You’re probably reading breakthrough COVID-19 case numbers the wrong way. Here’s why

RALEIGH, N.C. (WNCN) — Startled by headlines that show big numbers of breakthrough cases of COVID-19?

Don’t be alarmed. You just might be reading those figures the wrong way.

“Because these headlines are sort of focusing on the wrong numbers, they’re leading the general public maybe to focus on these wrong numbers,” said Dr. Lucy D’Agostino McGowan, an assistant professor of statistics at Wake Forest University.

“And I think it can certainly lead to vaccine hesitancy,” she said.

Why? Check out this simple example.

When thinking about whether you should get vaccinated, you want to know the probability of getting sick given you were vaccinated – this probability is quite low compared to the probability of getting sick given you were not vaccinated pic.twitter.com/IQUPI2gmkx

— Lucy D’Agostino McGowan (@LucyStats) August 4, 2021

One such headline claimed “Vaccinated people make up 75 percent of recent COVID-19 cases in Singapore.”

The problem, she says, is because they’re using the wrong denominator in figuring out those ratios.

“The bottom number, that denominator, is the one that I keep seeing with sort of the wrong number thrown in there,” McGowan said.

They’re dividing the number of breakthroughs by the total number of new cases in that time frame when they should be dividing it by the total number of vaccinated people.

Here’s how it works in North Carolina.

According to the most recent numbers from the state Department of Health and Human Services, nearly 7,300 breakthrough cases were reported between May and July 22.

While they account for about 7 percent of all cases during that time, that statistic is almost completely useless.

“It doesn’t tell us anything about how effective the vaccine is,” McGowan said.

Dividing that total of new cases by the number of people fully vaccinated — 4.8 million at the time the breakthroughs were counted — gives you a more useful figure, and works out to about 0.1 percent, or 1 in 1,000.

That lines up with findings from the Kaiser Family Foundation, which calls breakthrough cases “extremely rare” and says the “rate of breakthrough cases reported among those fully vaccinated is well below 1 percent in all reporting states.”

“There’s a positive outlook when you look at the numbers the right way,” McGowan said.

DHHS spokeswoman Catie Armstrong says the agency anticipates another update later this week.

But why does it take so long between updates?

The short answer is that calculating breakthrough cases is a bit more complicated than simply tallying new cases or hospital admissions. The state Division of Public Health matches information from the COVID Vaccine Management System and Federal Pharmacy Program with the database of COVID cases reported in the state.

She says that information is then studied to determine which cases meet the definition of a post-vaccination — or, breakthrough — case, and “this process can take time.”