That the effectiveness of a coronavirus vaccine is 95% does not mean that 5% of those vaccinated are infected
Yes, it is true the vaccine from Pfizer and BioNTec has 95% efficiency, but what exactly does that mean? What does it mean that Moderna’s is 94.5% or AstraZeneca’s is around 82.4%? Moreover, is it compatible that the Johnson & Johnson vaccine is the “great hope for the European vaccination process“and is only 66% efficient?
It is still curious that, despite the fact that we have been talking about efficiencies and their percentages for months, this concept is one of the most misunderstood of the entire pandemic. Against what is common to hear, 95% efficacy does not mean that 5 out of 100 vaccinated will contract the disease. Just look at the numbers from the Pfizer trial (which they begin to be contrasted with the data of Israel), to verify that those infected were much less: 0.04 among those vaccinated; that is, 100 times less than that supposed 5%.
Then, How should we interpret these efficiency percentages?
The efficacy of vaccines, explained
Actually, 95% efficiency refers to something else: that vaccinated people have a 95% lower risk of contracting COVID-19 than an unvaccinated person. That is, against vaccines such as flu vaccines that, each year, are around 40-60% effective, coronavirus vaccines do quite well. In fact, mRNAs have efficiencies comparable to the best vaccines we have.
However, the figure itself can be misleading because it is very difficult to compare.. Despite the war of press releases that we suffered at the time, the truth is that it is difficult to know which vaccine is more effective. In the first place, because each “efficacy” has to be understood in the light of what the clinical trial has considered a “case”.
To give us an idea, in the Pfizer and Moderna clinical trials, a case was defined as having a positive COVID-19 test and at least one symptom (even if it was mild). At the Johnson & JohnsonTo take another close example, a “case” is having a positive COVID-19 test and a moderate symptom (such as shortness of breath, abnormal blood oxygen levels, or abnormal respiratory rate) or at least two mild symptoms.
This (although, of course, they both counted the number of hospitalizations and deaths) makes it difficult to compare trials with each other. But there are even more problems because we have to take into account that each clinical trial was carried out in different countries, with people from different sociodemographic backgrounds and at different epidemiological moments.
Hence, when clinical trials are analyzed, it is done individually, one by one and according to their own terms and definitions. Also for this reason, regulatory agencies and health systems recommend some uses or others depending on this data. That is why we can say that, in general terms, the vaccines that are being approved have achieved very important results in terms of efficacy levels.
Not to mention that, to continue with the previous example, the three vaccines (Pfizer, Moderna and J&J) were 100% effective in preventing severe cases once immunity has developed (between six and seven weeks after the full regimen) and, in none of the trials, a COVID-19 vaccinated person died.
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