The recent flurry of papers on omicron trying to decouple the effect of its intrinsic virulence from prior immunity have been finding that prior infection (with delta) and unboosted vaccination have comparable risk reduction file hospitalization from Covid

It’s been a couple weeks so maybe there is better, less preliminary analysis available now

On hospitalization risk in the UK: https://www.imperial.ac.uk/mrc-global-infectious-disease-ana...

On reinfection/breakthrough risk: https://www.medrxiv.org/content/10.1101/2021.12.27.21268278v...