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?
Vaccine efficacy is: the relative change in having a disease in the vaccinated group. The comparison is against non-vaccinated people in the trial.
By its nature, researchers estimate vaccine efficacy through trials. The optimal condition is a randomised control trial. This is where trial volunteers are in groups. For example, one group receives the vaccine; the other receives a placebo.
Due to random assignment, researchers can estimate how good the vaccine is. Researchers look at what proportions of people have a disease in each group, and compare them. Different research projects may use different definitions of a case. A case is a person having the disease, or virus. They could use different tests, or confirm in labs after different…
