“Don’t viruses become less lethal the more they mutate?“
Yes, yes they do. Replication is never perfect and very rarely is a mistake in favor of whatever is being replicated.
Becoming less severe, and therefor less lethal to some hosts, generally IS favorable to the virus / in its best interest.
Consider: Infectiousness is determined not only at the cellular level — a better “key” to ACE2 receptors, for example, but also by infected persons’ behavior. If the virus-caused disease slaps victims into bed quickly, they are not out and about spreading it. If it kills often, in modern societies, social activity decreases. (Even older societies would quarantine, etc.) Conversely, if the infected persons generally feel good enough, once contagious, to be out and about for at least for a few days, the virus has an advantage.
Put another way, the virus “purpose” is not to kill. It could care less. Its “purpose” is to replicate and spread to new hosts.
Ideally, an airborne virus is efficient at the cellular level and so mild that it creates some coughs and sneezes but no real discomfort or impediment to peoples’ in-person interactions.
Now, with billions of mutations occurring, we still occasionally get a combination of considerable contagiousness and lethality. The Spanish Flu is a good example, in humans. The pig plague in (mostly) Asia is an example too.
But, back to best efficiency: The G614 variant of COVID that popped up last spring is a bit more contagious at the cellular level than the earlier D(something) variant. It was also reported early on as slightly milder, though I don’t know if that held up, real world. Overall, it WAS an improvement in favor of the virus.
Generally speaking, it is true the majority of mutations have no effect or are NOT favorable to an organism, and get pushed / weeded out. Unfortunately (listen up, certain FReepers!) a few mutations do give a pathogen an advantage. When you have billions of mutations occurring, the “success” rate of the mutations does not have to be very high. “One in a billion” more than suffices.
OTOH, as I understand it, the COVID-19 vaccines target the virus’ excellent ability to unlock our ACE2 receptors. If the virus was to mutate to beat the vaccine, it will likely lose its ability to attack our ACE2 receptors. Sorry, Charlie! (Vietnam reference, applied to different commies.)