Well, one important confounder is age (older people tend to be more vaccinated), but they control for that. Many other confounders cannot easily be controlled

Well, one important confounder is age (older people tend to be more vaccinated), but they control for that. Many other confounders cannot easily be controlled for, but is there any reason to assume that they're correlated with the outcome? It could be that vaccinated people are more careful in terms of meeting other people. At any rate, we had big RCTs at the beginning that demonstrated efficacy against the then prevailing variant (alpha, I assume). It'll be tricky to repeat an RCT at this stage.