My Ovine friend is correct, naturally...reverse the Q, is there an example of a virus losing virulence aside from the <story> of myxoma in ? Not that I'm aware of, certainly.
Viruses evolve quickly, even those thought to be "slow mutators" (which SARS2 isn't, really), and

they do so based on their inherent replication properties - the enzymes that copy their genomes and the nature of their genome - per se, the "environmental niche" (ie the host population, their genetics, immunity etc), and a plethora of other "selection pressures", which could

be NPIs, vaccines, drugs etc. These pressures are complex, but essentially boil down to stuff that prevents transmission to enough new hosts for the virus - the ultimate selfish gene - to thrive, or at the very least subsist. Whether a mutation can arise that will allow the virus