https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4873896/
> By far, the most widely studied trade-off involves transmission and virulence (Anderson and May, 1982; Frank, 1996; Alizon et al. 2009).
However, this is only true in the LONG RUN, when different variants have played out for a while.
In the short term, there could be a highly transimissible strain with a much higher mortality. It will eventually come to a point when the most common strains in a population are milder strains... but one of the reasons that happens is that hosts with immune systems unable to handle the strain die off... and those who survive do so because they can handle it better. So, at a later stage, it looks like the virus got milder, but:
- it doesn't generally apply to all strains
- the strains look mild later because only those for whom it was mild survive