system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly does "X% effective" mean when it comes to vaccines?

If a vaccine is 50% effective, does that mean that 50% of people won't get xyz virus if they have the vaccine, or that the vaccine only works 50% of the time the host comes in contact with the virus? Is it something else?

The websites I found for this question on google all lost me with big words I don't know within the first few paragraphs.
gpt
You can't really take vaccine statistics and apply them to a single person, thats not how it works. So you can't give any sort of percentage change to an individual's immunity; what 50% effective means is that when you compare a vaccinated group to an unvaccinated group then there is half as many infections in the vaccinated group than the control.