The 95% efficacy of COVID vaccines does not mean that there is a 5% risk of infection. The reason

 

As more Americans get vaccinated against COVID-19, there is some confusion about the extent to which vaccines protect against the disease.

According to clinical tests, the Pfizer and Moderna vaccines, for example, have efficacy rates of about 95 percent against COVID-19 symptoms in laboratory tests.

Does that mean that 5 percent of people who get vaccinated could get sick?

No, nor does it mean that vaccinated people have a five percent chance of contracting COVID-19 or that 95 percent of those vaccinated are protected against infection. What the statistics suggest is that the risk of infection after getting vaccinated is actually much lower.

Efficacy speaks to reducing risk, such that those who get immunized with the Pfizer or Moderna vaccine are about 95 percent less likely to get COVID-19 compared to those who don't get vaccinated. .

Still confused? Please discuss the following.

"If you could clone yourself, and you have one version of you that is vaccinated and another version that is not vaccinated, then the vaccinated version has a 95 percent lower chance of getting sick," Natalie Dean told Bloomberg. , who works as a biological statistician at the University of Florida (UF), specializing in infectious diseases.

About 36,000 people participated in the latest clinical trials with Pfizer's vaccine, in which half were vaccinated and the other half were injected with a placebo. In total, 170 people showed symptoms of COVID-19: 162 in the placebo group and eight in the group that got vaccinated.

So, in reality, the chance of having COVID-19 symptoms after vaccination is 0.04 percent (not five percent) after dividing the number of vaccinated people who got sick by the total number of the vaccinated.

However, even this explanation can be confusing, according to Dr. Akiko Iwasaki, a virologist at Yale University.

“If you're not sure that all 18,000 people were exposed to the virus, then yes, the efficacy of the vaccine would be incredible, right? But they are not, ”Iwasaki told Business Insider. "This is why we have to compare the real vaccine with the placebo, since the placebo vaccine is what tells us how much exposure and infection there could be, if they were not vaccinated."

It is more useful to compare the eight people who became infected after being vaccinated with the 162 who became ill without a vaccine.

"That's where that 95 percent comes from," Iwasaki said.

The same process applies to the Moderna vaccine and the Johnson & Johnson vaccine that only needs one dose.

That makes the Pfizer and Moderna vaccines "one of the most effective vaccines we have," Brianne Barker, a virologist at Drew University in New Jersey , told Live Science .

By comparison, the flu vaccine is 40 to 60 percent effective, depending on the version of the virus circulating in a given year, the Centers for Disease Control and Prevention (CDC) said. However, the flu vaccine prevented about 7.5 million illnesses, 105,000 admissions, and 6,300 deaths during the 2019-2020 season.

Johnson & Johnson's COVID-19 vaccine is just as effective in preventing severe COVID-19 infections when compared to other variants of the coronavirus, experts say. However, it is not easy to understand why the vaccine has a lower efficacy rate (72 percent) among North Americans when compared to the Pfizer and Moderna vaccines.

Pfizer and Moderna completed their clinical trials before the variants emerged in the UK, Brazil and South Africa.

Although clinical tests give us the efficacy rates of vaccines, they do not reveal the "effectiveness" of immunization, which is how vaccines protect against COVID-19 in the real world outside of the laboratory. However, new evidence suggests that vaccines are working well now that millions of people have been vaccinated.

A CDC study that appeared last week found that the risk of becoming infected with coronavirus decreased by 90 percent two weeks after receiving the second and final dose of the Pfizer or Moderna vaccine, and by 80 percent two weeks later. single-dose among nearly 4,000 essential healthcare workers, rescuers and other workers in six states.

Overall, the study indicates that vaccines can prevent both asymptomatic and symptomatic infections among people who got vaccinated.

Translation of Jorge Posada

 

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