1. Thread on masks (also posted on Twitter). Addresses:
- Why so much controversy?
- Airborne transmission
- RCTs of masks – strengths/limitations, community, healthcare settings
- Meta-analyses – gold standard or lazy lumping?
- Non-RCT evidence – why is it needed, what does it show?
- AOB
- Why so much controversy?
- Airborne transmission
- RCTs of masks – strengths/limitations, community, healthcare settings
- Meta-analyses – gold standard or lazy lumping?
- Non-RCT evidence – why is it needed, what does it show?
- AOB
09:16 AM - Feb 26, 2023
In response to Trish Greenhalgh.
2. Before I start, here’s my OLD mask thread from Twitter July 21, which ran to >100 tweets. Some of that is now rather dated but a lot still holds. I’ll paste the best from that into this new thread. Will take a while (~100 total)
https://twitter.com/trishg...
https://twitter.com/trishg...
09:17 AM - Feb 26, 2023
In response to Trish Greenhalgh.
3. WHY IS MASKING SO CONTROVERSIAL?
First, ‘masks’ and ‘masking’ are very broad terms. They cover different technologies (cloth, medical, respirator), wearers (public, patient, healthcare worker), settings (low v high risk) and requirements (mandated v free choice).
First, ‘masks’ and ‘masking’ are very broad terms. They cover different technologies (cloth, medical, respirator), wearers (public, patient, healthcare worker), settings (low v high risk) and requirements (mandated v free choice).
09:18 AM - Feb 26, 2023
In response to Trish Greenhalgh.
4. WHY CONTROVERSIAL?
Because of this multiplicity, statements like “masks [don’t] work” may be referring to very different interventions in different populations for different activities at different times, not to mention different diseases. The literature, in short, is a mess.
Because of this multiplicity, statements like “masks [don’t] work” may be referring to very different interventions in different populations for different activities at different times, not to mention different diseases. The literature, in short, is a mess.
09:18 AM - Feb 26, 2023
In response to Trish Greenhalgh.
5. WHY CONTROVERSIAL?
Second, masking is a cultural practice as well as a public health intervention. It has symbolic meaning. Pirates and burglars wear masks. To some, masking means you’re a ‘pansy’, ‘sheep’ etc. Masking offends libertarians.
https://www.bmj.com/conten...
Second, masking is a cultural practice as well as a public health intervention. It has symbolic meaning. Pirates and burglars wear masks. To some, masking means you’re a ‘pansy’, ‘sheep’ etc. Masking offends libertarians.
https://www.bmj.com/conten...
09:19 AM - Feb 26, 2023
In response to Trish Greenhalgh.
6. WHY CONTROVERSIAL?
There’s strong social pressure to mask in some settings to contribute to the greater good (“my mask protects you”). People may feel pushed into doing something which a) feels awkward, b) symbolises an allegiance they don’t identify with and c) they feel is ineffective.
There’s strong social pressure to mask in some settings to contribute to the greater good (“my mask protects you”). People may feel pushed into doing something which a) feels awkward, b) symbolises an allegiance they don’t identify with and c) they feel is ineffective.
bmj.com
Face coverings for covid-19: from medical intervention to social practice | The BMJ
Helene-Mari van der Westhuizen and colleagues argue that face coverings should be considered not as medical equipment but as a social practice informed by norms and expectations ### Key messages Covering the mouth and nose is a public health measure designed to capture respiratory droplets from th..
09:20 AM - Feb 26, 2023
In response to Trish Greenhalgh.
7. WHY CONTROVERSIAL?
Third, masking (whatever its benefits) has some downsides. It’s (depending on the design) uncomfortable. It can interfere with communication (especially if hard of hearing). If you work long shifts, it gets hot/sweaty behind the mask.
https://www.bmj.com/conten...
Third, masking (whatever its benefits) has some downsides. It’s (depending on the design) uncomfortable. It can interfere with communication (especially if hard of hearing). If you work long shifts, it gets hot/sweaty behind the mask.
https://www.bmj.com/conten...
bmj.com
Face masks can be devastating for people with hearing loss | The BMJ
Innovation is urgently needed to ensure clear communication in medical settings The covid-19 pandemic has literally changed the face of medical care: clinicians, nurses, other staff, and patients are all wearing masks. We are trying to communicate while keeping the required distance apart. With mas..
09:20 AM - Feb 26, 2023
In response to Trish Greenhalgh.
8. WHY CONTROVERSIAL?
In short, masking is something many of us feel strongly about. It’s not even possible to separate “the science” of masks from “the politics”, since science does not sit in some protected bubble away from the rest of society. Read on!
In short, masking is something many of us feel strongly about. It’s not even possible to separate “the science” of masks from “the politics”, since science does not sit in some protected bubble away from the rest of society. Read on!
09:21 AM - Feb 26, 2023
In response to Trish Greenhalgh.
9. AIRBORNE TRANSMISSION
Understanding why & to what extent masks work requires knowledge of how the SARS-CoV-2 virus is transmitted. With colleagues, I published this summary of evidence that it spreads *predominantly* via air. https://twitter.com/trishg...
Understanding why & to what extent masks work requires knowledge of how the SARS-CoV-2 virus is transmitted. With colleagues, I published this summary of evidence that it spreads *predominantly* via air. https://twitter.com/trishg...
09:22 AM - Feb 26, 2023
In response to Trish Greenhalgh.
10. AIRBORNE TRANSMISSION
I responded to critics of that paper here https://twitter.com/trishg... (plus see more in thread below).
I responded to critics of that paper here https://twitter.com/trishg... (plus see more in thread below).
09:22 AM - Feb 26, 2023
In response to Trish Greenhalgh.
11. AIRBORNE TRANSMISSION
For those who want depth and detail, here’s Chia Wang and team’s stunning review of the evidence for airborne transmission in Science (one of the very top scientific journals).
https://www.science.org/do...
For those who want depth and detail, here’s Chia Wang and team’s stunning review of the evidence for airborne transmission in Science (one of the very top scientific journals).
https://www.science.org/do...
09:23 AM - Feb 26, 2023
In response to Trish Greenhalgh.
12. AIRBORNE TRANSMISSION
In sum, it is *settled science* that SARS-CoV-2 is transmitted via the air. Settled, that is, in all branches of science *except* key groups of doctors, including some in powerful positions at the World Health Organisation. I’ll return to that paradox later.
In sum, it is *settled science* that SARS-CoV-2 is transmitted via the air. Settled, that is, in all branches of science *except* key groups of doctors, including some in powerful positions at the World Health Organisation. I’ll return to that paradox later.
09:23 AM - Feb 26, 2023
In response to Trish Greenhalgh.
13. AIRBORNE TRANSMISSION
Medical/surgical (~same thing) masks are designed to stop spatter (e.g. your dentist dribbling into your mouth). Most aren’t made to a specific quality standard. Respirators are designed to filter out airborne particles. They ARE made to a standard.
Medical/surgical (~same thing) masks are designed to stop spatter (e.g. your dentist dribbling into your mouth). Most aren’t made to a specific quality standard. Respirators are designed to filter out airborne particles. They ARE made to a standard.
09:23 AM - Feb 26, 2023
In response to Trish Greenhalgh.
14. AIRBORNE TRANSMISSION
If SARS-CoV-2 were spread mainly by droplets, we’d need to focus on handwashing and medical/surgical masks which stop wet drops. If mainly via the air, we need to focus on air quality and use the kind of face coverings that stop airborne particles (i.e. respirators).
If SARS-CoV-2 were spread mainly by droplets, we’d need to focus on handwashing and medical/surgical masks which stop wet drops. If mainly via the air, we need to focus on air quality and use the kind of face coverings that stop airborne particles (i.e. respirators).
09:23 AM - Feb 26, 2023
In response to Trish Greenhalgh.
15. AIRBORNE TRANSMISSION
All masks stop some airborne particles (even if the holes are bigger than the particles, fibres are criss-crossed and electrostatic forces kick in). But N95 (FFP2) respirators stop >95% of particles of SARS-CoV-2 size, and N99s (FFP3s) stop >99%.
All masks stop some airborne particles (even if the holes are bigger than the particles, fibres are criss-crossed and electrostatic forces kick in). But N95 (FFP2) respirators stop >95% of particles of SARS-CoV-2 size, and N99s (FFP3s) stop >99%.
09:24 AM - Feb 26, 2023
In response to Trish Greenhalgh.
16. AIRBORNE TRANSMISSION
A year ago, we wrote this article recommending that everyone upgrades their mask from either medical/surgical to respirator.
https://theconversation.co...
A year ago, we wrote this article recommending that everyone upgrades their mask from either medical/surgical to respirator.
https://theconversation.co...
09:24 AM - Feb 26, 2023
In response to Trish Greenhalgh.
17. AIRBORNE TRANSMISSION
Whilst respirators protect best, ANY mask is better than none. Efficiency at reducing transmission of respiratory aerosol:
N95 respirator 99%
Medical mask 59%
Bandana (single) 47% (double) 60%
Face shield 2%
https://tandfonline.com/do...
Whilst respirators protect best, ANY mask is better than none. Efficiency at reducing transmission of respiratory aerosol:
N95 respirator 99%
Medical mask 59%
Bandana (single) 47% (double) 60%
Face shield 2%
https://tandfonline.com/do...
09:25 AM - Feb 26, 2023
In response to Trish Greenhalgh.
18. AIRBORNE TRANSMISSION
The above, a laboratory study of artificial aerosols, should not be taken as direct evidence that these masks would be this effective/ineffective in preventing real-world SARS-CoV-2 transmission. But they *add to the mix of evidence*. More on that later.
The above, a laboratory study of artificial aerosols, should not be taken as direct evidence that these masks would be this effective/ineffective in preventing real-world SARS-CoV-2 transmission. But they *add to the mix of evidence*. More on that later.
09:26 AM - Feb 26, 2023
In response to Trish Greenhalgh.
19. AIRBORNE TRANSMISSION
Face shields are only 2% effective cos they’re designed to stop spatter, NOT filter air. Medical masks don’t fit snugly, so instead of breathing filtered air, you breathe what leaks in round the sides. This is why a doubled-over bandana works pretty well!
Face shields are only 2% effective cos they’re designed to stop spatter, NOT filter air. Medical masks don’t fit snugly, so instead of breathing filtered air, you breathe what leaks in round the sides. This is why a doubled-over bandana works pretty well!
09:26 AM - Feb 26, 2023
In response to Trish Greenhalgh.
20. AIRBORNE TRANSMISSION
While I believe cloth/medical masks have a place, some sfeel the benefits of respirators are so great, we should junk all lesser kinds of masking.
https://www.cidrap.umn.edu...
While I believe cloth/medical masks have a place, some sfeel the benefits of respirators are so great, we should junk all lesser kinds of masking.
https://www.cidrap.umn.edu...
09:27 AM - Feb 26, 2023
In response to Trish Greenhalgh.
21. AIRBORNE TRANSMISSION
Remember, respirators (and masks) only work if they fit you snugly. You don’t need a professional fitting any more than you need to have your underwear professionally fitted, but do check for gaps.
Remember, respirators (and masks) only work if they fit you snugly. You don’t need a professional fitting any more than you need to have your underwear professionally fitted, but do check for gaps.
09:27 AM - Feb 26, 2023
In response to Trish Greenhalgh.
22. RANDOMISED CONTROLLED TRIALS (RCTs) OF MASKS
RCTs of masks have continued to generate more heat than light. I’m going to reproduce what I said two years ago, which still holds (even more so now, since what has happened since could have been predicted from my earlier thread).
RCTs of masks have continued to generate more heat than light. I’m going to reproduce what I said two years ago, which still holds (even more so now, since what has happened since could have been predicted from my earlier thread).
09:28 AM - Feb 26, 2023
In response to Trish Greenhalgh.
23. RCTs
In the name of evidence-based medicine (EBM), the West got off on the wrong foot. We became obsessed with the holy grail of a definitive randomised controlled trial (RCT) that would quantify both the benefits and the harms of masks, just as you would for a drug.
In the name of evidence-based medicine (EBM), the West got off on the wrong foot. We became obsessed with the holy grail of a definitive randomised controlled trial (RCT) that would quantify both the benefits and the harms of masks, just as you would for a drug.
09:28 AM - Feb 26, 2023
24. RCTs
If you were raised in the EBM tradition, where “rigorous RCTs” are mother’s milk, it’s not easy to get your head round why this was a bad way to approach the problem. Looks like Prof G has lost it, dropped her standards, joined the dark side etc. Bear with me.
If you were raised in the EBM tradition, where “rigorous RCTs” are mother’s milk, it’s not easy to get your head round why this was a bad way to approach the problem. Looks like Prof G has lost it, dropped her standards, joined the dark side etc. Bear with me.
In response to Trish Greenhalgh.
09:28 AM - Feb 26, 2023
In response to Trish Greenhalgh.
25. RCTs
A RCT is a controlled experiment. Since people are randomly allocated to one or other group (‘arm’), any confounding variables are distributed evenly between the arms so they all cancel out (so long as the study is large enough and allocation is truly random).
A RCT is a controlled experiment. Since people are randomly allocated to one or other group (‘arm’), any confounding variables are distributed evenly between the arms so they all cancel out (so long as the study is large enough and allocation is truly random).
09:28 AM - Feb 26, 2023
In response to Trish Greenhalgh.
26. RCTs
Random allocation means that differences between the arms of a RCT are highly likely to be due to the intervention (in this case, masks) and not to confounders. But it doesn’t follow that a RCT is better, for any scientific question, than a non-RCT design. Why not?
Random allocation means that differences between the arms of a RCT are highly likely to be due to the intervention (in this case, masks) and not to confounders. But it doesn’t follow that a RCT is better, for any scientific question, than a non-RCT design. Why not?
09:29 AM - Feb 26, 2023
