COVID-19 sparked a polarising debate over the effectiveness of face masks in reducing the spread of respiratory infections. Central to this debate are two major reviews of the evidence: the 2023 Cochrane Review by Jefferson et al., and the 2024 ‘state of the science’ review led by Greenhalgh et al. These reviews reach notably different conclusions. Understanding the differences between these reviews is essential for anyone seeking an evidence-informed view on the role of face masks in public health.
Published last year, the review led by Professor Trisha Greenhalgh and published in Clinical Microbiology Reviews titled ‘Masks and respirators for prevention of respiratory infections: a state of the science review’ gave comfort to many on the side of wearing face masks. It was considered to be a response to, and referred to the study by, Professor Tom Jefferson et al. in the Cochrane Library titled ‘Physical interventions to interrupt or reduce the spread of respiratory viruses’.
Absence of evidence
Considered by many, especially those sceptical about the effectiveness of face masks, to be the best review of the evidence up to that point, the review by Jefferson et al. could find no robust evidence that face masks of any kind were effective per se or that respirator type masks were any more effective than the disposable face masks much in evidence during COVID-19. Greenhalgh et al. purport to provide evidence that face masks are effective and that respirator type face masks are even more effective.
As science is a perpetual discussion, due to the stochastic nature of the outcome of scientific studies and the phenomenon of regression towards the mean, it would be improper to claim that either study was the final word or that there was definitive evidence in either direction. Therefore, it is not helpful to state which of the studies provides the correct answer.
But it is possible to compare and contrast the methods used in these studies, to examine the claims they make and how they arrive at their conclusions. In that regard, the study by Jefferson et al. makes no bold claims other than, according to its analysis, there is an absence of evidence for the effectiveness of face masks. However, Greenhalgh and colleagues make bolder claims about the effectiveness of face masks. It is worth considering why.
Greenhalgh et al. reflects on the Jefferson et al. study early on, highlighting its method of considering only evidence available from randomised controlled trials (RCTs) and claiming this is “controversial”. This is an unusual claim for two reasons. First, the method of gathering and evaluating evidence used by the Cochrane Library whereby only individual studies of the highest quality are included is considered the gold standard method of conducting evidence syntheses. Second, as a cursory search on Google shows, Greenhalgh herself is no stranger to the Cochrane Library given that she has led and participated in several of its reviews.
The Cochrane cookbook – tried and tested
The Cochrane method involves the methods of systematic review and meta-analysis. The former is a transparent and replicable method of retrieving relevant studies involving the use of search terms and date limits to interrogate specified databases. For the studies retrieved, strict inclusion and exclusion criteria are set for which studies will be considered to hone many thousands of potential studies down to those specifically related to the research question. Next, the studies are evaluated for quality issues such as bias, adequacy of sample size and whether appropriate procedures have been used to follow the recruitment and dropout of participants.
Meta-analysis is a mathematical method of combining studies with similar interventions and outcomes. Steps are taken to minimise heterogeneity between the studies included and the degree of heterogeneity can be calculated and reported. Thus, the quality of the outcome of a Cochrane review itself can be evaluated.
In summary, the Cochrane Library uses an entirely transparent process, the protocol is subject to rigorous peer review prior to the study and, likewise, the final report. Clearly, while authors may have prejudices regarding the subject under review, the process of conducting such a review should be entered with an open mind and the methods ensure that the published outcomes are as free from bias as possible. This perfectly describes the methods applied in the review by Jefferson et al.
Starting with the answer
The review by Greenhalgh et al. did not follow Cochrane Library procedures. While selected groups of studies were meta-analysed, systematic methods were not used to retrieve studies in the first instance. Being a ‘state of the science’ review, the process used by Greenhalgh et al. is described as a “hermeneutic review” approach whereby the review team started with known sources, particularly those familiar to the team (which included experts in public health, epidemiology, infectious diseases, sociology, psychology and engineering).
There followed a process of iteratively refining the literature base by following up on references from key sources using Google Scholar. Thereafter, the authors actively sought recommendations from colleagues and researchers in relevant fields and crowdsourced references via posts on social media platforms. As a result they prioritised inclusivity and contextual understanding and eschewed methodological constraints (such as only including RCTs).
The problem with this approach is immediately obvious. The methods of retrieval are not transparent, systematic or replicable. The team, which one can presume to be biased in favour of face masks, far from approaching the issue with an open mind, began with studies its members considered showed that face masks were effective. They then mined the seams of articles in favour of face masks which were cited in those studies.
Let the echo chamber decide
The method of crowdsourcing further material is clearly biased as, with more to gain from an outcome in favour of face masks, the main respondents were likely mostly in favour of face masks and, subsequently, submitted studies reinforcing their bias. The review team had no way of checking for such bias. I searched in vain for non-experimental studies showing face masks were ineffective and found only two suggesting the possibility of publication bias in this field. Rigorous experimental studies such as RCTs must be registered prior to commencing the study under the AllTrials agreement signed by all leading academic publishers specifically to minimise publication bias. No such agreement governs non-experimental work.
Moreover, beyond these methodological issues, the lead author Greenhalgh was a leading campaigner of fanatical proportions for masking during COVID-19. Her Twitter feed repeatedly urged the wearing of face masks, at some points more than one at a time, even during exercise such as jogging. That at least one member of the review team was biased in favour of face masks is demonstrated at the end of the review where a headshot of each member is shown. Professor C Raina MacIntyre is shown with a face mask under her chin.
Proving a point
In my estimation, Greenhalgh and colleagues set out to prove that face masks were effective and conducted secondary analyses of data to prove their point. While the main issue affecting most of the population of the world during COVID-19 was the enforced wearing of disposable face masks, these are given very little consideration in the study. The discussion quickly moves to respirator type face masks which, at least, are theoretically more effective. The authors also used modelling data to support the effectiveness of face masks. Ironically, while critical of the ‘controversial’ Cochrane approach to evidence synthesis, Greenhalgh and co. use meta-analysis of selected data and are not averse to citing RCTs where the outcome favours face masks.
Both reviews offer insight into the complex and often polarised discussion surrounding the use of face masks for respiratory protection. The Jefferson et al. Cochrane review exemplifies a disciplined and transparent approach to evidence synthesis, while the Greenhalgh et al. review embraces a more interpretive, expansive method that seeks relevance and applicability beyond the confines of RCTs. However, when scientific credibility is under scrutiny, methodological rigour, transparency and neutrality are essential. In that regard, the contrast between these two reviews highlights not just differing conclusions but fundamentally different approaches to what constitutes reliable evidence. Sitting the Jefferson et al. and the Greenhalgh et al. reviews side by side, examining the methods, the possibility of bias amongst the teams and the conclusions reached, I doubt I am alone in concluding which of the reviews was the more robust and reliable.
Dr Roger Watson is Professor of Nursing at Saint Francis University, Hong Kong SAR, China. He has a PhD in biochemistry. He writes in a personal capacity.
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Greenhalgh, it would appear, starts with the answer, then works backwards to find out how to phrase the question.
Well put.
Not unlike the starting conclusion that manmade CO2 emissions cause warming aka climate change, and they are still searching for the elusive proof 30 years on!
Why is “reducing the spread of respiratory viruses” a good thing? Reducing from what to what? Over what time period? In what way does it improve overall quality of life? Does it increase life expectancy? Let’s say that through mask wearing, social distancing and “vaccinations” we completely eliminate respiratory viruses. What would be the consequences of that to health?
In addition, what kind of insanity thinks it is possible to eliminate any or all respiratory viruses? There are a few and they all mutate all the time and are airborne.
Absolutely ss!
Apparently the same insanity that thinks the U.K. reducing their CO2 emissions will save the whole world.
And they call us the fruitcakes and loonies.
If you believe in viruses then presumably everybody will be exposed to them, other than perhaps diseases that mostly kill the host. Far better to encourage people to be as healthy as they can than to fiddle around with some insignificant measure to “reduce the spread” (which surely has to be slow the spread rather than reduce it, at best).
Regardless of their efficacy, I have yet to see anyone use a mask properly.
Most will take some scruffy item out of a pocket or handbag, donning it as they enter the environment they are frightened of.
Whereas, they should be washing their hands before removing it from its sterile packaging, putting it on without fiddling with the mask itself, then washing their hands again and then repeating the process in reverse before disposing it in a suitable receptacle, remembering it is actually a potential bio-hazard.
One major giveaway that all this was theatre – there were zero instructions or protocols or anything about what to do with used masks. There were zero bio hazard bins anywhere outside hospitals In addition there were zero protections given to refuse collectors considering they would be in close contact to hundreds or thousands of used masks. It was also strange (or not) that there were no higher death rates among refuse collectors.
I smelled the rat when they said a mask made from a t-shirt, or a scarf across your face would suffice.
My own ‘rabbit-hole’ view is that someone on the BIT programme thought it would be an easy visual indicator of compliance… but, I should probably renew the foil in my hat!
Page 3 of this June 2020 WHO document talks about masks made from 12-16 layers of cotton…
… along with the warning “not for medical use” printed on the box
When the mandate came in I went to a haberdashers and bought gauze and elastic and made my own.
When the mandate came in I thought TC GTF.
What purpose did that serve, other than to comply in an individualistic manner?
I think it is a form of non conformity. Considering how many wore just a cloth face covering and nothing happened is useful evidence to hang onto for the next time this is triad.
In many professions good knowledge is shared and propagated easily so that almost anywhere on the planet there will be common understanding. I do not understand why it is not obvious to very ‘clever’ people that face masks were only for theatre and were worse than useless for preventing the spread of an airborne virus. Of course, the virus was characterised as being passed between people via droplets, and this assertion was pushed in a way that was very uncharacteristic of any professional group, and bore no relationship to any scientific discipline.
Please see the video below of Stephen Petty stating quite clearly why face masks will not work, and yet his presentation to a US Senate meeting has made no difference and has not been disseminated.
https://www.youtube.com/watch?v=aEwi_NUpk0k&t=169s
Stephen Petty, Expert, Industrial Hygienist gives solutions far more effective than masks
“We live in an unscientific age in which almost all the buffeting of communications and television-words, books, and so on-are unscientific. As a result, there is a considerable amount of intellectual tyranny in the name of science.”
R. Feynman
Re. the diagram.
I don’t believe that the size of the virus is relevant. I expect a fluid particle containing lots of viruses, water, etc. There is no reason why such a fluid particle couldn’t be smaller than the pore size.
In the video of Stephen Petty talking about health controls size is absolutely relevant. The droplet argument was intended as a justification for all the ludicrous personal restrictions including the masks and the 6ft rule. The fact that all respiratory viruses are airborne was supressed, and, we breath air not water. Stephen Petty explains quite clearly what the challenges are and what is possible and not possible, but it does require listening to what he has to say.
You could also ask if the UK Covid-19 Inquiry is starting from the answer…
Greenhalgh is seriously unbalanced (a modern day zealot). Not only do masks not work, they cannot work. They were designed to prevent liquid transmission not viruses. The Covid virus was at least 10x smaller than the gaps in the weave of the masks, never mind the fitting gaps.
The key point these zealot miss, is the immune system. Every day humans come into contact with millions of bacteria/virus etc which are countered by the immune system. Look after that and it will look after you.
Although immune systems fade with age, a person with good diet, exercise and sleep will be in a great position to fight off viruses. There were Drs in A&E covid wards who choose not to wear masks and yet they never caught it.
Masks were all about obedience and control.
I’d say it’s pretty simple. If an N95 mask or a face covering stopped you breathing in virus particles, it would also stop you breathing in air and cause you to suffocate and die.
Unless it’s a full face chemical warfare, fire brigade or meth lab style respirator, which nobody was wearing and we’re not talking about, face masks are nothing more than a splash guard.
Let’s see Trisha Greenhalgh’s financial records from 2019 onwards. Then we’ll know what’s what.
She could be Jim Davidson’s older brother. Uncanny…
https://www.youtube.com/watch?v=F46DhUqoJNQ
I never used them, and it seemed to me that it was like a cultural copy & paste tactic, as it has long been common practice for some workers on high density public transport, such as the Japanese metro system in Tokyo to wear such things. It fitted in with a typical “something must be done” attitude, grossly unscientific, with a bit of opportunism on top.
Have you ever read any of the “get out” labels printed on those so-called “masks” that were on sale. You needed a magnifying glass to read them, but they were there.
The label on the box of the one I bought said:
Fashion mask. Not a medical mask. Use for dust protection, sun protection, fashion.
I would no more cover my airways than any other form of self-harm. Snake oil.
Greenalgh personally admitted that her push for masking was never really because of COVID but because this was a cultural change she desired to force onto Britain ever since encountering widespread (or somewhat widespread) public mask wearing during a holiday in Japan. This means anything published by her to this effect is simply policy-based ‘evidence making’ by someone not willing to concede defeat yet.
There’s a medical term for her condition and it’s called mysophobia and there are also some less-than-flattering terms for her general bevahioural drive, namely, relentless egotism of an ageing babyboomer whose overriding concern is fear of her own, eventual death and who has thus long since passed her shelf-life as possibly useful member of society.
…policy-based ‘evidence making’
Superb phrase, whether original or not.
It’s not from me.
It was somewhat frequently used to describe all the – well – policy-based evidence making which was used to keep The Great Pandemic Show running during that time. Eg Chris Whitty’s (mock) shock disclosure that “Viruses mutate!” which came just in time to justify killing Christmas 2020 and go into the 3rd national lockdown from January onwards despite he had already known about the so-called Kent Variant since September 2020.
Correction: The “holiday in Japan” story was Susan Mitchie’s, not Trisha Greenalgh’s.
The whole face mask period for me meant that I couldn’t go on any public transport (not having transport myself), or into many shops, or visit friends, because I refused to get the clotshot, or flu jab, or wear a mask, even though the face mask period lasted a couple of years. It was hard for me to recognize people wearing the masks, so imagine the developmental damage it must have done to little children just learning about visual clues and facial expressions!
What surprised me the most was seeing people wearing masks even while walking outside in the fresh air along a country lane, or driving alone in their cars. I’m glad the madness has passed, and articles like this one by Dr. Roger Watson (nice friendly photo of him, DS) will help awaken people to never be fooled again.
Wasn’t she one of the trio who reported Aseem Malhotra to the GMC?
Snap.
Dr Greenhalgh was, it seems, one of the three doctors who used the coward’s mask of anonymity when they attacked the excellent Dr Aseem Malhotra by reporting him to the General Medical Council.
https://dailysceptic.org/2025/06/06/the-brave-doctors-who-stood-up-for-the-truth-about-the-covid-vaccines-deserve-an-apology/
So she clearly practised what she preached.
Ummm, what a dilemma you’ve put me in ; whether to trust Tom or Greenlagh?
Evidence & science is not a strong point of Greenlagh.
A pro-masker manipulating ‘data’ – surely not!