACE2 and infection.