For many Americans, wearing a mask has become a relic. But fighting about masks, it seems, has not.
Masking has widely been seen as one of the best COVID precautions that people can take. Still, it has sparked ceaseless arguments: over mandates, what types of masks we should wear, and even how to wear them. A new review and meta-analysis of masking studies suggests that the detractors may have a point. The paper—a rigorous assessment of 78 studies—was published by Cochrane, an independent policy institution that has become well known for its reviews. The review’s authors found “little to no” evidence that masking at the population level reduced COVID infections, concluding that there is “uncertainty about the effects of face masks.” That result held when the researchers compared surgical masks with N95 masks, and when they compared surgical masks with nothing.
On Twitter, longtime critics of masking and mandates held this up as the proof they’d long waited for. The Washington Free Beacon, a conservative outlet, quoted a researcher who has called the analysis the “scientific nail in the coffin for mask mandates.” The vaccine skeptic Robert Malone used it to refute what he called “self-appointed ‘experts’” on masking. Some researchers weighed in with more nuanced interpretations, pointing out limitations in the review’s methods that made it difficult to draw firm conclusions. Even the CDC director, Rochelle Walensky, pushed back against the paper in congressional testimony this week, citing its small sample size of COVID-specific studies. The argument is heated and technical, and probably won’t be resolved anytime soon. But the fact that the fight is ongoing makes clear that there still isn’t a firm answer to among the most crucial of pandemic questions: Just how effective are masks at stopping COVID?
An important feature of Cochrane reviews is that they look only at “randomized controlled trials,” considered the gold standard for certain types of research because they compare the impact of one intervention with another while tightly controlling for biases and confounding variables. The trials considered in the review compared groups of people who masked with those who didn’t in an effort to estimate how effective masking is at blunting the spread of COVID in a general population. The population-level detail is important: It indicates uncertainty about whether requiring everyone to wear a mask makes a difference in viral spread. This is different from the impact of individual masking, which has been better researched. Doctors, after all, routinely mask when they’re around sick patients and do not seem to be infected more often than anyone else. “We have fairly decent evidence that masks can protect the wearer,” Jennifer Nuzzo, an epidemiologist at Brown University, told me. “Where I think it sort of falls apart is relating that to the population level.”
The research on individual masking generally shows what we have come to expect: High-quality masks provide a physical barrier between the wearer and infectious particles, if worn correctly. For instance, in one study, N95 masks were shown to block 57 to 90 percent of particles, depending on how well they fit; cloth and surgical masks are less effective. The caveat is that much of that support came from laboratory research and observational studies, which don’t account for the messiness of real life.
That the Cochrane review reasonably challenges the effectiveness of population-level masking doesn’t mean the findings of previous studies in support of masking are moot. A common theme among criticisms of the review is that it considered only a small number of studies by virtue of Cochrane’s standards; there just aren’t that many randomized controlled trials on COVID and masks. In fact, most of those included in the review are about the impact of masking on other respiratory illnesses, namely the flu. Although some similarities between the viruses are likely, Nuzzo explained on Twitter, COVID-specific trials would be ideal.
The handful of trials in the review that focus on COVID don’t show strong support for masking. One, from Bangladesh, which looked at both cloth and surgical masks, found a 9 percent decrease in symptomatic cases in masked versus unmasked groups (and a reanalysis of that study found signs of bias in the way the data were collected and interpreted); another, from Denmark, suggested that surgical masks offered no statistically significant protection at all.
Criticisms of the review posit that it might have come to a different conclusion if more and better-quality studies had been available. The paper’s authors acknowledge that the trials they considered were prone to bias and didn’t control for inconsistent adherence to the interventions. “The low to moderate certainty of evidence means our confidence in the effect estimate is limited, and that the true effect may be different from the observed estimate of the effect,” they concluded. If high-quality masks worn properly work well at an individual level, after all, then it stands to reason that high-quality masks worn properly by many people in any situation should indeed provide some level of protection.
Tom Jefferson, the review’s lead author, did not respond to a request for comment. But in a recent interview about the controversy, he stood by the practical implications of the new study. “There’s still no evidence that masks are effective during a pandemic,” he said.
Squaring all of this uncertainty with the support for masking and mandates early in the pandemic is difficult. Evidence for it was scarce in the early days of the pandemic, Nuzzo acknowledged, but health officials had to act. Transmission was high, and the costs of masking were seen as low; it was not immediately clear how inconvenient and unmanageable masks could be, especially in settings such as schools. Mask mandates have largely expired in most places, but it doesn’t hurt most people to err on the side of caution. Nuzzo still wears a mask in high-risk environments. “Will that prevent me from ever getting COVID? No,” she said, but it reduces her risk—and that’s good enough.
What is most frustrating about this masking uncertainty is that the pandemic has presented many opportunities for the U.S. to gather stronger data on the effects of population-level masking, but those studies have not happened. Masking policies were made on sound but limited data, and when decisions are made that way, “you need to continually assess whether those assumptions are correct,” Nuzzo said—much like how NASA collects huge amounts of data to prepare for all the things that could go wrong with a shuttle launch. Unfortunately, she said, “we don’t have Houston for the pandemic.”
Obtaining stronger data is still possible, though it won’t be easy. A major challenge of studying the effect of population-level masking in the real world is that people aren’t good at wearing masks, which of course is a problem with the effectiveness of masks too. It would be straightforward enough if you could guarantee that participants wore their masks perfectly and consistently throughout the study period. But in the real world, masks fit poorly and slip off noses, and people are generally eager to take them off whenever possible.
Ideally, the research needed to gather strong data—about masks, and other lingering pandemic questions—would be conducted through the government. The U.K., for example, has funded large randomized controlled trials of COVID drugs such as molnupiravir. So far, that doesn’t seem to have happened in the U.S. None of the new studies on masking included in the Cochrane review were funded by the U.S. government. “The fact that we never as a country really set up studies to answer the most pressing questions is a failure,” said Nuzzo. What the CDC could do is organize and fund a research network to study COVID, much like the centers of excellence the agency has for fields such as food safety and tuberculosis.
The window of opportunity hasn’t closed yet. The Cochrane review, for all of its controversy, is a reminder that more research on masking is needed, if only to address whether pro-mask policies warrant the rage they incite. You would think that the policy makers who encouraged masking would have made finding that support a priority. “If you’re going to burn your political capital, it’d be nice to have the evidence to say that it’s necessary,” Nuzzo said.
At this point, even the strongest possible evidence is unlikely to change some people’s behavior, considering how politicized the mask debate has become. But as a country, the lack of conclusive evidence leaves us ill-prepared for the next viral outbreak—COVID or otherwise. The risk is still low, but bird flu is showing troubling signs that it could make the jump from animals to humans. If it does, should officials be telling everyone to mask up? That America has never amassed good evidence to show the effect of population-level masking for COVID, Nuzzo said, has been a missed opportunity. The best time to learn more about masking is before we are asked to do it again.