【COVID-19とマスク】マスクは有効ですか？なぜ一部の人達はマスクの効果否定するの？それらは害を及ぼしますか？どんなマスクを着るべきですか？COVID-19が空中浮遊していることがわかった今、マスクをどのように変更する必要がありますか？ いつマスク着用をやめられますか？(2021.8.15作成)

mm @tyonarock

マスクについてのスレ パンデミック初期に西側諸国がマスクの効果に対し「明確なエビデンスがない」ことに拘り被害を大きくしたことにも言及。 twitter.com/trishgreenhalg…

Trisha Greenhalgh @trishgreenhalgh

LONG THREAD on masks. Mute if not interested. 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. 1/

2021-07-12 03:42:21

Trisha Greenhalgh @trishgreenhalgh

LONG THREAD on masks. Mute if not interested. 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. 1/

Trisha Greenhalgh @trishgreenhalgh

Let’s start with observational data. Countries that introduced mandated masking within 30 days of the first case (mostly Asian) had *dramatically* fewer Covid-19 cases than those that delayed beyond 100 days (mostly Western). ajtmh.org/view/journals/… 2/ pic.twitter.com/YitQiYDvSZ

拡大

Trisha Greenhalgh @trishgreenhalgh

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. 3/

Trisha Greenhalgh @trishgreenhalgh

In this BMJ paper, we presented very limited and indirect empirical evidence (from non-Covid studies) and also *moral evidence* to argue for the precautionary principle: let’s all wear masks, *just in case*. pubmed.ncbi.nlm.nih.gov/32273267/ 4/

Trisha Greenhalgh @trishgreenhalgh

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. 5/

Trisha Greenhalgh @trishgreenhalgh

New drugs & vaccines may have toxic side effects worse than the disease itself. Hence, it’s appropriate to require definitive empirical evidence from RCTs of the benefit-harm balance before they're introduced. But critics inappropriately applied the same rules to mask studies. 6/

Trisha Greenhalgh @trishgreenhalgh

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. 7/

Trisha Greenhalgh @trishgreenhalgh

But that’s what happened. Tragically, WHO along with Public Health England, CDC and many other bodies around the world all focused on two things: a) the lack of incontrovertible, definitive evidence and b) speculation about possible harms. 8/

Trisha Greenhalgh @trishgreenhalgh

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. 9/

Trisha Greenhalgh @trishgreenhalgh

Asian countries framed the challenge differently. Recalling SARS (2003) and MERS (2012), they weren’t taking any chances. Masks *might* help in this new disease, so let’s wear them just in case. (e.g. Taiwan: cnbc.com/2020/07/15/how…) 10/

Trisha Greenhalgh @trishgreenhalgh

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?”. 11/

Trisha Greenhalgh @trishgreenhalgh

The putative harms of masks were twofold. First, self-infection. The idea was that the mask was dirty, and by touching it (while putting it on, or when your face itched under it) you might transfer virus to your hands and thence to your eyes etc. 12/

Trisha Greenhalgh @trishgreenhalgh

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. jamanetwork.com/journals/jaman… 13/

Trisha Greenhalgh @trishgreenhalgh

The meme that *touching your own mask* could kill you was an extraordinary fantasy in which many reputable scientists got swept up. Masking was depicted as a highly specialist activity, dependent on perfect donning/doffing procedures. The public simply weren’t up to it. 14/

Trisha Greenhalgh @trishgreenhalgh

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”. 15/

Trisha Greenhalgh @trishgreenhalgh

The other masks-are-harmful meme related to risk compensation. If you wear a mask, you’ll feel protected and take more risks. Like the driver who becomes more reckless when wearing a seatbelt, you’ll be slapdash about hand-washing and you’ll get too close to passers-by. 16/

Trisha Greenhalgh @trishgreenhalgh

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. bmj.com/content/370/bm… 17/

Trisha Greenhalgh @trishgreenhalgh

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. 18/

Trisha Greenhalgh @trishgreenhalgh

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. 19/

Trisha Greenhalgh @trishgreenhalgh

A RCT is a controlled experiment. Since people (or animals) are randomly allocated to one or another 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). 20/

Trisha Greenhalgh @trishgreenhalgh

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? 21/

Trisha Greenhalgh @trishgreenhalgh

Many reasons. Drugs are (arguably) a simple intervention, but masks are a highly complex one. As we all know, there are two key questions: do they protect the wearer from other people’s germs – and do they protect other people from the wearer’s germs (‘source control’)? 22/

Trisha Greenhalgh @trishgreenhalgh

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. 23/

Trisha Greenhalgh @trishgreenhalgh

More fundamentally, we’re not just interested in whether my mask protects either me or you from catching Covid during a short intervention period (say, one month). We’re interested in how masking impacts on the *exponential spread* of an accelerating pandemic. 24/