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Patricia Mary “Trisha” Greenhalgh OBE FRCP FRCGP FMedSci is a British professor of primary health care and a practising general practitioner. Wikipedia
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LONG THREAD on masks. Mute if not interested.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
Do masks work? Why do some people claim they don’t work? Do they cause harm? What kinds of masks should we wear? How does masking need to change now we know that Covid is airborne? When can we stop wearing them?
Get your popcorn.
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Was this association or causation? Early on in the pandemic, we didn’t know. But – important point – nobody in these Asian countries seemed to come to harm from wearing a mask.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Critics of that early paper were right that the empirical evidence was weak. But they didn't engage—and 16 months later have still not engaged—with the moral arguments. They continue to argue that the best course of action in the face of empirical uncertainty is to do nothing.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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A bit of cloth over the face simply doesn’t have the same risks as a novel drug or vaccine, and *doing nothing* could conceivably cause huge harm. Arguing for “caution” without engaging with the precautionary principle was scientifically naïve and and morally reckless.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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For many mission-critical weeks in early 2020, these bodies persisted in saying “there’s not enough evidence of benefit” and (without evidence) “there could be harms”, and insisting that these arguments justified inaction.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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The most fundamental error made in the West was to frame the debate around the wrong question (“do we have definitive evidence that masks work?”). We should have been debating “what should we do in a rapidly-escalating pandemic, given the empirical uncertainty?”.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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This never made sense. If your mask contains virus, it’s likely come from you, so you’re already infected. There was never any evidence that people touch their faces more when masked. They touch them less.https://t.co/l2jUQh5990
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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There was an alternative, common-sense view. Your cotton mask is no more likely to kill you than your cotton T-shirt which you pull over your head. In mid-2020, @jeremyphoward came up with the slogan “it’s a bit of cloth, not a land mine”.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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There was never one jot of evidence for risk compensation. But as Eleni Mantzari and team showed, scientists *talking up* risk compensation as a purely hypothetical problem led to significant negativity towards masks.https://t.co/TCcDN8So9h
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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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 Greenhalgh has lost it, dropped her standards, joined the dark side etc. Bear with me.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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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 does *not* follow that a RCT is better, for any scientific question, than a non-RCT design. Why not?
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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The RCT design can’t cope with this. It’s easy to design a study where the primary outcome is infection in wearers, but how would a RCT of source control work? I consent to wearing a mask, but the whole town must consent to be tested (at baseline & repeatedly) for infection.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Take the number 1 and double it, and keep going. 1 becomes 2, then 4, etc. After 10 doubles, you get 512. After 10 more doubles, you get 262144. Now instead of doubling, multiply by 1.9 instead of 2 (a tiny reduction in growth rate). After 20 cycles, the total is only 104127.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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These two issues—the near-impossibility of using RCTs to test hypotheses about source control and over-reliance on “statistically significant effects” within a short-term intervention period—is why a RCT of masks is *highly likely to mislead us*.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Note: as a long-term survivor of a poor-prognosis cancer, I owe my life to RCTs of drugs and surgery. RCTs are fantastic for testing both treatments and vaccines, and have led to many lives being saved in the pandemic. But they are problematic for testing masks.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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In sum, RCTs of masks were difficult to do, and participants randomised to wearing masks didn’t comply well. But these RCTs were in the context of – for example – a flu outbreak on a university campus in a country that had never seen a deadly pandemic of anything.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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DANMASK key flaws in one tweet: no CONSORT statement, no ethical approval, inappropriate setting (there was almost no Covid circulating at the time!), under-powered sample, wrong primary outcome, wrong intervention period, inaccurate test, misinterpretation of own findings.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Yanni Li and colleagues did a meta-analysis of case-control studies. Acknowledging the limitations of such designs, Li et al found that masks reduced SARS-CoV-2 infections by (on average) ~70% in healthcare workers and ~60% in lay people. https://t.co/FUZy3LJw4y
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
35/ pic.twitter.com/LcD58AqvbY
Hence we need to treat the Li meta-analysis finding with care. On its own, it’s not definitive; combined with other evidence, it strengthens the case for masks. And another recent case-control study affirms Li’s findings.https://t.co/mtnsULU9x6
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Evidence from observational studies (see tweet 2 in this thread) is pretty consistent, though causality is hotly contested. But broadly speaking, in countries and regions where mask mandates were introduced, the rate of spread of the virus subsequently fell substantially.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Laboratory evidence is often dismissed as “low methodological quality” by the EBM crowd. Actually, lab studies can be done well or badly (full disclosure: my first doctorate is in lab science). High-quality lab studies are as important as high-quality RCTs.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Lydia Bourouiba studies how far the stuff that comes out of your mouth and nose when you cough and sneeze travels. Answer: a long way. Common sense says that something placed across the mouth and nose will block some of these flying germs.https://t.co/UPEig8m3ov
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Here’s one lab study for example. Efficiency at reducing the transmission of an artificial respiratory aerosol:
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
-N95 respirator 99%
-Medical grade mask 59%
-Polyester neck gaiter (single) 47% (double) 60%
-Face shield 2%https://t.co/55xd0LEHZx
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For a long time, WHO and other bodies denied that SARS-CoV-2 was airborne. We wrote a paper on why this error happened (TL;DR: we hypothesised powerful people with entrenched mental models and reputations to lose).https://t.co/qP4h0g7Fo5
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Specifically, we need to shift our mechanistic model from one that focuses on projectile clouds of droplets that come from coughs and sneezes to one that sees the *very air we breathe* as laden with infectious particles. Detail here:https://t.co/U7FmVDL5sL
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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March 2020 now seems ancient history. At that time, we were drawing on the Bourouiba study (tweet 43) and similar to argue that *any* bit of cloth over your face would serve as effective source control because it blocked gobby droplets and sneezy droplets.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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If it was all large droplets, masks would work mainly as source control. But given that SARS-CoV-2 travels deep into the lungs in tiny airborne particles, masks need to protect against *inhaling air that others have exhaled*.https://t.co/z1yoU2XOYs
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
53/ pic.twitter.com/uGfmWTYNdo
Let’s talk about the complex transmission dynamics of SARS-CoV-2, specifically the over-dispersion of R (80% of Covid cases come from only 10-20% of infected people – the super-spreaders). https://t.co/040ClqILEA
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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In a nutshell, we need to *reframe* the key mask research question from “what is the effect size of masks on individuals?” to “how might universal masking impact on the highly non-linear transmission dynamics of this curious virus – which behaves VERY differently to flu?”.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Not only do we need to keep wearing masks, but we need masks that protect us against *airborne transmission* as well as against droplets. Yes it’s a pain. Yes prolonged masking is a political hot potato bc “freedom”. But don’t shoot the messenger: I didn’t invent this virus.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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There has been a huge amount of research on what makes a perfect mask. In our paper we reviewed 3 factors: how well it filters, how well it fits, and whether you actually wear it (which in turn depends on how comfortable it is).https://t.co/z1yoU2XOYs
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
61/ pic.twitter.com/oHO5kUHsMT
Mask design is now very niche (preprint here), but note: “Despite the complexity of the design of a very good mask, it is clear that for the larger aerosol particles, *any* mask will provide substantial protection to the wearer & those around them.”https://t.co/k4R7JgzpcC
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Two tricks to improve the fit of a medical mask: a) wear a cloth mask OVER it (“double masking”), b) knot the ear loops before going round your ears. (I have a third: use a section of old tights / panty hose instead of the cloth in (a)). https://t.co/HUzNV3KriL pic.twitter.com/7R5LTM61k0
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
What about valves in respirators? Respirators were designed to protect healthy healthcare workers from germs breathed out by patients. The valve was there to make breathing easier. But if the HCW is infected, the valve acts as an exhaust pipe, releasing virus into the air.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Now for healthcare workers (HCWs). Big problem here right now. As I’ve argued, the SARS-CoV-2 virus is airborne. Airborne virus is a particular problem in hospitals and other healthcare facilities because that’s where you go when you’re not well.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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In short, the only HCWs who were given high-grade protection (including the more efficient respirator masks) were those conducing so-called AGPs. But since *breathing* (especially when unwell) generates aerosols, it makes no sense to have a two-tier protection system.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Next question: should fully-vaccinated people still wear masks? USA has famously said they don’t have to, so let’s look at both sides of the argument.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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On the other hand, even 95% protection isn’t perfect. It means that if 20 vaccinated people all leave their masks off, that’s equivalent to one unvaccinated person. And if we’re unlucky, one of those 20 people will be a super-spreader.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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As @zeynep has argued, we wear masks for 3 reasons: to protect ourselves, to protect others, and to model the kind of behaviour that is appropriate in a global pandemic. Creating a vax’d vs unvax’d apartheid will undermine this ethic.https://t.co/BqUpWgaBHs
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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Since airborne virus floats away in the outdoors, we usually only need to wear them *indoors*. And the most important settings are where we risk being exposed to high viral loads (i.e. crowded, poorly-ventilated places—especially hospitals).https://t.co/pLcasMPZ0O
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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There are lots of other questions I could have addressed – such as masking in kids, more on the politicization of masks, viricidal masks and other novelty designs, future research. But you’ve probably had enough by now. I’ve shared only about 40 papers of 2500.
— Trisha Greenhalgh (@trishgreenhalgh) July 11, 2021
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A PS to my long thread on masks (currently my pinned tweet): A large community-based, very pragmatic, RCT in Bangladesh showed statistically significant reduction in COVID cases with surgical masks and a substantial reduction with cloth masks. Detail:
— Trisha Greenhalgh (@trishgreenhalgh) September 10, 2021
83/https://t.co/Rzaula2knv
CITRAP “what can masks do?” part 2 here:https://t.co/cYw7zdeA1k
— Trisha Greenhalgh (@trishgreenhalgh) November 4, 2021
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Latest guidance from US CDC on community masks (December 21): https://t.co/95iI7ux18T
— Trisha Greenhalgh (@trishgreenhalgh) December 9, 2021
Adding this to the end of my long thread on the evidence for community masking – read from top of this thread if you haven't seen it already.
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TIL = total inward leakage (vertical axis). This is jargon for "air getting in round the sides of your badly-fitting mask".
— Trisha Greenhalgh (@trishgreenhalgh) December 11, 2021
High TIL is bad.
Aim for PURPLE: wear a FFP2 & use a bit of tape to close the gaps. GREEN is an untaped surgical mask – useless!
89/https://t.co/tottSJAYjH pic.twitter.com/6VrPuOxfVi
Adding this superb thread on re-using respirator masks to my pinned tweet thread on masks:
— Trisha Greenhalgh (@trishgreenhalgh) January 2, 2022
91/https://t.co/CYn0CHlEdQ
A few tweets on masks for kids (thanks @dgurdasani1).
— Trisha Greenhalgh (@trishgreenhalgh) January 2, 2022
US schools were 3.5 x more likely to have COVID-19 outbreaks if they did not have a mask requirement at the start of school compared with schools that required universal masking on day one.
92/https://t.co/znBoUXom5v
Re-using N95 masks: it's OK if they're not damaged you let them dry out between wearings.https://t.co/SEam4zXGED
— Trisha Greenhalgh (@trishgreenhalgh) January 10, 2022
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Masterly critique of “that DfE schools study” which was widely reported as showing that masks “don’t work”. (Narrator: masks *do* work, if worn properly in the right settings).
— Trisha Greenhalgh (@trishgreenhalgh) February 17, 2022
/97 https://t.co/Fbz54edZby
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weehingthong 
What do these foreign devils know. Our Pakej Penjagaan Covid-19 (PPC) sent by the Health Ministry/KKM under #KeluargaMalaysia contains 4 black cloth washable masks. They don’t need to be double masked. It is the picture of our glorious leader imprinted on the masks that protect us.