Actual scientists i follow on Twitter are consistent in saying there is no sign of strains with different virulence. I believe this is a myth. I would be interested to see any actual research showing otherwise, if you can find it (not NYT articles!).
There's a more infectious mutation that overtakes the original form wherever it spreads, including Asia, although it's not more severe. I guess Asian countries would have an advantage because they faced the original form first and got their act together, while Europe and the East Coast of the US faced the more infectious variant early on. (The West Coast, on the other hand, started out with the original form, probably because the virus came directly from Asia. The variant didn't even show up in the Bay Area until May, for example.)
> The D614G mutation displays only three independent emergences that qualify for inclusion in our analyses. While this limits our power to detect a statistically significant association with transmissibility, the low number of recurrent mutations leading to the D614G allele suggests that, rather than being a driver a of transmission itself, it arose early and went up in frequency by hitchhiking with one of the deepest branches in the global phylogeny as the SARS-CoV-2 population expanded.
People are still working on this, and it should become clearer in time.
This research came out months ago but no one is mentioning this.