Here’s why you shouldn’t compare efficacy numbers for the 3 COVID-19 vaccines

RALEIGH, N.C. (WNCN) — The three COVID-19 vaccines on the market have different efficacy rates that were determined under different circumstances, making it difficult to compare those oft-misunderstood figures.

The efficacy of a vaccine measures the reduction in risk that a vaccinated person in a clinical trial has over someone who gets a placebo.

In other words, an efficacy rate of 90 percent means the risk of a person getting that vaccine is 90 percent lower than a person in the control group — meaning they are 10 times less likely than those getting placebos to get sick.

The common misconception is that a vaccine with 95 percent efficacy leaves 5 percent of vaccinated people still getting sick.

The Moderna and Pfizer vaccines, which require two doses, each had efficacies that exceeded 90 percent while the single-dose Johnson & Johnson product was lower.

“We’re told the efficacy was about 95 percent,” said Dr. David Wohl, an infectious disease specialist at the University of North Carolina School of Medicine. “What that means is all the people who got COVID-19 in those trials, 95 percent of them were in the placebo arm.”

But the trials that determined those figures were so different that it’s difficult, if not impossible, to make direct comparisons.

The Pfizer trials measured symptomatic COVID-19 a week after the second dose while that time period for the Moderna vaccine was 14 days. The Johnson & Johnson Vaccine had 85 percent efficacy against severe cases of COVID-19 four weeks after the shot and 66 percent efficacy against moderate or severe cases of the disease 28 days after inoculation.

“I think there’s a danger in comparing those two, because that is very dependent on the way the trial was run and the way you measure who got COVID,” said Dr. Cameron Wolfe, an associate professor of medicine at the Duke University School of Medicine.

“And in fact, the trials actually did them quite differently, though, measured in different people, different times of the year, in different parts of the epidemic in different countries,” he added. “And they measured COVID in different ways, believe it or not, across those different trials. They were not set up the same way. So it’s actually really difficult to sit down and look at those two statistics and compare them as easy as you might think.”

Another key issue is sample size. Wolfe says the tens of thousands of people taking part in COVID-19 vaccine trials were “far more (people) than what we ever put through standard vaccine trials.

“This is not a lack of safety whatsoever,” he added.

But there is a difference between 30,000 people in a Pfizer trial and the millions of people who have received one or both doses of the vaccine, he said, and encouraged people who had an unusual reaction to report it to the Centers for Disease Control and Prevention.

“But let’s put that in context,” he said. “We vaccinate people, we’re taking, we’re looking at risk, and we’re looking at benefit. And the risk of getting COVID still in the United States is still way higher than what we would have wanted. The risk of having a complicated COVID complication is far higher than the risk of getting significant side effects to the vaccine. That’s the calculus we need to make.”

The best vaccine, Wolfe said, is the one you can get the quickest.

“Don’t get hung up in the vaccine efficacy statistics,” he said. “They’re going to be a little bit different because the trials were different and they were measured different. The right answer is to get it into your shoulder. That’s the right answer for all of us.”

Copyright 2021 Nexstar Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.