Do the masks work? It’s a matter of physics, biology, and behavior

Zoom in / Asian woman in protective face mask using smartphone while moving on urban bridge in city against crowd of people

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

Adams’ advice was in line with messages from other US officials and the World Health Organization. Days later, however, US public health leaders reversed course. Mask use was once a global pandemic control strategy, but whether that strategy worked is now a matter of heated debate—especially after a major new analysis, released in January, appeared to conclude that masks remain an unproven strategy to limit the transmission of COVID -19 and other respiratory viruses.

“There is no evidence yet that masks are effective during a pandemic,” the study’s lead author, physician and epidemiologist Tom Jefferson, told an interviewer recently.

Many public health experts strongly disagree with that claim, but the study has drawn attention, in part, because of its pedigree: It was published by Cochrane, a nonprofit that aims to bring more rigorous scientific evidence to the practice of medicine. The group’s highly regarded systematic reviews influence 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, “Natural interventions to stop or reduce the spread of respiratory viruses,” is an updated version of a paper published in fall 2020. It came at a time when the debate about COVID-19 is still simmering among scientists and policymakers , and the general public.

For some, the Cochrane review provided vindication. “Mask mandates have been a failure,” conservative columnist Bret Stephens wrote in the New York Times last week. “Those skeptics who were furiously derided as bigots and occasionally censored as ‘misinformers’ for contrary orders were right.”

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 orders in an effort to contain not only COVID-19, but also other respiratory viruses, including the flu and RSV.

The polarized debate hides a darker picture. Whether or not masks “work” is a multi-layered question—one that involves a mix of physics, infectious disease biology, and human behavior. Many scientists and doctors say the Cochrane review’s findings were, in a strict sense, correct: High-quality studies known as randomized controlled trials, or RCTs, typically don’t show much benefit for mask wearers.

But if that means masks no Work is a tougher question—one that has revealed stark divisions among public health researchers.

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

However, early in the pandemic, some researchers saw evidence that SARS-CoV-2 spread through smaller particles, which can stay airborne and better slip around or through surgical and cloth masks. “Recommendations for screening masks — as many have suggested — will not reduce the transmission of SARS-CoV-2,” respiratory protection experts Lisa Brosseau and Margaret Sietsema wrote in an April 2020 article for the Center for Infectious Disease Research and Policy at 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 sweeping public recommendation issued by any government agency without a single source of data or information to support it.” he said on a podcast in June. (The Minnesota center receives funding from 3M, which makes surgical masks and respirators.)

In a recent interview with Undark, Brosseau emphasized that she believes cloth and surgical masks have some protective benefit. But she and others, including Osterholm, urged policymakers to emphasize tight-fitting respirators like N95s over looser-fitting cloth and surgical masks. This is because there is clear evidence that respirators can effectively trap these tiny particles. “A well-fitting, good-quality respirator will trap the virus, almost entirely, and greatly reduce your exposure to it,” said Linsey Marr, an engineering professor at Virginia Tech who studies airborne virus transmission.

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