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What Does It Actually Mean?

Kill That Virus!Kill That Virus!

Vaccine Efficacy is a very confusing term for many people, even more so for those who have not yet been vaccinated or who do not plan to be vaccinated for some reason.

We all want to understand what vaccine efficacy means. If a vaccine against COVID-19 is 95% effective, does that mean 5% who get the vaccine will get the virus? Is a vaccine that is 95% effective a better vaccine than one that has a lower efficacy?

The short answer to these questions is no. Data suggests there is a much lower risk of infection after vaccination with any of the vaccines we have available today (as of this writing-June 2021).


Vaccine efficacy is all about risk reduction. People who get vaccinated with the Pfizer or Moderna shots will benefit from about a 95% lower risk of developing COVID-19 compared to those who are not vaccinated.

A simpler way of looking at vaccine efficacy would be if a set of twins was divided into 2 groups and one was vaccinated and the other was not, then the twin that was vaccinated would be 95% less likely to get sick than the one that was not.


For a late-stage Pfizer vaccine trial, 36,000 people were chosen to participate. One-half (18,000) received the Pfizer shot and the other 18,000 received a placebo. The result was that a total of 170 people developed symptomatic COVID-19. 162 of those people were in the placebo group and 8 in the vaccine group.

So, if we look at the odds of getting symptomatic COVID-19 after vaccination, we divide the number of vaccinated people who got sick (8) to the total number of vaccinated people (18,000) which is in reality 0.04%, not 5%. (8 divided by 18,000% =.04%).

But the above equation is also misleading. 0.04% would be an incredible vaccine efficacy only if it could be ensured that all 18,000 people were exposed to the virus. But, they were not.

The more helpful way to show vaccine efficacy for scientists is to look at the 170 people who got sick. If we compare the 162 who got sick without a vaccine to the 8 people who got infected after vaccination, the 162 in the placebo group or 95.29% got the virus. 8 of the 170 that were in the vaccine group got the virus and that is 4.7%. Round the numbers to 95% and 5% and that is where the 95% comes from.

Both the Pfizer and Moderna vaccines show an approximate 95% efficacy rate.


Even though there are currently some problems with the Johnson & Johnson vaccine, it also has a very high vaccine efficacy. The main reason the rate is lower than Pfizer and Moderna vaccines has to do with when the J&J clinical trials took place.

The Pfizer and Moderna companies completed their clinical trials before the new variants emerged from the U.K., Brazil, and South Africa. Johnson & Johnson was performing against these new coronavirus variants that appear to be more contagious and can evade vaccines to a small degree.


Clinical trials produce efficacy rates, but they cannot reveal the effectiveness of any shot. Effectiveness is how well a vaccine protects in the real world outside of the lab setting.

Fortunately, as we continue to get emerging evidence from the millions of people in our general population that have been vaccinated, it appears the vaccines are performing as well in the real world as in the lab.

Overall, the evidence thus far suggests that these vaccines can prevent both asymptomatic (without symptoms) and symptomatic infections among those vaccinated.


Vaccinate the WorldVaccinate the World

The main way to stop this pandemic is to have as many people as possible vaccinated as soon as possible.

It doesn’t matter which vaccine is available. Vaccines work! With few exceptions, polio, smallpox, and measles have been almost completely eradicated due to the vaccines developed to prevent them.

The less chance a virus gets to enter into a body, the less chance it has to mutate and develop a new strain. Wearing masks and social distancing, as well as washing hands, will at least provide some protection from those we meet who are unwilling or unable to be vaccinated.

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