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


As more Americans line up to receive their COVID-19 vaccine, there is confusion about how shots are protected against the disease.

In clinical trials, for example, Pfizer and Moderna shots showed an efficacy rate of about 95% against symptomatological COVID-19 in a laboratory setting.

This means that 5% of vaccinated people can still get sick, right?

No — It does not mean that vaccinated people are 5% more likely to get COVID-19, or 95% are protected from the disease. The data suggest that the risk of infection after vaccination is actually much lower.

Efficacy tells us about risk reduction, so people who are vaccinated against Pfizer or Moderna are about 95% less likely to develop COVID-19 than those who are not vaccinated. ..

Are you still confused? Think of it this way.

“If you can clone yourself, have one vaccinated version, and one unvaccinated version, the vaccinated version is 95%. Less likely to get sickNatalie Dean, a biostatistician specializing in infectious diseases at the University of Florida, told Bloomberg.

Decomposing math helps

About 36,000 people were involved in Pfizer Late vaccine trial About half received the shot and the other half received the placebo. In total, 170 people developed symptomatological COVID-19 — 162 in the placebo group and 8 in the vaccine group.

Therefore, in reality, the probability of developing symptomatic COVID-19 after vaccination is actually 0.04% (not 5%) after dividing the number of sick vaccinated people by the total number of vaccinated people. )is.

However, even this explanation can be misleading, according to Dr. Akiko Iwasaki, a virus expert at Yale University.

“If we were to expose all 18,000 people to the virus, yes, the effectiveness of the vaccine would be incredible, right? But they are not“Iwasaki told Business Insider. “That’s why we need to compare the vaccine and placebo groups, because the placebo group shows how much exposure and infection they would have had if they had not been immunized with the vaccine.”

It is more useful to compare 8 people who became infected after vaccination with 162 people who became ill without the vaccine. Scientists do this by finding differences in the proportion of people who get sick in the placebo and vaccine groups.

“95% is born from that,” says Iwasaki.

The same process applies to Moderna and the one-shot Johnson & Johnson vaccine.

Therefore, the Pfizer and Moderna vaccines are ” The most effective vaccine we haveBrianne Barker, a virologist at Drew University in New Jersey, told Live Science.

For comparison Influenza vaccine According to the Centers for Disease Control and Prevention, it is 40% to 60% effective, depending on the version of the virus that is prevalent in a particular year. Nevertheless, the flu vaccine prevented an estimated 7.5 million flu illnesses, 105,000 hospitalizations and 6,300 deaths during the 2019-2020 season.

According to experts, Johnson & Johnson’s COVID-19 shots are just as effective in preventing severe COVID-19 infections when compared to other coronavirus candidates. However, it’s easy to see why late-stage trials showed that shots were less effective (72%) among Americans compared to Pfizer and Moderna.

Because it had Very different clinical trials Than the shots of Pfizer and Moderna. Johnson & Johnson opposed a new coronavirus variant that appears to be more contagious and slightly evading the vaccine. Pfizer and Moderna completed clinical trials before the emergence of variants from the United Kingdom, Brazil and South Africa.

Clinical trials show the efficacy rate of the vaccine, but the “efficiency” of the shot is not known. This shows how well the vaccine can protect against COVID-19 in a real environment outside the lab. But new evidence suggests that vaccines are functioning as well, now that millions of people in the general population have been vaccinated.

A CDC study posted last week found that the risk of coronavirus infection 90% reduction Two weeks after receiving the second and final dose of Pfizer or modelna vaccine, and once among nearly 4,000 healthcare workers, first responders, and other essential workers in six states. 80% 2 weeks after.

Overall, it suggests that the vaccine can prevent both asymptomatic (symptomatic) and symptomatic infections in vaccinated people.

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