Yves here. Lambert is very happy to see that the team led by the not-so-stupid Trish Greenhalgh from Oxford has hit the ground running on a strange and damaging campaign against the use of masks, with a debunked (but only after much discussion) Cochrane Report original article. for example. Greenhalgh and his fellow scientists took the commonsensical step of excluding studies that did not study the effectiveness of masking, such as those that had participants take off their masks indoors, or worse, treated various public mask-wearing programs as if they produced appropriate mask use. The key sentence from the post below: “Most RCTs of community mask wearing were actually tests of mask wearing advice.”
I was going to embed the lesson below, which both KLG and Ignacio took as a judge, at the end of the post, but the file size is too big. You can find it here.
And please share this post widely!
By Trish Greenhalgh, Professor of Primary Care Health Sciences, University of Oxford; C Raina MacIntyre, Professor of Global Biosecurity, NHMRC L3 Research Fellow, Head, Biosecurity Programme, Kirby Institute, UNSW Sydney; and David Fisman, Professor in the Department of Epidemiology, University of Toronto. Originally published on The Conversation
When a Texan farm worker caught bird flu from cattle recently, social media was abuzz with rumors. Although bird flu is not a human pandemic, scientists and policy makers around the world are determined to prepare as best they can when this pandemic occurs – a difficult task, as the science is messy, policy must be realistic and human values do not. align regularly.
Time for the mask to enter the conversation. At the beginning of an epidemic caused by a novel or newly mutated virus, there may be no vaccine, no solid information about how bad things will happen and no specific treatment. Slowing down until more is known will be important.
Getting more people to wear masks may nip the outbreak in the bud, prevent an epidemic or reduce its impact. Wearing a mask is not easy, but not like it’s a lock.
But does the mask work? A review of masks and respirators, which looked only at clinical trials, concluded that there was insufficient evidence to assess whether wearing a mask reduces the risk of spreading or acquiring respiratory infections. However, we do not agree with that.
The review, carried out by the non-profit Cochrane Collaboration, failed to influence the latest guidelines issued by the US Centers for Disease Control and Prevention (CDC) in response to concerns about bird flu transmission to humans. The CDC recommended proper-fitting respirators — as well as overalls and safety glasses — for anyone working with potentially infected cattle until the threat of bird flu is over.
Is this latest guidance based on sound evidence? According to our new review of the evidence, yes. As a Cochrane team, we pooled data from randomized controlled trials (RCTs) and analyzed the pooled data – the so-called meta-analysis.
Unlike them, we also examined non-RCT evidence, including a number of laboratory studies that showed that respiratory diseases, including the common cold, COVID, influenza, measles and TB, are more airborne.
Laboratory evidence has shown that different mask materials are better or worse at filtering out small particles, and are more breathable – especially when it’s humid. This explains why a cloth or paper mask that is wet with moisture from the air becomes difficult to breathe through and may be less protective.
Although medical masks are usually tied loosely to the face (which is why air can pass through the filter), respirators fit well and when worn at work should be checked to ensure that all air inhaled or exhaled passes through a high-quality filter.
All of this non-RCT evidence is very important in the design of RCTs. Because respiratory germs float in the air, to be effective the mask must be made of high-purity materials and must fit. It should not be removed while indoors or the person will be quickly exposed to infectious particles in the air.
It follows that we should not expect RCTs of poorly designed masks, ill-fitting masks or masks that are only worn for a certain period of time, to show an effect. We should also not expect that the mere advice to wear masks will have any effect unless it is followed.
Finally, when comparing respirators and masks in places where there is a high risk of infection, such as a hospital, the respirator needs to be worn continuously until the person leaves the building, not just to be seen occasionally when performing so-called “aerosol-generating processes” – such as inhaling a patient an opening.
If we consider these important details of the RCT design, rather than simply comparing any mask-off, mask-off trials, we find that masks are effective, and respirators are even more effective, in reducing the spread of respiratory disease. And we can explain why some previous reviews seemed to indicate that this was not the case.
Most of the RCTs of community mask wearing were actually trials of mask wearing advice. In both RCTs and observational studies (similar to real-world tests), there was a dose-response effect: the more people wore their masks, the more effective the masks were. And when there is an epidemic coming, people tend to wear their masks.
The Bottom Line
When we looked at the RCTs, we found that masks were protective in the public, and N95 respirators (masks made using high-quality filter materials and designed to fit close to the face to protect against airborne contaminants) were superior to masks in healthcare workers, especially when respirators were worn continuously on the job. Non-RCT evidence also shows that masks work and respirators work better.
Let’s hope we don’t refer to another epidemic. But since we’re thinking about what’s possible, the focus of our latest review is the mask function. Along with improving indoor air quality and avoiding crowded, poorly ventilated spaces, they provide the best way to avoid catching respiratory diseases. And our findings support previous advice to not only wear a mask but to wear the best mask available.
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