Behind the numbers: what does it mean if a Covid vaccine has ‘90% efficacy’? | David Spiegelhalter

Confusion surrounds the vaccines' effectiveness. The leading Cambridge professor clarifies the data behind the trials

Imagine 100 people are ill with Covid-19. 90% efficacy" means if only they'd had the vaccine, on average only 10 would have got ill. Vaccine efficacy is the relative reduction in the risk: whatever your risk was before, it is reduced by 90% if you get vaccinated. There is a lot of confusion about this number: it does not mean there is a 10% chance of getting Covid-19 if vaccinated - that chance will be massively lower than 10%.

Researchers estimate efficacy by comparing numbers of new cases in vaccinated and unvaccinated people, best done through a randomised control trial". All volunteers receive an injection but, at random, either the actual vaccine or a placebo.They don't know which they are getting.