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“There’s still no evidence that masks are effective during a pandemic,” the study’s lead author, physician, and epidemiologist Tom Jefferson, recently told an interviewer.
Many public health experts vigorously disagree with that claim, but the study has caught attention, in part, because of its pedigree: It was published by Cochrane, a not-for-profit that aims to bring rigorous scientific evidence more squarely into the practice of medicine. The group’s highly regarded systematic reviews affect clinical practice worldwide. “It’s really our gold standard for evidence-based medicine,” said Jeanne Noble, a physician and associate professor of emergency medicine at the University of California, San Francisco. One epidemiologist described Cochrane as “the Bible.”
The new review, “Physical interventions to interrupt or reduce the spread of respiratory viruses,” is an updated version of a paper published in the fall of 2020. It dropped at a time when debates over COVID-19 are still simmering among scientists, politicians, and the broader public. //
The polarized debate conceals a murkier picture. Whether or not masks “work” is a multilayered question—one involving a mix of physics, infectious disease biology, and human behavior. Many scientists and physicians say the Cochrane review’s findings were, in a strict sense, correct: High-quality studies known as randomized controlled trials, or RCTs, don’t typically show much benefit for mask wearers. //
In a recent interview with Undark, Brosseau stressed that she thinks cloth and surgical masks have some protective benefit. But she and others, including Osterholm, have urged policymakers to emphasize tight-fitting respirators like N95s, rather than looser-fitting cloth and surgical masks. That's because there’s clear evidence that respirators can effectively ensnare those tiny particles. “A well-fitting, good quality respirator will trap the virus, almost all of it, and will greatly reduce your exposure to it,” said Linsey Marr, an engineering professor at Virginia Tech who studies the airborne transmission of viruses. //
Huang's analysis found that mask mandates were associated with substantially dampened COVID-19 spikes, although the benefit waned over time in some counties. The reason behind that waning was unclear, but could perhaps be could be due to fatigue with the mandates, the researchers suggested. Similar studies have often—but not always—found a positive effect.
Whether the masks were responsible for those benefits, though, was hard to pin down, Huang said. It’s possible that other factors—such as other policies implemented alongside mask mandates, or greater social distancing—actually kept COVID-19 rates lower, rather than the masks themselves. “I think it’s very difficult,” Huang said, “to make a causation conclusion.” //
“Strictly speaking, they're correct that there's no statistically significant effect,” said Ben Cowling, an epidemiologist at the University of Hong Kong whose research is cited in the Cochrane review. “But when you look at the totality of evidence, I think there's a pretty good indication that masks can protect people when they wear them.”
In particular, Cowling said, mechanistic studies—like those conducted with mannequins—do offer strong evidence that respirators cut down on the passage of viral particles.
Huang, the Penn biostatistician, is among others who argue that, in many RCTs examining mask use, the sample sizes are just too small. Even if masks are effective, that may not show up as a statistically meaningful result. “When the effect is moderate, or small, we really need a large sample size to find a significant difference,” said Huang. Many of these RCTs, she said, simply weren’t large enough to find some potentially meaningful signal.
And even if the effect is modest, during peak periods of a pandemic, small advantages can have a large impact by reducing the number of sick patients seeking hospital care at the same time. “From a public health perspective," said Cowling, "reducing the reproductive number by even 10 percent could be valuable." //
That's the thing,” said Shira Doron, a physician and the chief infection control officer at Tufts Medicine. A respirator, used perfectly and continuously, may work to reduce the spread of COVID-19. But if there’s a public health intervention that requires strict adherence, and almost nobody seems willing or able to follow it, is that actually an effective intervention at all? What does it even mean to say that it works? //
Cowling, who heads the Department of Epidemiology and Biostatistics at the University of Hong Kong’s School of Public Health, expressed doubts about that kind of policy. He argued that the evidence is clear that widespread masking, deployed during a pandemic surge, may help to flatten the curve and save lives. “That's the exact scenario that public health measures are designed for,” he said. But “that's not the way they've been used in the last years,” he added.
"What's happened in many parts of the world is that measures are brought in and kept in place,” Cowling said, “far longer than they're needed."