Here's how they actually ran their trials. 30k people participated and either did or did not get the vaccine. They then waited for about 100 of them to develop covid symptoms and tested those people for covid. Of those who were positive, 95% of them had not taken the vaccine. They didn't test any of the other 30k people who never developed symptoms.

Even setting aside that a 100-person sample size in no other scientific study would be considered viable, they're using the fact that "only 5% of the people we tested had taken the vaccine" to claim that the vaccine is 95% effective, which is preposterous.