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Alarmed about breakthrough COVID cases? Here’s how experts say to read the numbers

·4 min read

The COVID-19 pandemic has turned many of us into armchair statisticians and epidemiologists as we pore over infection trends and calculate our risk and, not surprisingly, often misread the numbers.

That failing turns up in media reports, too, and it’s causing dismay among those who really are experts in assessing the pandemic’s shifting statistics.

I recently received an op-ed submission from Lucy D’Agostino McGowan, a Wake Forest University assistant professor of statistics, and her colleague, Matthew Fox, a professor of epidemiology at Boston University, seeking to clear up a growing misconception regarding people who are vaccinated but still become infected.

Given my armchair status, I couldn’t quite follow their analysis, so I called them up for the layman’s version.

D’Agostino McGowan and Fox say some news reports on infection percentages emphasize the wrong denominator – the bottom number in a fraction. As a result, they say people are overly alarmed by the number of fully vaccinated people who are turning up among the infected.

For instance, the two professors noted a recent headline about infections in Los Angeles that said: “1 in 5 COVID-19 infections in LA in June were in fully vaccinated people.” They weren’t scared by the headline, but they were bothered by the math. By stressing the wrong denominator, the headline leaves the impression that COVID vaccines are much less effective than they are.

D’Agostino McGowan describes the confusion this way: “If you’re trying to decide on getting vaccinated, you don’t want to look at the percentage of sick people who were vaccinated. You want to look at the percentage of people who were vaccinated and got sick.”

But that misplaced emphasis keeps happening. The Washington Post reported last week on an internal CDC analysis that focused on an outbreak at a festival in Provincetown, Mass. The Post story began with: “A sobering scientific analysis published Friday found that three-quarters of the people infected during an explosive coronavirus outbreak fueled by the delta variant were fully vaccinated.”

That sounds sobering, but D’Agostino McGowan and Fox say it’s not quite what it suggests. What matters is the number of people who attended the festival who were vaccinated, and that isn’t known.

The CDC analysis said there were 469 COVID infections tied to the festival that attracted thousands of people and 74 percent of those infected were fully vaccinated. “The percent is somewhat concerning,” said Fox, “but until you know how many people were there and how many were vaccinated and unvaccinated, you can’t really say the vaccine is less effective.”

When the CDC offered new guidance last week that fully vaccinated people should resume wearing masks in some situations, it was because it had found that vaccinated people who become infected can transmit the illness as readily as unvaccinated people who become infected. It wasn’t because it found that vaccines were less effective overall.

There is also another confusing aspect to assessing infections among the vaccinated. D’Agostino McGowan noted that as an increasing percentage of the population is vaccinated, the share of vaccinated people among the infected will go up, not down.

While the vaccines can’t prevent all infections, particularly since the arrival of the delta variant, they are providing a powerful defense. Only 25 states monitor and report breakthrough infections, but a Kaiser Family Foundation analysis of those states found the incidence of breakthrough infections was well below 1%.

D’Agostino McGowan said the delta variant may increase the number of breakthrough infections, but those infections need to be seen for what they are – very rare.

“The vaccines are very effective,” she said, “but reports are not making that as clear as it should be.”

Associate opinion editor Ned Barnett can be reached at nbarnett@ news observer.com, or 919-829-4512.

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