Since the advent of the first vaccines, there has been some uncertainty about their effectiveness. What does it mean that a vaccine is 90% effective?

"Imagine 100 people being sick with Covid-19. "90% efficiency means that if they had been vaccinated, on average only 10 would have been sick," David Spiegelhalter, chairman of the Winton Center for Risk and Evidence Communication in Cambridge, told the Guardian.

"The effectiveness of the vaccine is relative risk reduction: Whatever your risk before, it is reduced by 90% if you are vaccinated. "There is a lot of uncertainty about this number: it does not mean that you have a 10% chance of getting Covid-19 if you get vaccinated - it is massively lower than 10%," he added.

"Researchers are evaluating the effectiveness of comparing the number of new cases in vaccinated and unvaccinated people. All volunteers receive an injection, but, randomly, either the current vaccine or a placebo. "They do not know who they are getting."

Shën.red. Placebo - a substance or treatment designed to have no therapeutic value.

To better explain how vaccines work, Spiegelhalter took the example of Pfizer / BioNTech vaccine research: with 162 cases in 22,000 people taking placebo. "Researchers randomly assigned people, so we can be sure that the vaccine caused this change."

Since 8/162 = 5%, the vaccine efficacy is estimated at 95%.

Continuing his explanation, Spiegelhalter stressed that "the evidence did not focus on assessing efficacy in preventing serious illness and death," however "the data suggest that efficacy may be even higher for these more serious outcomes."