Americans should be able to rely on the Centers for Disease Control and Prevention (CDC) for honest and accurate information about communicable illnesses and strategies for dealing with them. But time and again during the COVID-19 pandemic, CDC Director Rochelle Walensky has proven herself untrustworthy. The latest example is Walensky's slippery response to criticism of a study that she has repeatedly cited to justify the CDC's controversial recommendation that K–12 schools require students to wear face masks as a safeguard against COVID-19.
That study, which the CDC published on September 24, looked at "school-associated COVID-19 outbreaks"—defined as two or more confirmed cases among students or staff members within a 14-day period—in two Arizona counties from July 15 through August 31. "After adjusting for potential described confounders," the researchers reported, "the odds of a school-associated COVID-19 outbreak in schools without a mask requirement were 3.5 times higher than those in schools with an early mask requirement."
As I noted at the time, the study did not take into account local vaccination rates or COVID-19 safeguards that schools adopted in addition to mask mandates. The failure to consider those variables by itself makes it impossible to draw any firm conclusions about the explanation for the difference described by the researchers.
It is plausible that schools with "early mask requirements" tended to be located in neighborhoods with relatively high vaccination rates. It is also plausible that they were especially likely to take other precautions, such as improved ventilation and physical distancing. Those factors could help explain why the schools with mask mandates were less likely to report outbreaks. Since the researchers did not control for those variables, their study cannot tell us what role mask requirements played.
In a December 16 article published by The Atlantic, David Zweig noted those issues and several other potential problems with the study, including the choice of outbreaks rather than infection rates as the outcome variable, a bias in testing of "close contacts," and the fact that some schools were open twice as long as others during the study period. More generally, the scientists Zweig interviewed said the magnitude of the purported effect was highly implausible and inconsistent with other research on the benefits of masking. Noah Haber, a postdoctoral fellow at Stanford University's Meta-Research Innovation Center who co-authored a recent systematic review of research on COVID-19 mitigation measures, described the Arizona study as "so unreliable that it probably should not have been entered into the public discourse."
Walensky nevertheless latched on to the study as validation of the CDC's support for "universal masking" in K–12 schools. During a Face the Nation interview two days after the study was published, she said it "demonstrated that places that had no [mask mandates] in place were three and a half times more likely to have outbreaks than places that did have [mask mandates] in place." During a White House briefing two days later, she said "jurisdictions that had masking [requirements] early in their school year…were three and a half times less likely to have outbreaks." In a tweet that afternoon, she said the Arizona data "reinforce the benefits of masks" in "preventing #COVID19 outbreaks in schools." During another White House briefing on October 13, she reiterated that "schools without a mask requirement were three and a half times likelier to have a COVID-19 outbreak than schools that require[d] masks."
Last week Walensky took viewer questions during a Fox News segment. "You have consistently cited one study in Arizona as justification for mask mandates in schools," one viewer, Dave Joyce, noted. "Yet there's reporting in The Atlantic that shows that the study is deeply flawed. Will you follow the science and stop relying on faulty studies and end mask mandates for children in schools?"
Despite her heavy reliance on the Arizona study, Walensky did not acknowledge, let alone rebut, any of the points raised by Zweig or the experts he quoted. Instead she said this:
There have been study after study, not only in this country but in other countries, that have demonstrated that our layered prevention strategies, including masks in schools, are able to keep our schools safely open. What I would say now is that we have the capacity with vaccines available now to children above the age of 5 that we would encourage parents to get their children vaccinated. Importantly, we want to be able to keep our schools open, and the best way to do that is to use those layered prevention strategies, and that includes not only vaccinating our children and our adults but also to continue to mask, certainly in the context of this very transmissible omicron variant.
Note that Walensky refers to "layered prevention strategies" rather than mask mandates specifically. That's because the research to which she is alluding typically did not attempt to isolate the impact of mask requirements. By and large, the studies did not even compare schools with mask mandates to schools without them. As the CDC put it, the studies showed that "transmission within school settings is typically lower than—or at least similar to—levels of community transmission, when prevention strategies are in place." These studies did not show, and because of their design could not show, that masking or any other specific safeguard was necessary for schools to "safely" reopen.
When it first issued its guidance for schools, the CDC's best attempt at a more rigorous analysis was a large study of Georgia schools published in May, which found no statistically significant evidence that requiring students to wear masks reduced infection rates, even before vaccines were widely available. In a preprint study posted the same month, Brown University economist Emily Oster and four other researchers analyzed COVID-19 data from Florida, New York, and Massachusetts for the 2020–21 school year. "We do not find any correlations with mask mandates," they reported. But they noted that "all rates [were] lower in the spring, after teacher vaccination [was] underway."
In short, the CDC decided to recommend "universal masking" of K–12 students without any solid evidence that the policy had an important impact on COVID-19 transmission, let alone that its benefits outweighed the substantial burdens it imposes. The CDC is now trying to retroactively validate its decision by citing subsequent research that remains far from conclusive.
School districts that still require masking are relying on the CDC's judgment. So is the Biden administration when it argues that deviating from the agency's advice is not just unwise but illegal. Both assume that we can count on the CDC to fairly assess the scientific evidence. The agency's handling of this issue, along with its many other missteps, misstatements, and weakly justified reversals, shows we can't.