What does "efficacy" really mean for a vaccine?

What does efficacy really mean for a vaccine?

With the imminent arrival of Johnson & Johnson, the number of vaccines approved for administration increases to four, each with a different efficacy. Here's what it means and why the numbers aren't so easily comparable

(Image: Pixabay) Pfizer-BioNTech and Moderna almost perfect, Oxford-AstraZeneca so-so, Johnson & Johnson could do better. Looking at the numbers of the effectiveness of the Covid vaccines we currently have available (and those to come) can easily - and understandably - lead to the conclusion that there are series A drugs and series B drugs. Forgetting for a moment the question of side effects, it is in fact reasonable to think that between a vaccine whose efficacy is close to 95% and one which stops at 66%, it would be preferable, being able to do so, to choose the former. As often happens, however, things are more complicated than this: the value of effectiveness, in fact, must be understood and contextualized, and the brutal comparison of the values ​​declared by the manufacturers can often be misleading and lead to errors.

How efficacy is measured ...

The efficacy of anti-Covid vaccines is measured starting from the results of phase 3 clinical trials (the previous ones, phase 1 and 2, in fact serve to to establish only that the vaccine candidate, and more generally any drug, is safe). Phase 3 trials work like this: one half of the participants are given a placebo and the other half the candidate vaccine. After that, the volunteers are sent back to the real world and monitored over the next few months to count how many of them contract the disease.

Let's take for example the phase 3 clinical trial conducted by Pfizer: the trial included about 43 thousand people, and after the administration of the vaccine (or placebo) 170 of them contracted the disease; of these, 162 were in the placebo group, and 8 were in the vaccine group. It is from these numbers that the researchers measured an effectiveness of 95%. If all cases had registered in the placebo group, the vaccine would have been 100% effective; conversely, if they had been equally distributed in the two groups, the vaccine would have been 0% effective. Which, translated, would have meant that the vaccine did not in any way change the probability of getting sick.

... and what does it mean

In light of this, how should the value of the 'effectiveness? Having a 95% effective vaccine does not mean that out of 100 people to whom it is administered, 5 will get sick with Covid: on the contrary, the sense of the measure is related to individual protection and must be read in probabilistic terms. That is: in the case of a 95% effective vaccine, each vaccinated patient is 95% less likely than an unvaccinated subject to be infected every time he is exposed to the virus.

Why efficacy is not easily comparable

At this point we can return to the initial question: how much can we trust the declared efficacy values ​​to draw up a hypothetical ranking of the quality of the vaccines we have available? Not too much, according to what we know. The crucial point, in fact, lies in the fact that each clinical trial is a story unto itself, since the trials were conducted at different times and places.

Pfizer and Moderna, for example, have conducted their own trials mainly in the United States during last summer, at a time when the circulation of the virus was all in all under control. Johnson & Johnson, on the other hand, conducted the Phase 3 clinical trial between autumn and winter, when the numbers of infections were much higher, and mainly in South Africa and Brazil, where variants of the virus other than the one for which the vaccine was developed (just think that 67% of the infections found during the trial in South Africa were related to variant B.1.351). Nonetheless, the vaccine proved to be 64% effective: a very good figure, if we consider, for example, that the 2018 flu vaccine was less than 50% effective and still proved useful in reducing the number. of deaths and hospitalizations.

The efficacy measured by clinical trials is a value that must be read carefully: a precise comparison would only make sense if the trials were conducted in exactly the same way, with the same criteria of inclusion, in the same population and at the same time. A more precise estimate will perhaps only be given by the data on real efficacy, that is, the one measured starting from the administrations carried out in the general population.

Alternative parameters

The efficacy, however, it is not the only quantitative value that allows us to evaluate the goodness of a vaccine. Many experts even argue that it is not even the best, since prevention tout court from infection is not the only parameter to be evaluated. The goal of vaccinations, especially in an emergency situation like the one we are facing, is not necessarily to arrive at the scenario in which there are no more infections, but rather, and more realistically, to reduce hospitalizations, the onset of serious symptoms. and deaths. And if you look at things from this point of view, the scenario changes a lot: all the vaccines we have available, and hopefully even those that will arrive, are extremely powerful in reducing hospitalizations, severe symptoms and deaths. In all clinical trials, of all vaccines, there were infections in both the placebo and the immunized groups; but unlike placebo, no infected patient in the vaccinated group developed severe symptoms, ended up in hospital or lost their lives.

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