Coronavirus vaccines: What does 95% efficacy mean? and why in practice it may be lower

Vaccinating as many people as possible against the coronavirus is vital in order to put a “brake” on the covid-19 pandemic. But to do this, many scientists consider it necessary to have the greatest possible transparency in advance about vaccines , so as not to create “post-noise” that would reinforce anti-vaccine views.

In this case, one of the things that needs more clarification is what exactly it means that in their clinical trials the vaccines of Pfizer / BioNTech and Moderna showed – greater than expected – an efficiency of about 95% (while the Russian “Sputnki-5” on 90%) and whether this means that almost everyone who gets these vaccines will be “invulnerable” to the coronavirus.

“These are vaccines that change the terms of the game. “We were all expecting efficacy of 50% to 70%,” vaccine researcher Dr. Gregory Poland of the Mayo Clinic told the New York Times.
Laboratory efficacy is different from that of vaccines

But as NYT science biologist and author Carl Zimmer points out, a 95% efficiency in testing does not mean it will be the same in practice after mass vaccinations. Test efficiency is a statistical indicator that results from specific mathematical calculations.

The pharmaceutical company vaccinates some people, while giving others a placebo vaccine, without any participants knowing whether they have actually been vaccinated or not (“blind” test). Vaccinated and pseudo-vaccinated people then live their normal lives and the company monitors how many will develop covid-19 symptoms and be diagnosed positive for coronavirus (170 in the case of Pfizer / BioNTech). It then calculates which percentage of patients had been vaccinated and which had not, as well as the relative difference between the two percentages. In the case of both vaccines, the vast majority of those who became ill had not been vaccinated. This difference, expressed as a percentage (eg 95%), shows efficacy and is a convincing indication that the vaccine “works”

What experience shows in previous vaccines
However, as experience with previous vaccines has shown, the effectiveness of trials is different from the effectiveness of a vaccine in the real world (effectiveness) and it is important, according to experts, that these two types of efficacy are not confused. The 95% does not represent the real chance of someone becoming infected with the coronavirus, nor does it equate in practice – as many mistakenly think – to the fact that 95 out of 100 people who are vaccinated will certainly not become infected or get sick.

“Effectiveness shows how well the vaccine works outside of the real world,” said Naor Bar-Zeev, an associate professor of epidemiology at Johns Hopkins University School of Public Health in Baltimore.

The effectiveness of coronavirus vaccines in trials (efficacy) may prove to be just as impressive in practice, but based on previous vaccination experience, it will probably be somewhat less-unknown how much. This is partly because the few thousand people participating in clinical trials are not representative of the population. For example, “out there” there are people with a variety of chronic health problems that can affect the effectiveness of the vaccine.

Unknown asymptomatic vaccinated

On the other hand, the Pfizer / BioNTech and Moderna vaccine trials, as well as others, were designed to show whether they protect people from getting sick with covid-19. If a volunteer in the test had a fever or cough, get tested for coronavirus. However, the tests did not “catch” those infected with the coronavirus, but without symptoms. It is therefore possible that some people who were vaccinated in the trials became infected but without realizing it, neither themselves nor the pharmacists (who have no way of knowing, since they did not have a coronavirus test in all those who participated in the trials, but only in those who showed symptoms along the way).

If there are such cases of vaccinated asymptomatic carriers of the coronavirus, they can continue to transmit the virus, as asymptomatic ones are also known to be contagious, even if – according to some estimates – they have a lower viral load. In fact, if someone has been vaccinated and recklessly thinks that it is no longer possible to get sick or even become infected with the coronavirus, then they will relax the preventive measures, e.g. no longer wearing a mask or keeping your distance from others.

“Vaccination programs save lives”
“In this case, there could be a paradoxical situation, things could get worse,” warns Dr. Bar-Zeev, despite the vaccination.

Another crucial factor that will determine the true effectiveness of the vaccine will be what percentage of the population will do so. Scientists know that even a vaccine with amazing efficacy can make little difference in the world if only a few people are vaccinated.

“Vaccines do not save lives. “Vaccination programs save lives,” said Professor David Paltiel of Yale University School of Public Health. He led a recent modeling study published in the journal Health Affairs and concluded that, in terms of reducing cases, hospital admissions and deaths from covid-19, the extent of the vaccination program is the same. impact on vaccine efficacy in trials.

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