We know because we have randomized, double-blind studies and millions upon millions of doses and millions upon millions of infections. We know statistically you're far less likely to end up in the hospital or die from Covid if you're vaccinated, and have millions of examples to prove this out.

That doesn't mean you will never get sick or die from Covid if you're vaccinated, just that you're far less likely to, and as such, the vaccine works. All of these are well understood if you don't bury your head in the sand or fill it with extremist garbage.

Sure, perhaps we've come to expect too much from vaccines, like with the efficacy of things like the polio vaccine which has effectively eradicated the disease. We were all hoping for a silver bullet that can end the pandemic. But that doesn't mean the vaccine doesn't work or that we would be better off without it.