The Cochrane Review on Masks

Cochrane reviews are usually useful to help synthesize a number of published studies. Doing so both has benefits and pitfalls. This particular study is one of the darlings of the anti-mask crowd. Most have probably not read it or do not have the skills to interpret it properly.

The authors of the study were quite clear in their conclusions that “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 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” i.e., we don’t know.

The editor-in-chief of the Cochrane library specifically commented on the people making claims that this study shows that masks don’t work.

Many commentators have claimed that a recently-updated Cochrane Review shows that ‘masks don’t work’, which is an inaccurate and misleading interpretation.

It would be accurate to say that the review examined whether interventions to promote mask wearing help to slow the spread of respiratory viruses, and that the results were inconclusive. 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. 

The review authors are clear on the limitations in the abstract: ‘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.’ Adherence in this context refers to the number of people who actually wore the provided masks when encouraged to do so as part of the intervention. For example, in the most heavily-weighted trial of interventions to promote community mask wearing, 42.3% of people in the intervention arm wore masks compared to 13.3% of those in the control arm. 

The original Plain Language Summary for this review stated that ‘We are uncertain whether wearing masks or N95/P2 respirators helps to slow the spread of respiratory viruses based on the studies we assessed.’ This wording was open to misinterpretation, for which we apologize. While scientific evidence is never immune to misinterpretation, we take responsibility for not making the wording clearer from the outset. We are engaging with the review authors with the aim of updating the Plain Language Summary and abstract to make clear that the review looked at whether interventions to promote mask wearing help to slow the spread of respiratory viruses.

Anyone trying to use this study to argue against masks either hasn’t read it, doesn’t understand the science, or is intentionally trying to be deceptive.


This gets even more interesting. I was going to start a rebuttal for the next anti-mask article from the Brownstone Institute, The Latest in the War on Science. This was an eye opener. It was co-authored by the primary author of the Cochrane Review, Tom Jefferson. Why would someone authoring the review also be writing for a very anti-science organization?

That led me to looking for conflicts of interest, which were easy to find. Several stood out that I quote:

TJ is occasionally interviewed by market research companies about phase I or II pharmaceutical products for which he receives fees.

TJ is Director Medical Affairs for BC Solutions, a market access company for medical devices in Europe.

TJ is a relator in a False Claims Act lawsuit on behalf of the United States that involves sales of Tamiflu for pandemic stockpiling. If resolved in the United States’ favor, he would be entitled to a percentage of the recovery.

The above conflicts show that he makes money related to pharmaceuticals and medical devices, which wouldn’t be needed as much if masks work.

Since 2020, TJ receives fees for articles published by The Spectator and other media outlets.

What’s to be said about The Spectator?

TJ was funded to co-author rapid reviews on the impact of Covid restrictions by Collateral Global Organisation (2021-22).

One only need to look at the types of articles he has written, oddly enough, often coauthored by Carl Heneghan, who wrote the Brownstone article with him, to see his bias.

TJ receives revenues from

Guess who the writers are on the site. The same two. One only needs to look at some of the titles on the site to see their bias. I would argue that the Cochrane Review was not a science-based approach at all. Jefferson claims that there is a war on science. I agree with that, but he’s the one attacking.

Other Articles About the Cochrane Review

Unpacking Cochrane’s Update on Masks and COVID-19 (2023)

“The DANMASK-19 study had several flaws: it was underpowered; was not able to evaluate the impact of masks as source control (i.e., filtering viral particles directly from the source, the infected wearer); used SARS-CoV-2 antibody testing to detect infection instead of antigen testing, which is used to identify acute illness (so that infection could have happened at any time in the past, not necessarily during the study period); and was conducted at a time of low SARS-CoV-2 circulation.”

“One of the important criticisms of the Cochrane review is that the role of masks as source control—beyond their role of wearer protection (personal protective equipment)—is an effect that the large RCTs that Cochrane analyzes are not good at estimating.”


“In the DANMASK-19 RCT, the authors estimated that no more than 5% of the general population used masks at the time of the study, thus masks were not a significant contribution to source control in the community.4 The study was not powered to detect a wearer protection efficacy of less than 50%.”

“There are more than a dozen excellent observational epidemiological studies that demonstrate the positive effect of masking, with very sound data and statistical methods, that did not meet the inclusion criteria of the Cochrane framework.”