One thing that's never looked into is the microbiome. Most respiratory and gut viruses rely for cellular entry on some pathway that overlaps with microbiome bacterial metabolic and "immune" chemistry; thus, they only succeed if they are either passively or actively supported by these pathways.

Leaving aside the general importance of the gut microbiome in innate immune competence, which is more universal, there is a lot of regional variation in microbiotic motifs that could influence viral cellular entry fitness. Childhood diet regulates individual "default" microbiomes, with self-reinforcing effects for regionally dominant commensal strains. SARS-CoV-2 depends heavily on TMPRSS2, which is expressed everywhere but quite a bit in the mouth, to uncap the spike protein to allow for binding - has anyone looked into whether certain bacteria (or cigarette smoking, another regional distinction) makes TMPRSS2 less easy to access? Not as far as I know.