Here’s Why the Science Is Clear That Masks Work

The debate over masks’ effectiveness in fighting the spread of the coronavirus intensified recently when a respected scientific nonprofit said its review of studies assessing measures to impede the spread of viral illnesses found it was “uncertain whether wearing masks or N95/P2 respirators helps to slow the spread of respiratory viruses.”

Now the organization, Cochrane, says the way it summarized the review was unclear and imprecise, and that the way some people interpreted it was wrong.

“Many commentators have claimed that a recently updated Cochrane Review shows that ‘masks don’t work’, which is an inaccurate and misleading interpretation,” Karla Soares-Weiser, the editor in chief of The Cochrane Library, said in a statement.

“The review examined whether interventions to promote mask wearing help to slow the spread of respiratory viruses,” Soares-Weiser said, adding, “given the limitations in the primary evidence, the review is not able to address the question of whether mask wearing itself reduces people’s risk of contracting or spreading respiratory viruses.”

She said that “this wording was open to misinterpretation, for which we apologize,” and that Cochrane would revise the summary.

Soares-Weiser also said, though, that one of the lead authors of the review even more seriously misinterpreted its finding on masks by saying in an interview that it proved “there is just no evidence that they make any difference.” In fact, Soares-Weiser said, “that statement is not an accurate representation of what the review found.”

Cochrane reviews are often referred to as gold standard evidence in medicine because they aggregate results from many randomized trials to reach an overall conclusion — a great method for evaluating drugs, for example, which often are subjected to rigorous but small trials. Combining their results can lead to more confident conclusions.

Masks and mask mandates have been a hot controversy during the pandemic. The flawed summary — and further misinterpretation of it — set off a debate between those who said the study showed there was no basis for relying on masks or mask mandates and those who said it did nothing to diminish the need for them.

Michael D. Brown, a doctor and academic who serves on the Cochrane editorial board and made the final decision on the review, told me the review couldn’t arrive at a firm conclusion because there weren’t enough high-quality randomized trials with high rates of mask adherence.

While the review assessed 78 studies, only 10 of those focused on what happens when people wear masks versus when they don’t, and a further five looked at how effective different types of masks were at blocking transmission, usually for health care workers. The remainder involved other measures aimed at lowering transmission, like hand washing or disinfection, while a few studies also considered masks in combination with other measures. Of those 10 studies that looked at masking, the two done since the start of the Covid pandemic both found that masks helped.

The calculations the review used to reach a conclusion were dominated by prepandemic studies that were not very informative about how well masks blocked the transmission of respiratory viruses.

For example, in one study of hajj pilgrims in Mecca, only 24.7 percent of those assigned to wear masks reported using one daily, but not all the time (while 14.3 percent in the no-mask group wore one anyway). The pilgrims then slept together, generally in tents with 50 or 100 people. Not surprisingly, given there was little difference between the two groups, researchers found no difference from mask wearing and declared their results “inconclusive.”

In another prepandemic study, college students were asked to wear masks for at least six hours a day while in their dormitories, but they were not obligated to wear them elsewhere. Researchers found no difference in infection rates between those who wore masks and those who did not. The authors noted this might be because “the amount of time masks were worn was not sufficient” — obviously, college students also go to classes and socialize where they may not wear masks.

Yet despite their inconclusiveness, the data from just these two studies accounted for roughly half of the calculations for evaluating the impact of mask wearing on transmission. The other six prepandemic studies similarly suffered from low masking adherence, limited time wearing them and, often, small sample sizes.

The only prepandemic study reviewed by Cochrane reporting high rates of mask adherence started during the worrying H1N1 season in 2009 in Germany, and found mask wearing reduced spread if started quickly after diagnosis and if a mask was worn consistently (though its sample size, too, was small).

So what we learn from the Cochrane review is that, especially before the pandemic, distributing masks didn’t lead people to wear them, which is why their effect on transmission couldn’t be confidently evaluated.

Soares-Weiser told me the review should be seen as a call for more data, and said she worried that misinterpretations of it could undermine preparedness for future outbreaks.

So let’s look more broadly at what we know about masks.

Crucially, the question of whether a mask reduces a wearer’s risk of infection is not the same as whether wearing masks slows the spread of respiratory viruses in a community.

To use randomized trials to study whether masks reduce a virus’s spread by keeping infected people from transmitting a pathogen, we need randomized comparisons of large groups, like having people in one city assigned to wear masks and not to in another. As ethically and logistically difficult as that might seem, there was one study during the pandemic in which masks were distributed, but not mandated, in some Bangladeshi villages and not others before masks were widely used in the country. Mask use increased from 10 percent to 40 percent over a two-month period in the villages where free masks were distributed. Researchers found an 11 percent reduction in Covid cases in the villages given surgical masks, with a 35 percent reduction for people over age 60.

Another pandemic study randomly distributed masks to people in Denmark over a month. About half the participants wore the masks as recommended. Of thoseassigned to wear masks, 1.8 percent became infected, compared to 2.1 percent in the no-mask group — a 14 percent reduction. But researchers could not reach a firm conclusion about whether masks were protective because there were few infections in either group and fewer than half the people assigned masks wore them.

Why aren’t there more randomized studies on masks? We could have started some in early 2020, distributing masks in some towns when they weren’t widely available. It’s a shame we didn’t. But it would have been hard and unethical to deny masks to some people once they were available to all.

Scientists routinely use other kinds of data besides randomized reviews, including lab studies, natural experiments, real-life data and observational studies. All these should be taken into account to evaluate masks.

Lab studies, many of which were done during the pandemic, show that masks, particularly N95 respirators, can block viral particles. Linsey Marr, an aerosol scientist who has long studied airborne viral transmission, told me even cloth masks that fit well and use appropriate materials can help.

Real-life data can be complicated by variables that aren’t controlled for, but it’s worth examining even if they aren’t conclusive.

Japan, which emphasized wearing masks and mitigating airborne transmission, had a remarkably low death rate in 2020 even though it did not have any shutdowns and rarely tested and traced widely outside of clusters.

David Lazer, a political scientist at Northeastern University, calculated that before vaccines were available, U.S. states without mask mandates had 30 percent higher Covid death rates than those with mandates.

Perhaps the best evidence comes from natural experiments, which study how things change after an event or intervention.

Researchers at Mass General Brigham, one of Harvard’s teaching hospital groups, found that in early 2020, before mask mandates were introduced, the infection rate among health care workers doubled every 3.6 days and rose to 21.3 percent. After universal masking was required, the rate stopped increasing, and then quickly declined to 11.4 percent.

In Germany, 401 regions introduced mask mandates at various times over three months in the spring of 2020. By carefully comparing otherwise similar places before and after mask mandates, researchers concluded that “face masks reduce the daily growth rate of reported infections by around 47 percent,” with the effect more pronounced in large cities and among older people.

Brown, who led the review’s approval process, told me that mask mandates may not be tenable now, but he has a starkly different feeling about their effect in the first year of a pandemic.

“Mask mandates, social distancing, the other shutdowns we had in terms of even restaurants and things like that: if places like New York City didn’t do that, the number of deaths would have been much higher,” he told me. “I’m very confident of that statement.”

So the evidence is relatively straightforward: Consistently wearing a mask, preferably a high-quality, well-fitting one, provides protection against the coronavirus.

It’s also true that the highly contagious Omicron variant is much harder to avoid, especially because even people masking consistently can catch it from others in their social circle. Fortunately, Omicron arrived after vaccines and treatments were available.

Then why all the fuss?

Masks have become a symbol of frustration over shortcomings in the pandemic response. Some see a lack of mask mandates or a failure to wear masks as an abandonment of the clinically vulnerable. The pandemic’s burden has indeed fallen disproportionately on them.

Others have come to think mandates represent illogical rules. To be sure, we did have many illogical rules: mandating masks outdoors and even at beaches, or wearing them to enter a restaurant but not at the table, or requiring children as young as 2 to mask in day care but not during nap time (presumably, the virus also took a nap). Some mask proponents and public health authorities have also used weak studies to make overblown or imprecise claims about masks’ effectiveness.

So how should we evaluate an interview in which the lead author of the Cochrane review, Tom Jefferson, said of masks that the review determined “there is just no evidence that they make any difference”? As for whether N95s are better than surgical masks, Jefferson said, “makes no difference — none of it.”

It’s no surprise that Jefferson says he has no faith in masks’ ability to stop the spread of Covid.

In that interview, he said there is no basis to say the coronavirus is spread by airborne transmission — despite the fact that major public health agencies have long said otherwise. He has long doubted well-accepted claims about the virus. In an article he co-wrote in April 2020, Jefferson questioned whether the Covid outbreak was a pandemic at all, rather than just a long respiratory illness season. At that point, New York City schools had been closed for a month and Covid had killed thousands of New Yorkers. When New York was preparing “M*A*S*H”-like mobile hospitals in Central Park, he said there was no point in mitigations to slow the spread.

In an editorial accompanying a 2020 version of the review — the review is in its sixth update since 2006 — Soares-Wiser noted a lack of “robust, high-quality evidence for any behavioral measure or policy” and said that “when protecting the public from harm is the objective, public health officials must act in a precautionary manner to take action even when evidence is uncertain (or not of the highest quality).”

Jefferson, however, said in the interview that “the purpose of the editorial was to undermine our work.” Soares-Wiser strongly denied this, and asserted that her warning in that editorial would apply to this update as well.

As Marr notes, a respiratory virus outbreak with even higher death rates would cut these arguments tragically short. We need to be better prepared in many ways for the next pandemic, and one way is to continue to collect data on mask wearing, despite the challenges.

That, along with an honest assessment of what was done right and what might have been done better, could go a long way in resolving people’s questions and doubts.

Masks are a tool, not a talisman or a magic wand. They have a role to play when used appropriately and consistently at the right times. They should not be dismissed or demonized.

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