The COVID-19 pandemic has turned many of us into chair statisticians and epidemiologists as we look at infection trends and calculate our risk and, unsurprisingly, we often misinterpret the numbers.
This failure is also showing up in media reports, and it sows consternation among those who are truly experts in assessing the changing statistics of the pandemic.
I recently received an opinion piece from Lucy D’Agostino McGowan, assistant professor of statistics at Wake Forest University, and her colleague, Matthew Fox, professor of epidemiology at Boston University, seeking to dispel a growing misconception regarding people who are vaccinated but still infected.
Given my chair status, I couldn’t quite follow their analysis, so I called them for the layman version.
D’Agostino McGowan and Fox say some reports on infection percentages focus on the wrong denominator – the bottom number in a fraction. As a result, they say people are too alarmed at the number of fully vaccinated people showing up among those infected.
For example, the two professors noted a recent headline on infections in Los Angeles that read: “1 in 5 COVID-19 infections in Los Angeles in June were in fully vaccinated people. They weren’t afraid of the title, but they were shy about the math. By emphasizing the wrong denominator, the title 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 to get vaccinated, you don’t want to look at the percentage of sick people who have been vaccinated. You want to look at the percentage of people who were vaccinated and who got sick. “
But this misplaced emphasis continues to occur. The Washington Post reported last week on an internal CDC analysis that focused on an outbreak at a festival in Provincetown, Mass. The Post’s story began with: “A sobering scientific analysis released on Friday found that three-quarters of those infected in an explosive coronavirus outbreak fueled by the delta variant have been fully vaccinated.
It’s sobering, but D’Agostino McGowan and Fox say that’s not quite what it suggests. What matters is the number of people who attended the festival and were vaccinated, and this is not known.
CDC analysis indicated that there were 469 festival-related COVID infections which drew in thousands and 74% of those infected were fully vaccinated. “The percentage is a bit of a concern,” Fox said, “but until you know how many people were there and how many were vaccinated and not vaccinated, you can’t really tell the vaccine is less effective.”
When the CDC proposed new guidelines last week that fully vaccinated people should resume wearing masks in certain situations, it was because it found that vaccinated people who were infected could transmit the disease as easily as unvaccinated people who were infected. It is not because he found that the vaccines were less effective overall.
There is also another confusing aspect in assessing infections among vaccinees. D’Agostino McGowan noted that as an increasing percentage of the population is vaccinated, the share of those vaccinated among those infected will increase, not.
Although vaccines cannot prevent all infections, especially since the arrival of the delta variant, they are 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 to be well below 1%.
D’Agostino McGowan said the delta variant may increase the number of breakthrough infections, but these infections must be seen for what they are – very rare.
“The vaccines are very effective,” she said, “but the reports don’t make it as clear as it should be.”
Associate Opinion Writer Ned Barnett can be reached at nbarnett @ news observer.com, or 919-829-4512.