Cedars-Sinai Blog

COVID-19 Vaccine Efficacy: Clearing Up Confusion

Ethan Smith, PharmD, from Cedars-Sinai clears up COVID-19 vaccine efficacy confusion.

Before COVID-19, most of us didn't talk much about vaccine brands and efficacy. The pandemic changed that.

Vaccines made by Pfizer-BioNTech, Moderna and Johnson & Johnson are now being distributed in the U.S. and featured in the news every day, along with talk of efficacy rate. But what does efficacy really mean, and how is it measured? What's your risk of getting a serious case of COVID-19 after getting any of these shots?

We asked Ethan Smith, PharmD, Cedars-Sinai's program coordinator in Antimicrobial Stewardship, to explain.

"All these vaccines are extremely effective at preventing the really bad outcomes we can see with COVID-19."

What's the most common misunderstanding about vaccine efficacy?

Ethan Smith: If a vaccine has an efficacy rate of 66%, many people think that means they'll still have a 34% risk of getting sick with COVID-19 after being immunized. That's not true! Their risk will actually be much lower.

A 66% efficacy rate means your risk of getting sick, whatever it was before getting vaccinated, is reduced by 66%. Let's say you were at high risk to begin with, due to your age or underlying conditions. Even if your risk was 25%, which is high, the vaccine would reduce your risk of becoming sick to 8.5% (a 66% reduction from 25%).

But here's the most important thing by far: Johnson & Johnson's vaccine reduced the risk of hospitalization and death to zero during clinical trials—as good as the Pfizer and Moderna vaccines.

All these vaccines are extremely effective at preventing the really bad outcomes we can see with COVID-19. I feel confident encouraging people to take whichever is made available to them. Any of these vaccines is a game-changer for the individual, and, if enough people get immunized, the whole community.

How do scientists determine vaccine efficacy?

ES: Efficacy is determined in clinical trials, which divide participants into two groups: those getting the shot, and those who do not get the shot but instead get a placebo (inactive substance). None of the participants know which group they are in.

At the end of the trial, researchers tally how many people in the vaccinated group got sick compared to how many people in the unvaccinated group did. That's what's known as vaccine efficacy.

Let's look at a hypothetical example. Let's say 2,000 people take part in a vaccine trial. Of these, 1,000 get the vaccine and 1,000 get the placebo. Now let's say that by the end of the trial, a total of 500 people get sick: 475 from the placebo group and 25 from the vaccinated group. Expressed as a percentage, 47.5% of unvaccinated people got sick, versus just 2.5% of vaccinated people.

From there, we figure out efficacy with an easy formula: the risk in the unvaccinated group, minus the risk among the vaccinated, divided by the risk among unvaccinated.

47.5 – 2.5 / 47.5  = 0.95

Our hypothetical vaccine's efficacy is 95%.

How do I know what my individual risk is?

ES: It's hard to put a number on that, because it depends on lots of factors. For everyone eligible to get a shot, all these vaccines significantly reduce the risk of symptomatic illness, and current evidence suggests they all but eliminate the odds of ending up in the intensive care unit.

What's the difference between vaccine efficacy and vaccine effectiveness?

ES: In the media, you'll often see the terms used interchangeably. Both refer to how well the vaccine prevents COVID-19. Efficacy is the term for clinical trial results. Effectiveness is the term used for how the vaccine is performing in the real world.

Regulators continue to gather data as the vaccine is distributed among the population, which includes more diversity—including individuals who are immuno-compromised. Usually, we expect efficacy and effectiveness to be similar. That's what's happening so far with the COVID-19 vaccines, which is great news.

Anything else you'd like people to know?

ES: COVID-19 has turned out to be a dangerous, unpredictable illness and has caused a lot of suffering. The more people get vaccinated, the more we can move on with our lives, and be with our friends and family again.

Currently, the CDC does not express a preference for any specific authorized COVID-19 vaccine over another. The most important thing you can do to protect yourself, your family and your community is to get the COVID-19 vaccine that is available to you soonest, rather than waiting for a specific vaccine manufacturer. They all work and millions of people have received COVID-19 vaccines with significant monitoring to make sure that they are safe. These vaccines cannot give you COVID-19.