Masks - randomized controlled trials

Physical interventions to interrupt or reduce the spread of respiratory viruses
Jefferson et al.
This is an update of a Cochrane Review last published in 2020. We include results from studies from the current COVID‐19 pandemic.

Selection criteria:
We included randomised controlled trials (RCTs) and cluster‐RCTs investigating physical interventions (screening at entry ports, isolation, quarantine, physical distancing, personal protection, hand hygiene, face masks, glasses, and gargling) to prevent respiratory virus transmission.

Main results:
We included 11 new RCTs and cluster‐RCTs (610,872 participants) in this update, bringing the total number of RCTs to 78. Six of the new trials were conducted during the COVID‐19 pandemic…

Wearing masks in the community probably makes little or no difference to the outcome of laboratory‐confirmed influenza/SARS‐CoV‐2 compared to not wearing masks…

We are very uncertain on the effects of N95/P2 respirators compared with medical/surgical masks on the outcome of clinical respiratory illness…The use of a N95/P2 respirators compared to medical/surgical masks probably makes little or no difference for the objective and more precise outcome of laboratory‐confirmed influenza infection…

One previously reported ongoing RCT has now been published and observed that medical/surgical masks were non‐inferior to N95 respirators in a large study of 1009 healthcare workers in four countries providing direct care to COVID‐19 patients.

Authors’ conclusions:
The pooled results of RCTs did not show a clear reduction in respiratory viral infection with the use of medical/surgical masks. There were no clear differences between the use of medical/surgical masks compared with N95/P2 respirators in healthcare workers when used in routine care to reduce respiratory viral infection…



From the Proceedings of the National Academy of Sciences.

We directly analyze the effect of mask wearing on SARS-CoV-2 transmission, drawing on several datasets covering 92 regions on six continents, including the largest survey of wearing behavior (𝑛=�= 20 million) [F. Kreuter et al. , Home - UMD Global CTIS Open Data (2020)]. Using a Bayesian hierarchical model, we estimate the effect of mask wearing on transmission, by linking reported wearing levels to reported cases in each region, while adjusting for mobility and nonpharmaceutical interventions (NPIs), such as bans on large gatherings. Our estimates imply that the mean observed level of mask wearing corresponds to a 19% decrease in the reproduction number R . We also assess the robustness of our results in 60 tests spanning 20 sensitivity analyses. In light of these results, policy makers can effectively reduce transmission by intervening to increase mask wearing.

From the US Navy about a covid breakout on the USS Theodore Roosevelt

…Service members who reported taking preventive measures had a lower infection rate than did those who did not report taking these measures (e.g., wearing a face covering, 55.8% versus 80.8%; avoiding common areas, 53.8% versus 67.5%; and observing social distancing, 54.7% versus 70.0%, respectively)…

Masks work significantly better than denial.


“Very uncertain”? “Probably”? Doesn’t sound very conclusive to me.



Uncertain: not able to be relied on; not known or definite.

Very uncertain: we have no idea what the hell we’re talking about.


True. At the same time, as the authors wrote, “The pooled results of RCTs did not show a clear reduction in respiratory viral infection with the use of medical/surgical masks.”

As has been pointed out in numerous threads, RCTs (randomized controlled trials) are the ‘gold standard’.



Most important and relevant section of the above study:

The evidence summarised in this review on the use of masks is largely based on studies conducted during traditional peak respiratory virus infection seasons up until 2016. Two relevant randomised trials conducted during the COVID‐19 pandemic have been published, but their addition had minimal impact on the overall pooled estimate of effect.

In other words, the results of this study have largely nothing to do with Covid-19.

No. They are saying that the results from the covid studies weren’t different than the previous work and so didn’t change the results (“had minimal impact on the overall pooled estimate”).


I even know someone in total denial of how badly educated he is online reading charlatans who has not had Covid yet. He wears a mask. Now without the vaccines he is stuck wearing a mask.

The gold standard is dependent on the RCT being performed adequately. I find meta-analyses to be very difficult to critically review because many studies are being summarized in the review and it is extremely time consuming to quality check each one. One typically has to assume that the authors doing the review did their due diligence, otherwise it is “garbage in, garbage out”. I’ll give you an example of why I have doubts about your linked meta-review. This is Table 1 from your linked paper. One of the studies they used is Afelali 2020. I picked this on because it had one of the strongest findings against the usefulness of masks and was a major contributor (due to the number of subjects tested) to the conclusion that masks have no benefits.

Afelali 2020 studied mask use during the Hajj in Mecca, Saudi Arabia. They had one group use masks (intervention group) and a control group that did not use masks. To make a long story short, this is what the authors themselves state in the discussion:

"The most important limitation of this RCT was that despite much effort to encourage adherence with our protocol, compliance was limited. " Facemask against viral respiratory infections among Hajj pilgrims: A challenging cluster-randomized trial

Oh oh. How valid can the experiment be if your subjects are NOT following the rules? More detail is provided in the Results section:

“Overall facemask use was low, even in the intervention tents, with only 24.7% of participants using facemasks daily. Conversely, in the control tents 14.3% participants used facemasks daily. More participants in the intervention group had used a facemask anytime in the weeks before the actual Hajj compared to those assigned in the control group (27.4% vs. 24.2%, p < 0.01).”

Read that again as it is pretty astounding. Less than a quarter of the supposed mask-wearing intervention group wore masks daily! And 15 % of the supposed nonMask control group wore masks daily. Furthermore, while the intervention group used masks significantly more often per week than the control group, that difference was all of 27.4% to 24.2%!!!

Is anyone surprised that masks were found to be ineffective? This example makes me very skeptical of quality control used by the authors of your meta-review. Garbage in, garbage out.


Tom Jefferson, senior associate tutor at the University of Oxford, is the lead author of a recent Cochrane review that has ‘gone viral’ on social media and re-ignited one of the most divisive debates during the pandemic – face masks. The updated review titled “Physical interventions to interrupt or reduce the spread of acute respiratory viruses” found that wearing masks in the community probably makes little or no difference to influenza-like illnesss transmission.

Demasi: This Cochrane review has caused quite a stir on social media and inflamed the great mask debate. What are your thoughts?

Jefferson: Well, it’s an update from our November 2020 review and the evidence really didn’t change from 2020 to 2023. There’s still no evidence that masks are effective during a pandemic…

In early 2020, when the pandemic was ramping up, we had just updated our Cochrane review ready to publish…but Cochrane held it up for seven months before it was finally published in November 2020. Those seven months were crucial. During that time, it was when policy about masks was being formed. Our review was important, and it should have been out there…

There is just no evidence that they make any difference. Full stop. My job, our job as a review team, was to look at the evidence, we have done that. Not just for masks…

Demasi: May I just ask a finer point on masks… it’s not that masks don’t work, it’s just that there is no evidence they do work…is that right?

Jefferson: There’s no evidence that they do work, that’s right. It’s possible they could work in some settings….we’d know if we’d done trials. All you needed was for Tedros [from WHO] to declare it’s a pandemic and they could have randomised half of the United Kingdom, or half of Italy, to masks and the other half to no masks. But they didn’t.


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From the actual study:

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 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.

Our confidence in these results is generally low to moderate for the subjective outcomes related to respiratory illness, but moderate for the more precisely defined laboratory‐confirmed respiratory virus infection, related to masks and N95/P2 respirators. The results might change when further evidence becomes available. Relatively low numbers of people followed the guidance about wearing masks or about hand hygiene, which may have affected the results of the studies.


Valid conclusions are important to any study. I can see how the ‘low numbers of people [following] guidance’ concerning the very things tracked in the study might affect results!



Confirmation bias at work. If a person is anti-mask, they grab the headline, and dismiss the fact that study participants did not follow the protocol, making the study worthless. It’s like the “Prague meeting” story, that proponents of the invasion of Iraq clung to, and kept propagating, years after it had been discredited by the FBI, Czech Intellegence, and the Iraqi intelligence officer himself.



Not worthless, in that in the real world where people don’t follow protocols very well (like in the studies) you would also expect the masks to not be very effective.

As noted earlier, this is a conclusion that remains true both pre- and post-covid.


The only conclusion that could be drawn from that study is a mask that is not worn is not effective.



I’m tired of beating around the bush. This is utter garbage.

The study was hopelessly flawed, in that the study participants didn’t follow the study protocols. You simply can’t draw any conclusions about masks from such a study. To claim that there is some valid conclusion flies in the face of facts and logic.

Here’s an analogy - give a bunch of kindergarteners some paints. Then ask them to mix blue and yellow to see what they get. But being kindergarteners, they’re mixing all sorts of different colors, not just blue and yellow. Is it possible to look at the jumble of resulting colors and conclude that mixing blue and yellow produces purple or orange or brown? After all, plenty of the study participants got those colors as a result of their paint mixing. No. We know that mixing blue and yellow paint produces green. This particular study is invalid - it produces no conclusions - because the participants didn’t follow the protocol. They didn’t mix blue and yellow. They mixed up other random colors instead.

That’s what happened with this mask study. Participants who were supposed to wear masks didn’t. Participants who weren’t supposed to wear masks did. Therefore, the study wasn’t a study of mask effectiveness at all. Perhaps it could be re-used as a study of human behavior. But a study of human behavior is not a study of masks.

But in spite of all of this, some idiot headline writer somewhere wrote a headline that completely misrepresents the study. And all sorts of crazy folks have jumped on this stupid headline trying to claim that it proves something when, even according to the study authors, it proves nothing. It provides evidence for nothing. It is useless. No, it is worse than useless, as it has become fodder for feeding lies to countless people who want to hear what they want to hear rather than the truth.



That’s rather to the point, isn’t it? In real world settings it is quite difficult to find an effect of masks.


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I suppose the OR surgeons for decades have been getting it all wrong, right?


From the abstract:
“Wearing masks in the community probably makes little or no difference to the outcome of laboratory‐confirmed influenza/SARS‐CoV‐2 compared to not wearing masks…”


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Caution is advised when harnessing a study’s abstract as an opinion. You should remember…or be aware if you weren’t already…that an abstract isn’t an independent, objective summary of a paper. It’s composed by the author(s) in a way that’s designed to present the study in a favorable light to get a second look at initial peer review, and stand a better chance of publication. It’s reasonable to expect it to comport somewhat with the findings and authors’ conclusions and you wouldn’t expect it to be quite as exuberant as an institution press release…but it’s not intended to be a stand alone piece of work.

In this instance, as mentioned upstream, the authors’ themselves dont appear to agree with the initial statement…although, I can see why a more accurate statement along the lines of …“Wearing masks incorrectly…or failing to wear them at all…probably makes little or no difference…” wouldn’t be an attention grabber.