system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly does it mean that a vaccine/booster etc is X% effective?
gpt
Typically they are referring to “Relative risk reduction”.  Take 100 people and give them vaccine,  100 people who don’t get it (or better,  get a placebo).  After some period of time,  see how many in each group got the virus.

Let’s say 2 in the treatment group got the ‘vid and 20 in the placebo group.

2/100=0.02

20/100=0.2

So the relative risk reduction is= (0.2-0.02)/0.2 = 0.9

So the relative risk reduction is 90%. You can think of it as having a 90% less chance of catching the virus if you get the vaccine compared to if you don’t.