Basically, and this is my moderately self-educated layperson understanding:
Immune escape is the process where viruses basically evolve to either evade antibodies that your body has created, or do things to prevent your body from creating effective antibodies in the first place. This is a really common thing with a lot of viruses, including some other coronaviruses. We were pretty lucky that, until omicron, Covid didn’t have the greatest immune escape- but now it does. For whatever reason, omicron infection alone doesn’t give you great immunity against omicron variants or other variants. Having been vaccinated before your first omicron infection helps some, but not as much as it did with earlier variants.
Now, the thing about the immune system is that even when it can’t completely stop a virus, it can often help make it more mild over time. Unfortunately, for Covid, and especially for omicron, the opposite seems to be true. On average, reinfections are worse than earlier infections. Again, the vaccine seems to make this a little better, but not a lot.
The fact that reinfections are worse isn’t unknown in other diseases either. Antibody dependent enhancement is a known thing, and we know a lot of the effects of Covid seem to come from the immune system- so maybe ADE is the cause? It seems like that’s unlikely because the vaccine worked so well- but also, it seems like there are a bunch of other diseases that get worse for someone after they’ve had Covid. That would make it seem like whatever is happening to the immune system after an infection might be bigger than just Covid itself.
There are a bunch of ideas about why this might be- including reservoirs of virus lingering in the body, damage from Covid making it harder to fight off other infections, and the lockdown just generally making our immune systems a bit less effective for lack of use. One of the ideas for why this might be happening is this idea of “t-cell exhaustion” which, as I understand it, is basically just a kind of broad term for “t cells don’t seem to be working like they should and we don’t know why” with a few specific hypotheses about the possible mechanisms. I don’t think this is something that’s like, broadly accepted and well understood yet so much as it is a plausible non-specific idea for one way to explain some observed data.
At any rate, whatever is happening seems to be related to a natural Covid infection and either the spike protein encoded by the current vaccines isn’t enough to cause the immune system problems, or vaccination doesn’t produce enough of them over a long enough period of time to cause the problem.
I know that “ the data is weird” isn’t a very satisfying answer but it seems to be where we’re at.