Even if ketones, being on a ketogenic diet, or taking exogenous ketones doesn't treat or prevent you getting from the virus [COVID-19], it may help with recovery.

Basically, when you get the NLRP3 inflammasome assembling, you're promoting inflammation. BHB (ketones), in vitro and in various animal systems, have been shown to inhibit the formation of the NLRP3 inflammasome, so they decrease the amount of these pro-inflammatory cytokines.

Because there's so little known about the coronavirus, we actually don't know whether any of the ketone biology that we see in other inflammatory situations would also apply in the situations of the coronavirus because it has its own unique immune response fingerprint.