How do you find out if a vaccine will work?

Why a vaccine effectiveness study should be double-blinded, randomized controlled trial? How do you calculate the effectiveness of a vaccine and what does it mean in the real world?

Lately, two major companies (Pfzier and Moderna) announced results of their vaccine effectiveness, both claiming to be over 90% effective from interim results of their phase 3 trials. At the time of writing this, there are 13 phase 3 trials currently…