system
Given the following post, respond as if you're commenting on Reddit:
human
What does the coronavirus vaccine effectiveness rate mean?

What does it mean that (the coronavirus) vaccine is XX% effective? 

As I understand it, after the vaccine is administered, the body produces antibodies. So why is one vaccine 60% effective and another 98% effective? Does this mean that after the administration of the former vaccine, only 60% of the patients produce antibodies? 

If so, does checking the antibody test at the appropriate time after the vaccine confirm that the person is protected and that they are in the right percentage of vaccine efficacy?
gpt
Let’s say you have two different vaccine formulations; Vaccine X and Vaccine Y, and you want to test how effective they are. 

To test the vaccines, you give 1000 people vaccine X, another 1000 people Vaccine Y, and another 1000 people a placebo. The placebo is just salt water, it does nothing. 

You follow the 3000 test subjects for a period of 6 months, and you count how many in each group get infected during that period of time.

Of the 1000 people who got the placebo, which does nothing, 100 of them eventually get infected with the virus. This forms the baseline against which you measure effectiveness of the vaccines. 

Of the 1000 people who got Vaccine X, only 5 of them get infected. 5 instead of 100 is a 95% reduction in new infections compared to placebo. So the vaccine is “95% effective at preventing infection”.

Of the 1000 people who got Vaccine Y, only 40 of them got infected. 40 instead of 100 is a 60% reduction in new infections compared to placebo. So the vaccine is “60% effective at preventing infection”.

Both vaccines are clearly effective at reducing the chance that a person will get infected, but Vaccine X has proven to be more effective than Vaccine Y.