[rossdavidh]

Well, that's _mostly_ true, but the very fact that one virus variant displaces another over the course of a month or two, while by no means definitive proof of anything, is at the very least suggestive that for one reason or another it is better able to spread.

It _could_ be a neutral mutation that just spread due to a "founder affect" or something, but it would not be in any way surprising if a virus which is brand new in humans, evolved in the direction of adapting better to that new environment. What spreads better in bats, may not be the same as what spreads better in humans, and there is no particular reason to expect that this virus wouldn't be mutating in the direction that allows it to adapt to its new host.

Now, that could even be a good thing, if it was adapting in the direction of not triggering the host's immune system as strongly, since it is largely an immune system overreaction that kills the host. But, while by no means definitive, I don't think we can say that there is _no_ evidence to support the idea that strains have any impact on infection. the very fact that the initial strain has been largely displaced by others, suggests at least the possibility that it has.