[RandomBK]

It depends on whether you consider vaccines and human intervention as part of the 'natural' feedback process that tends to make viruses less severe.

In other words, the high-level theory is that more severe viruses either kill their hosts, or make them so sick that they stop interacting with other potential hosts. A form of social distancing, if you will.

In both cases, the virus is triggering some form of behavior change in its host that reduces its reproduction, providing evolutionary pressure towards milder viruses.

One could argue that the development and wide distribution of a vaccine is just one form of host behavior change, and should be factored in.