I have expressed before my befuddlement at why we are seeing the regional variation in the epidemic. Why was it so bad early on in the Northeast, but mild in most of the rest of the country? Why have cases, though thankfully not deaths, spiked in Southern and Southwestern states now? Why is there almost no epidemic in some Southeast Asia countries, while others have significant cases? Is there just an immutable law of coronavirus that it will eventually infect the same % of the population everywhere? Since it doesn’t necessarily appear to be population based, that is, based on something we might trace to genetic differences, it would seem more likely based on climate or other similar factors. To date, analyses on temperature and humidity have been ambiguous. Being indoors more because of heat or cold? Someone suggested SE Asia may have lower rates because of use of chloroquine to ward off malaria. Lockdown policies seem to make little difference. I have wondered about comparative rates of common coronavirus infection. This paper is intriguing, only because it suggests that this strain diverged somewhat recently from other coronaviruses. (Medrxiv Paper) The researchers trace the history of the evolution of this virus strain. Similar strains have been around for a while, some infecting humans, others animals, at least that we know of. One thing that hurts understanding now is the paucity of coronavirus research over the years. It just hasn’t been much of a threat. So we don’t know which strains for sure may or may not have infected humans at some point and we know very little about prevalence. I wondered if perhaps some of the close predecessors to the current strain have been circulating in Asia, at least, for some time and that the populations in SE Asia have built up some greater cross-reactive immunity. But then why did China have a relatively more severe epidemic. You would assume the same predecessor strains would circulate there. Some widespread surveys for both antibody and T cell reactivity to similar regions across the predecessor strains and other coronavirus strains would be helpful. Meantime, this is one of the most head-scratching aspects of the epidemic to me.