What Does Vaccine Efficacy Rate Mean?

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Dave reached out to ideastream's caller line to ask "What does the efficacy rate actually mean? When I hear the efficacy rate of the vaccine, whether it's over 90 percent or over 70 percent. I don't know exactly what that means."

Vaccine efficacy is different than its actual effectiveness, but MetroHealth’s infectious disease specialist Dr. Amy Ray said the terms are sometimes used interchangeably.

Vaccine efficacy is the percentage reduction in a disease in a group of people who received a vaccination in a clinical trial.

It is calculated by dividing the number of total COVID-19 cases in a clinical trial by the number of cases from the control group, who didn’t get the vaccine.

“Efficacy means: Does the vaccine result in a measurable response, and if so, how much of a response?” Dr. Ray said. "So it's actually measuring, quantitatively, antibody levels."

Vaccine effectiveness is different. It measures how well the vaccine works in a real-world environment.

 When people hear that Pfizer and Moderna’s vaccines are 95 percent effective, they might think they are 95 percent protected when they’re vaccinated, Ray said.

The actual rate for the individual may vary, Ray said, because each person is different.

The new variants of the virus may also have an impact on efficacy, she said.

Johnson & Johnson’s vaccine was less effective in South Africa, where there are variants of the virus, Ray said.

And while the Johnson & Johnson vaccine is given in one shot, and is about 66 percent effective against symptomatic COVID-19, its effectiveness rate goes up to about 85 percent when looking at severe illness due to infection, Ray said.

“If I were unvaccinated ... I might actually even opt for the Johnson & Johnson for a one-and-done approach. But I certainly would not allow the 95 percent efficacy of the Pfizer and Moderna dissuade me from getting immunized.”

Health experts say the Johnson & Johnson vaccine appears to be really good at keeping people alive and out of the hospital. And that efficacy of the vaccine for Johnson & Johnson is actually pretty similar to what we see with the flu vaccine.

Dr. Ray strongly recommended getting whichever vaccine you are offered first, regardless of the efficacy rate.

“Decreasing the rate of COVID-19 is the way to stop the virus. So taking the vaccine is the single best way to prevent transmission," Ray said.