My layman's understanding (and memory) goes something like this - there are 2 or 3 layers to our immune system, and an infection causes all of them to swing into action.

A vaccine triggers the antibodies, which is like layer 1. Because layer 1 is already active, the other layer/s are actually damped down or at least not triggered, which actually gives a real-life infection a much greater chance of harming or killing you.

Note the mRNA vaccines are to reduce symptoms, rather than prevent infection and transmission. They may well reduce infection and transmission BECAUSE they reduced the symptoms, so you don't cough and sneeze so much etc. But those symptoms are signs your body is fighting an infection; turning them off is like turning off the red flashing light on the dashboard and continuing to drive as normal.

So in the past they've tested ferrets (apparently they have very similar respiratory systems) and found various vaccines created a nice, robust antibody response. So the vaccine was a success. Woot.

Except when challenged with the actual wild virus, every ferret died.