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The Science of Masking Kids at School Remains Uncertain

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At the end of May, the Centers for Disease Control and Prevention published a notable, yet mostly ignored, large-scale study of COVID transmission in American schools. A few major news outlets covered its release by briefly reiterating the study’s summary: that masking then-unvaccinated teachers and improving ventilation with more fresh air were associated with a lower incidence of the virus in schools. Those are common-sense measures, and the fact that they seem to work is reassuring but not surprising. Other findings of equal importance in the study, however, were absent from the summary and not widely reported. These findings cast doubt on the impact of many of the most common mitigation measures in American schools. Distancing, hybrid models, classroom barriers, HEPA filters, and, most notably, requiring student masking were each found to not have a statistically significant benefit. In other words, these measures could not be said to be effective.

In the realm of science and public-health policy outside the U.S., the implications of these particular findings are not exactly controversial. Many of America’s peer nations around the world — including the U.K., Ireland, all of Scandinavia, France, the Netherlands, Switzerland, and Italy — have exempted kids, with varying age cutoffs, from wearing masks in classrooms. Conspicuously, there’s no evidence of more outbreaks in schools in those countries relative to schools in the U.S., where the solid majority of kids wore masks for an entire academic year and will continue to do so for the foreseeable future. These countries, along with the World Health Organization, whose child-masking guidance differs substantially from the CDC’s recommendations, have explicitly recognized that the decision to mask students carries with it potential academic and social harms for children and may lack a clear benefit. To date, the highly transmissible Delta variant has not led them to change this calculus. (Many experts I spoke with told me that while the Delta variant represents a major and concerning new development in the Covid pandemic, it probably shouldn’t change our thinking on a mask requirement for schools.)

Here in the United States, the message looks different. On July 9, a little more than a month after the study was published, the CDC released updated guidance for schools, including the recommendation that masks should be worn indoors by all individuals (age 2 and older) who are not fully vaccinated. Ten days later, the American Academy of Pediatrics took the guidance a step further and said that everyone in school over age 2 should wear masks, regardless of vaccination status. (The CDC later matched the AAP’s guidance.)

The extreme political heat around the issue of masks in schools is making it hard to have a coherent conversation about the science. At the risk of generalizing, much of blue-state America is strongly in favor of masks in schools, while much of red-state America is opposed. In Florida, Tennessee, and elsewhere, local school-board meetings are verging on violence as parents and officials fight over the question. But with tens of millions of American kids headed back to school in the fall, their parents and political leaders owe it to them to have a clear-sighted, scientifically rigorous discussion about which anti-COVID measures actually work and which might put an extra burden on vulnerable young people without meaningfully or demonstrably slowing the spread of the virus. In that context, the best practices for mask use in schools — elementary schools in particular — are much less obvious than CDC guidance and news headlines about keeping schools safe might have you believe.

The study published by the CDC was both ambitious and groundbreaking. It covered more than 90,000 elementary-school students in 169 Georgia schools from November 16 to December 11 and was, according to the CDC, the first of its kind to compare COVID-19 incidence in schools with certain mitigation measures in place to other schools without those measures. Scientists I spoke with believe that the decision not to include the null effects of a student masking requirement (and distancing, hybrid models, etc.) in the summary amounted to “file drawering” these findings, a term researchers use for the practice of burying studies that don’t produce statistically significant results. “That a masking requirement of students failed to show independent benefit is a finding of consequence and great interest,” says Vinay Prasad, an associate professor in........

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