On March 28, 2020, as COVID-19 cases began to shut down public life in much of the United States, then-Surgeon General Jerome Adams issued an advisory on Twitter: The general public should not wear masks. “There is scant or conflicting evidence they benefit individual wearers in a meaningful way,” he wrote.

Adams’ advice was in line with messages from other US officials and the World Health Organization. Days later, though, US public health leaders shifted course. Mask-wearing was soon a pandemic-control strategy worldwide, but whether this strategy succeeded is now a matter of heated debate—particularly after a major new analysis, released in January, seemed to conclude that masks remain an unproven strategy for curbing transmission of COVID-19 and other respiratory viruses.

“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.

For some, the Cochrane review provided vindication. “Mask mandates were a bust,” conservative columnist Bret Stephens wrote in The New York Times last week. “Those skeptics who were furiously mocked as cranks and occasionally censored as ‘misinformers’ for opposing mandates were right.”

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Meanwhile, masks continue to be recommended by the US Centers for Disease Control and Prevention, which describes them as “a critical public health tool.” And this winter, some school districts issued short-term mandates in an effort to curb not just COVID-19, but other respiratory viruses, including influenza and RSV.

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.

But whether that means masks don’t work is a tougher question—one that has revealed sharp divisions among public health researchers.

The principle behind masks is straightforward: If viruses like SAR-CoV-2 or influenza can spread when droplets or larger particles travel from one person’s nose and mouth into another person’s nose and mouth, then putting up a barrier may slow the spread. And there’s certainly evidence that surgical masks can block some relatively large respiratory droplets.

Early in the pandemic, though, some researchers saw evidence that SARS-CoV-2 was spreading via tinier particles, which can linger in the air and better slip around or through surgical and cloth masks. “Sweeping mask recommendations—as many have proposed—will not reduce SARS-CoV-2 transmission,” respiratory protection experts Lisa Brosseau and Margaret Sietsema wrote in an April 2020 article for the Center for Infectious Disease Research and Policy at the University of Minnesota.

Their colleague Michael Osterholm, a prominent epidemiologist, was more blunt: “Never before in my 45-year career have I seen such a far-reaching public recommendation issued by any governmental agency without a single source of data or information to support it,” he said on a podcast that June. (The Minnesota center receives funding from 3M, which manufactures both surgical masks and respirators.)

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.