Vaccine Efficacy Estimates

What does a ‘95% effective’ vaccine mean?

News reports shared trial findings of different vaccinations against COVID-19. For example, a BBC headline on 16th November was:

Moderna: Covid vaccine shows nearly 95% protection

This article looks at what statements about ‘95% protection’ and similar claims mean. This is another way of phrasing vaccine efficacy. It also shows how to calculate approximate uncertainty around those central estimates.

What is vaccine efficacy?