Vaccines aren’t 100% effective if that’s what you mean by work.
I can break it down for you if you like:
200 people in a test, 100 get there vaccine, 100 don’t.
Let’s say in the placebo group 50 people test positive for Covid while in the vaccine group 20 people do.
That means it has 60% efficacy.
Then out of the 50 people who tested positive in the placebo group 10 have severe symptoms but in the vaccine group only 2 have severe symptoms.
That means it’s 80% effective against severe Covid.
That’s what they mean. They don’t literally mean that one person would definitely have got severe symptoms, just by the efficacy of the vaccine you can extrapolate it.
NB- my %s aren’t correct