After some reading, it appears it's more of a mixed bag. Evolutionary pressure from vaccines can have that effect, as appears to have been the case with Omicron, but it's not guaranteed, and some variants appear to have been more virulent than the original virus. See the "Relative Severity of Variants" section of this paper: https://www.nature.com/articles/s41579-022-00841-7?utm_sourc...