Do Masks Prevent COVID?

Virtually all infectious disease experts and epidemiologists would unhesitatingly answer the question posed by the headline of this article in the affirmative, and yet there are some armchair experts who demur. They seem not to grasp the maxim, “The absence of evidence is not evidence of absence.” In other words, the fact that there are no “placebo” controlled trials of the effectiveness of parachutes for people jumping out of airplanes is not evidence that they don’t work.

And yet, it’s remarkable how many people fall into that trap. A recent example is an awful, angry article about a meta-analysis performed by the Cochrane Library that, according to one journalist, showed that wearing masks doesn’t prevent respiratory diseases. The author, John Tierney, concludes:

This verdict ought to be the death knell for mask mandates, but that would require the Centers for Disease Control (CDC) and the rest of the public-health establishment to forsake ‘the science’ — and unfortunately, these leaders and their acolytes in the media seem as determined as ever to ignore actual science.

First of all, on its face, that statement is highly implausible.

Second, let’s talk about who’s ignoring the science. Beginning with the headline and subhed and extending to the quotation above from his article, Tierney distorts the findings of the Cochrane analysis.

Here are Tierney’s headline and subhed:

“Approximately Zero”

“Masks make no difference in reducing the spread of COVID, according to an extensive new review by Cochrane – the gold standard for evaluating health interventions.”

Not so fast. The Cochrane authors identify these as the “key messages” of their analysis: “We are uncertain whether wearing masks or N95/P2 respirators helps to slow the spread of respiratory viruses based on the studies we assessed,” and “[h]and hygiene programs may help to slow the spread of respiratory viruses.”

And expanding on those key messages, here are the first four sentences of the Cochrane authors’ conclusions:

The high risk of bias in the trials, variation in outcome measurement, and relatively low adherence with the interventions during the studies hampers drawing firm conclusions. There were additional RCTs [randomized, controlled trials] during the pandemic related to physical interventions but a relative paucity given the importance of the question of masking and its relative effectiveness and the concomitant measures of mask adherence which would be highly relevant to the measurement of effectiveness, especially in the elderly and in young children.

There is uncertainty about the effects of face masks. 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. (Emphasis added.)

Moreover, in the authors’ discussion of their findings concerning the effectiveness of the highest quality, most effective masks – N95/P2 respirators – there is no mention at all of COVID specifically:

N95/P2 respirators

Four studies were in health care workers, and one small study was in the community. Compared with wearing medical or surgical masks, wearing N95/P2 respirators probably makes little to no difference in how many people have confirmed flu (5 studies; 8,407 people); and may make little to no difference in how many people catch a flu-like illness (5 studies; 8,407 people), or respiratory illness (3 studies; 7,799 people). 

Michael Osterholm, the director of the Center for Infectious Disease Research and Policy (CIDRAP) and Regents Professor at the University of Minnesota, and his colleagues provide a scholarly, two-part discussion of the nuances of wearing masks to prevent respiratory infections (here and here). 

Perhaps the most persuasive evidence is found in the table below that illustrates the degree of protection against an aerosol afforded by different types of masks when worn by both source and receiver. It demonstrates “that an N95 FFR [filtering faceplate respirator, such as an N95], even if not fit-tested, offers more benefits as both source control and personal protection than any mask,” as measured by time before an infectious dose is received:

But there’s more. Much more.

As Drs. Katelyn Jetelina and Kristen Panthagani point out in an excellent article on the ability of masks to prevent the transmission of viral respiratory diseases:

Viral transmission in a population is exponential. Even if masks only reduce the risk of transmission for each individual by a small fraction, when a community masks, those small effects compound exponentially across a population, making a big dent in cases. Just like compounding interest — a small change in the percentage makes a big difference down the road.

Those observations (and the table above) provide what might be called the theoretical basis for predicting the effectiveness of masks, but the authors go on to provide actual real-world evidence of protection against COVID:

• “Mask wearing corresponded to a 19% decrease in the R(0) in one study. In other words, masks helped reduce transmission.
• “In Bangladesh villages were randomized to be provided free masks. Villages that got the intervention had more than double the mask usage than villages that didn’t (13% vs. 42%). This resulted in a 9% reduction in cases in the mask-wearing villages. 
• “In the U.S., a 10% increase in mask wearing was associated with greater control of transmission.
• “In Germany, mask mandates reduced spread by 45%.”

Finally, they discuss the palpable weaknesses of the Cochrane meta-analysis so beloved by Tierney, including:

• The trials in the Cochrane analysis “had a number of problems and, given the limited number of RCTs [randomized controlled trials] on COVID-19, do not represent the totality of evidence.”
• The Cochrane analysis encompassed trials with different viruses: “When a virus is less contagious, an effect is harder to detect. Many of the RCTs evaluated influenza, which is far less contagious than COVID-19. This means that if we combine them, the impact of masks may be underestimated. (Another scientist, separate from this review, removed the flu studies and reran the meta-analysis. He found masks protected against SARS-CoV-2.)
• The Cochrane analysis combined studies performed in widely disparate settings. “Studies ranged from suburban schools to hospital wards in high‐income countries, crowded inner city settings in low‐income countries, and an immigrant neighborhood in a high‐income country.”

There’s much more that Jetelina and Panthagani could have added, including the CDC’s compilation of studies of mask efficacy, which shows that most have found positive effects from masking. Among them is a study of an outbreak aboard the aircraft carrier U.S.S. Theodore Roosevelt, where sailors lived and worked in close quarters, where masks produced a 70% reduced risk of infection.

Another study of 33,000 pupils in eight school districts in Massachusetts found an infection rate of 11.7% for unmasked compared to 1.7% for masked children.

I could go on, but I will leave it to readers to judge (1) the efficacy of masks, and (2) Mr. Tierney’s veracity.

The bottom line: Don’t take everything you read – especially ideological journalists’ accounts of published scientific or medical articles – at face value. Be circumspect about which interpretations you accept, whose advice you take, and whether there exists a consensus among experts.

Henry I. Miller, a physician and molecular biologist, is the Glenn Swogger Distinguished Fellow at the American Council on Science and Health. He was the founding director of the FDA’s Office of Biotechnology.