[coffeebean]

Originally Posted by Choro&Swing

People just go about their lives in the way they would if they hadn’t got the vaccine—or a placebo. If they wore masks before, they continue the way they were; they don’t go out of their way to be reckless. After all, they may have received a placebo! This only works because there are a LOT of people doing this and because there is a serious attempt to have similar people on both sides.

In essence, this is how it works. The drug company waits until 100 people who received placebos have caught the virus. When that happens, they count how many people who received the vaccine have caught the virus. If it was 5, then the vaccine is 95% effective. Again, this only works because of statistics.

It helps that we are in an epidemic situation where a lot of people are getting sick. If it’s a vaccine for something much rarer, it takes a lot longer to figure out if the vaccine works.

Thank you. This is the answer to my question in my OP.