Hard information regarding serious illness / death from Omicron is scanty; it's still early. I've read indications that the vaccines (and/or previous infection with another variant, such as Delta) do seem to provide substantial protection against serious illness/death, but I don't have a link handy. IIRC at least some of the data we're working with is based on lab experiments of how various immune system components react to Omicron, which of course is less definitive than clinical outcomes, but is worth something.
> Why would a variant which has apparently evolved to evade the vaccine continue to react to prior vaccination?
This is over-simplified. In particular, "react to prior vaccination" is conflating a bunch of different things, involving antibodies, B cells, T cells, etc. My understanding is that (1) such data as we have seems to generally point toward protection-from-infection being badly eroded but protection-from-illness/death being only moderately eroded, (2) if you're boosted, protection is less badly eroded, (3) all of this seems to fit with our general understanding of how the immune system works. I don't have the domain knowledge to unpack that further.
> Why would a variant which has apparently evolved to evade the vaccine continue to react to prior vaccination?
This is over-simplified. In particular, "react to prior vaccination" is conflating a bunch of different things, involving antibodies, B cells, T cells, etc. My understanding is that (1) such data as we have seems to generally point toward protection-from-infection being badly eroded but protection-from-illness/death being only moderately eroded, (2) if you're boosted, protection is less badly eroded, (3) all of this seems to fit with our general understanding of how the immune system works. I don't have the domain knowledge to unpack that further.