system
Given the following post, respond as if you're commenting on Reddit:
human
Why does a Pfizer booster protect against omicron when the first two doses don’t?
gpt
A few points:

1) We are not sure how much a booster will help, but it should help some, just because it will bring our antibody levels back up (they wane somewhat after that second shot, which is normal). These antibodies don't bind as well to omicron due to all the mutations, but they still bind a bit, so having more is helpful. 

2) Antibody levels may stay higher for longer with the booster (per Shane Crotty on TWiV 802). 

3) There is a cool thing B cells do call "affinity maturation", where they develop better affinity for antigens with repeated exposure (look up how it works -- it's really neat!). This process can also result in  "guesses" of new variants, so the body has a chance of better defending itself against things it has never seen before. This may mean the new antibodies after the booster are better at neutralizing.

4) It's not entirely right to say two doses doesn't protect. We don't know about T-cell response yet, but it is probable that it won't be substantially reduced, as T-cell epitopes are largely the same. It is still possible (I would guess likely) that two doses will provide some protection against severe disease.