But the Arizona study at the center of the CDC’s back-to-school blitz turns out to have been profoundly misleading. “You can’t learn anything about the effects of school mask mandates from this study,” Jonathan Ketcham, a public-health economist at Arizona State University, told me. His view echoed the assessment of eight other experts who reviewed the research, and with whom I spoke for this article. Masks may well help prevent the spread of COVID, some of these experts told me, and there may well be contexts in which they should be required in schools. But the data being touted by the CDC—which showed a dramatic more-than-tripling of risk for unmasked students—ought to be excluded from this debate. The Arizona study’s lead authors stand by their work, and so does the CDC. But the critics were forthright in their harsh assessments. Noah Haber, an interdisciplinary scientist and a co-author of a systematic review of COVID-19 mitigation policies, called the research “so unreliable that it probably should not have been entered into the public discourse.”…

Ketcham and others also criticized the Arizona study’s use of school-related outbreaks, rather than cases per student per week, as the relevant outcome. The authors defined an outbreak as being two or more COVID-19 cases among students or staff members at a school within a 14-day period that are epidemiologically linked. “The measure of two cases in a school is problematic,” Louise-Anne McNutt, a former Epidemic Intelligence Service officer for the CDC and an epidemiologist at the State University of New York at Albany, told me. “It doesn’t tell us that transmission occurred in school.” She pointed to the fact that, according to Maricopa County guidelines, students are considered “close contacts” of an infected student—and thus subject to potential testing and quarantine—only if they (or that infected student) were unmasked. As a result, students in Maricopa schools with mask mandates may have been less likely than students in schools without mandates to get tested following an initial exposure. This creates what’s known as a detection bias, she said, which could grossly affect the study’s findings. (Jehn and McCullough called it “highly speculative to make the assumption that identified close contacts are more likely to be tested than other students.”) McNutt believes that masks are an important prevention tool in the pandemic, but she maintained that the Arizona study doesn’t answer the specific question it purports to answer: whether mask mandates for students reduce the spread of SARS-CoV-2.