How do you know the vaccine reduced severity? My household of 3 all had it and 2 weren’t aware of it. I had minor symptoms which is the only reason we found out

How do you know the vaccine reduced severity? My household of 3 all had it and 2 weren’t aware of it. I had minor symptoms which is the only reason we found out. This was back in 2020 so we were all unvaccinated. The “b comes after a, so a must imply b” fallacy seems to be everywhere and nobody is calling it out.