system
A chat on subreddit /r/NoStupidQuestions
human
No vaccine is a force field which prevents a virus from entering your body.

A vaccine trains your immune system to recognize and fight a virus. 

So if a virus does enter your body (in which case you have the disease and can pass it on), your immune system knows how to fight it quickly. You’ll have the disease for shorter, and it won’t impact you anywhere near as severely. 

It’s like giving your immune system years of boxing lessons.  If you take boxing lessons, some loser on the street still can get into a fight with you and punch you. But you’re better prepared to win.
gpt
That's actually a good analogy which works for vaccines administered outside of a pandemic.

When you introduce a vaccine in a midst of a pandemic, not only do many people not have enough time to develop a robust immunity but you "train the virus" if you will, to bypass vaccine immunity on people that aren't fully protected.