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A CDC study released in September, did not say that 85% of people who wear masks get infected with the coronavirus. In fact, it did not even attempt to figure out what percentage of people who wear a mask get infected with the coronavirus.
Rather, the study looked at the behavior of 154 symptomatic people who had tested positive for the coronavirus in July around the country and 160 people who reported symptoms but tested negative in July.
The study found that, of those 154 people, 85% said they had worn a mask either "always" or "often" over the 14 days prior to the onset of their illness. That's where the 85% figure comes from.
Of the 160 people in the study who had tested negative, however, 88.7% said they had worn a mask either "always" or "often." So there's really no difference between people who wore masks and those who didn't.
The suggestion is that the CDC found that 85% of all people who wear masks get the coronavirus. But the CDC was just looking at the behavior of these 314 symptomatic people who sought out testing at 11 particular sites around the country in July.
Here's how one of the co-authors, Christopher Lindsell, co-Director of the Health Data Science Center at Vanderbilt University Medical Center, described the study's data on masks.
"The data suggest that among a group of patients who are already showing symptoms that prompted them to get testing for the virus, there was no statistical evidence of a difference in mask wearing behavior between those who tested positive and those who tested negative," Lindsell said in an email. "This is very different from the question of whether wearing masks prevents you becoming infected with the virus, and it is also different to the question of how many or what percentage of people who wear masks contract the virus. The study was not designed to answer these questions."
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