It's highly unlikely the booster trials were on eight mice.
edit: Found the source of the eight mouse thing; it's a misreading of https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-..., page 21. It's not the safety trial, it's the immunogenicity comparison on a small population of older mice previously given the primary vaccine. (The previous slide, #20, has a heading of "Pre-Clinical Studies"; #21 is describing the early studies before the human trials.)
Page six debunks the idea it's only been tested on mice entirely; it cites a trial of 512 human adults for the Moderna BA.4/BA.5 Omicron booster.