We're hearing 90% effective but to accurately calculate effectiveness, the you need to know how many people normally develop symptoms with covid-19. We don't truly have that data. I've read it is anywhere from 20-80% of people who catch it don't develop symptoms meaning it could be any number, no one knows.

That means no drug company truly knows how effective their vaccine is because the results of the vaccine might be the same results as among the normal population.