I listen to TWiV, and the prominent virologists there are suggesting that we don't actually know. In some of the latest episodes, they discuss frankly the fact that yearly boosters may or may not actually have any meaningful impact for normal healthy folks. They get into the details of why we know annual influenza boosters are effective, and that we really don't know whether COVID boosters will have the same effect.
https://en.wikipedia.org/wiki/This_Week_in_Virology