Pfizer’s vaccine efficacy is estimated at 95 percent. Does this mean, as is sometimes heard, that five out of every 100 people vaccinated will get sick from Covid? Thankfully, this is not the way this figure should be understood. A few definitions may be helpful to avoid such misunderstandings. Worldwide, the laboratory has selected 43,000 volunteers. Half of them, chosen at random, were vaccinated. The other half were “vaccinated” with a placebo: salt water. The volunteers could not know if they had really been vaccinated. It was then waited for 170 of them to experience symptoms of Covid and for their test results to be positive. Of these, eight had been vaccinated and 162 had received the placebo. Thus, the number of vaccinated patients was twenty times less than those who were not vaccinated. The risk of getting sick if you are vaccinated is therefore 5% of the risk of getting sick if you are not vaccinated. In other words, the risk of getting sick has been reduced by 95%, which is expressed as 95% clinical efficacy. This clinical trial has to be done before the vaccine is released, because an efficacy of more than 50 percent is required to obtain market authorization: 95 percent is therefore a very good score.

We are much more interested in real-world efficacy: the question now is how much the risk of disease is reduced in the real world for a vaccinated person. It’s quite different from a clinical trial, which mainly measures an individual’s degree of protection. The real effectiveness depends on the number of people vaccinated in the population: the more people vaccinated, the less the virus circulates, and the fewer infections and therefore the fewer people who get sick. Moreover, the duration of protection provided by the vaccine, which is still poorly known, is very important in reality, whereas it is of little importance in a clinical trial, which lasts only a short time. Actual efficacy can only be assessed after the vaccine has been released, through sensitive epidemiological investigations: it will take time to know it in the case of covid vaccines.

**Benefits for all**

There are two other kinds of efficiency to be added. Let’s not forget that vaccination is first and foremost a public health measure, which aims not only to limit the risk of disease for the vaccinated individual, but also for society as a whole, a significant proportion of which is not vaccinated (sometimes for good reasons). We can then estimate the indirect effectiveness, i.e. the reduction in risk that unvaccinated individuals benefit from those who are vaccinated and who do not contaminate them. Finally, there is overall effectiveness, perhaps the most important and most difficult to estimate: the decrease in average risk in the total population (vaccinated or not) compared to what that risk would be if no one were vaccinated. So these are four different notions of efficacy. In all cases, Covid vaccines will be extremely useful even if their

overall efficiency will likely be less than 95%. Even a value of 50% would prevent half of all diseases, lead to a significant decrease in the circulation of the virus in the population and save a large number of lives. As always, one must be careful with numbers. Let’s imagine that in a population there are ten times as many vaccinated people as unvaccinated people. Imagine that the risk of disease for a vaccinated person is five times less than for a non-vaccinated person. Since there are ten times as many people who are vaccinated, the number of vaccinated patients will be twice as many as the number of non-vaccinated patients. This does not mean that vaccination is ineffective.

Do not hesitate! As soon as you have the opportunity, vaccinate yourself!

Etienne Ghys