What does vaccine efficacy tell us?

On November 9, Pfizer and BioNTech impressively reported that their vaccine was found to be more than 90% effective. That’s impressive by vaccine standards — better than the average flu vaccine and the effective 50% set according to WHO. That’s good in itself as it saves thousands of people every year from hospitalization. But without diverging further into the social and economic effects of vaccination, let’s explore how we get to the number.

The study was conducted in a Random Control Trial (RCT) with over 40,000 participants from a range of backgrounds. Randomization is a fundamental…