Analysis: 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 that 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.

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