> 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.
Getting booster shots doesn't simply give you more antibodies, it gives you better ones; each round finds new ways to match the spike protein.
Yesterday I read that the customised omicron specific boosters being developed by pfizer and moderna are not going ahead as the existing boosters are deemed good enough, though I can't find the link for that now.
Regarding "better antibodies", that doesn't make any sense to me. More in number perhaps, but better? It's the exact same formula being given as boosters.
https://www.tandfonline.com/doi/full/10.1080/22221751.2021.2...
Similarly, immunity to omicron might not be good at the other variants (which are worse for you), which is why reusing the existing boosters instead of making new ones isn't that bad an idea.
"One major caveat of the current study is that we used an in vitro assay system and the pseudotyped viruses but not the real viruses were used in the assay."