RALEIGH, N.C. (WNCN) — It sounds like a good thing: Pfizer says its COVID-19 vaccine for children between 5 and 11 is more than 90 percent effective.
And it is.
With kids in that age group now getting vaccinated, it’s important to know what that number means.
It takes a little math to figure it out.
What the efficacy rate tells you is how much less likely a vaccinated person is to catch the disease, compared to a non-vaccinated person.
Pfizer’s efficacy rate of 90.7 percent among those children means the risk of a kid catching COVID after getting the vaccine is more than 90 percent lower than a person in the control group.
In other words, they are 10 times less likely than those getting placebos to get sick.
It’s also important to know because there are new efficacy numbers for the Johnson & Johnson vaccine for adults, with a real-world study published this week showing it has been 73.6 percent effective.
Here’s a hypothetical example of where the efficacy comes from.
Let’s say you’re conducting a trial with 200 people — way fewer than most trials. (Pfizer says there were about 3,100 children between 5 and 11 who took part in its safety study.)
Of those 200, 100 get the vaccine and the other 100 get a placebo.
Let’s say five people in the vaccine group get sick, compared to 25 in the placebo group.
To determine the efficacy, you divide the percentage of sick people among vaccinated people (5/100, or 0.05) by the percentage of sick people among the placebo group (25/100, or 0.25). Then you subtract that number from 1.
One common misconception: A vaccine efficacy of 80 percent does not mean that 20 percent of vaccinated people still get sick.
CBS 17’s Joedy McCreary has been tracking COVID-19 figures since March 2020, compiling data from federal, state, and local sources to deliver a clear snapshot of what the coronavirus situation looks like now and what it could look like in the future.