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Face masks effectively limit the probability of SARS-CoV-2 transmission

science.sciencemag.org

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Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#121

Post by carlmr »

wrycoder wrote: post #115
rsj_hn wrote: post #75

Earlier quoted context omitted.

Indeed this is the money paragraph: "The effectiveness of masks, however, is still under debate. Compared to N95/FFP2 respirators which have very low particle penetration rates (around ~5%), surgical and similar masks exhibit higher and more variable penetration rates (around ~30-70%) (2, 3). Given the large number of particles emitted upon respiration and especially upon sneezing or coughing (4), the number of respi…

I suspect that in locations with a higher prevalence of masking the population is generally more vigilant: more hand washing, more social distance, stay at home when ill, avoid obviously ill people, and so on. May be a case of correlation, not causation. It would be interesting to have a trial where everyone was wearing N95s, but we're not likely to see that.

>It would be interesting to have a trial where everyone was wearing N95s, but we're not likely to see that.

Bavaria and Austria had FFP2/KN95 only while the rest of Germany had the choice of surgical masks or FFP2, where almost everybody picked surgical masks (cheaper, not as annoying to wear).

Now the rest of Germany switched to only FFP2 in public transport.

So we do have some population studies. It didn't seem to make much of a difference. I'd say the factor overwhelming almost every other measure is the weather. More people outside means dropping case numbers. More people at home means rising case numbers.

It's been true for influenza and cold, and it's still holding up for Corona.

If any measure would make sense it would be to allow people to meet outside while forbidding it at home. They only forbade at home without allowing outside meetups, which made home parties more likely.

Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#122

Post by caddemon »

rsj_hn wrote: post #75
tgsovlerkhgsel wrote: post #39

This entire study seems to be centered around a theoretical, mathematical model, and I didn't see any attempt to actually validate that model. Basically, the authors seem to assume that the virus behaves according to their formulas, and show that under their assumptions, face masks work, but don't actually prove that their assumptions match reality - or did I miss something?

Indeed this is the money paragraph: "The effectiveness of masks, however, is still under debate. Compared to N95/FFP2 respirators which have very low particle penetration rates (around ~5%), surgical and similar masks exhibit higher and more variable penetration rates (around ~30-70%) (2, 3). Given the large number of particles emitted upon respiration and especially upon sneezing or coughing (4), the number of respi…

The RCTs I have seen with basic surgical masks also seem to focus only on protection of the wearer, whereas on a societal level wearing even a shitty mask may be helping to prevent spread by presymptomatic people. I think it is unlikely the positive outcomes in mask wearing communities are purely due to confounders, although of course they probably also play a role.

Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#123

Post by tryonenow »

splitrocket wrote: post #5
tryonenow wrote: post #2

>Surgical masks with particle collection efficiencies around ~50% cannot prevent the release of millions of particles per person and their inhalation by others (green dots in Fig. 1, B and D). In other words, the human-emitted respiratory particle number is so high that we cannot avoid inhaling particles generated by another person even when wearing a surgical mask So the entire premise of the modeling in the paper r…

Virus particles do not travel naked in the air, instead they are colloidally suspended in liquid, both droplets and aerosols. So, if your filter catches the droplets and aerosols, which are much larger in size than the actual virus, the filter works.

Aerosols are explicitly defined as particulate with long settling time, and are differentiated from droplets by their size, with arbitrarily defined cutoffs on the order of 1um [0]. The pore size of surgical masks is in the range of 16-146um [1] (surprisingly hard to find). So whether or not the masks filter aerosolized covid-19 is a valid question and these downvotes are merely reflexive conditioning.

Even the question of whether surgical masks reduce infection after surgery was recently controversial, long before covid. This science is not settled. And it's in line with at least two sources which demonstrated negligible or no reduction in covid spread from surgical masks.

Plus, cloth mask pore size is orders of magnitude larger than than that of surgical masks and whether they block aerosols on the order of 1um is extremely dubious.

0. https://wwwnc.cdc.gov/eid/article/12/11/06-0426_article

1. https://www.researchgate.net/publication/242297437_The_Relat...

Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#124

Post by Mediterraneo10 »

bellyfullofbac wrote: post #102
slver wrote: post #71

Earlier quoted context omitted.

Bless your soul, but what I've learned from the way we handled the pandemic worldwide (and are still handling it) is that information that's readily available and should in theory reach everyone, flows ridiculously slowly, even among specialists and political figures, it's easily corrupted and distorted, and a loud ignorant minority counts as much in the public discourse, as the voices of millions of reasonable peopl…

Yeah, it's disappointing to see how I still see people wearing their masks incorrectly, e.g. just over the mouth and not covering their noses. Sometimes I think human beings are advanced, but then I walk into a public toilet and I see evidence how many are still no better than animals. It's a simple technology, urinate into the hole where the water can wash it away and we'll remain hygienic, but a lot of people still…

There are a couple of defensible reasons that people don’t put their mask over their nose outdoors. Firstly, if you wear glasses, then on humid days your glasses can fog up to a degree that you can’t even cross the street safely.

Secondly, in some countries the public has chafed against laws requiring outdoor mask wearing in places that are not even remotely crowded. (In my own country, the same scientists advising the government on the COVID response have also opposed this law and urged the government to overturn it, arguing that it is clearly unscientific and only undermines the government’s credibility in getting the population to observe the public-health measures that really matter.) At that point, wearing a mask in a half-ass way is a way for the population to signal their discontent. It has happened that such laws were lifted when ruling parties saw mass flaunting of them, because such strictness might cost them the next election.

Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#125

Post by adrr »

anonymousiam wrote: post #35

Here's the manufacturer's (3M) label: WARNING: This respirator helps protect against certain particulate contaminants but does not eliminate exposure to or the risk of contracting any disease or infection. Misuse may result in sickness or death. For proper use, see supervisor, or User Instructions. https://multimedia.3m.com/mws/media/1837275O/3m-9132-healthc...

Because they are just NIOSHA certified and not FDA. Only certain N95s are have gone through testing and certification to be cleared for medical use.

Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#126

Post by splithalf »

caddemon wrote: post #122
rsj_hn wrote: post #75

Earlier quoted context omitted.

Indeed this is the money paragraph: "The effectiveness of masks, however, is still under debate. Compared to N95/FFP2 respirators which have very low particle penetration rates (around ~5%), surgical and similar masks exhibit higher and more variable penetration rates (around ~30-70%) (2, 3). Given the large number of particles emitted upon respiration and especially upon sneezing or coughing (4), the number of respi…

The RCTs I have seen with basic surgical masks also seem to focus only on protection of the wearer, whereas on a societal level wearing even a shitty mask may be helping to prevent spread by presymptomatic people. I think it is unlikely the positive outcomes in mask wearing communities are purely due to confounders, although of course they probably also play a role.

I suspect mask wearing increases social distancing. That would explain much of the data but satisfy neither political party.

Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#127

Post by caddemon »

takeda wrote: post #47
brobdingnagians wrote: post #22

This isn't a study; it is a mathematical model [1]. > Here, we develop a quantitative model of airborne virus exposure that can explain these contrasting results and provide a basis for quantifying the efficacy of face masks. We show that mask efficacy strongly depends on airborne virus abundance. Based on direct measurements of SARS-CoV-2 in air samples and population-level infection probabilities, we find that the…

The study about mask being ineffective is about cloth mask. That's why the hospital masks are no longer made of cloth.

That study also focuses on influenza which does not transmit quite the same way as COVID. A bunch of these links were the same thing being shared in the early days of COVID by doctors claiming that masks did not work. Obviously most healthcare organizations have changed their recommendations in light of new evidence specific to this pandemic.

Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#128

Post by tgsovlerkhgsel »

rsj_hn wrote: post #75
tgsovlerkhgsel wrote: post #39

This entire study seems to be centered around a theoretical, mathematical model, and I didn't see any attempt to actually validate that model. Basically, the authors seem to assume that the virus behaves according to their formulas, and show that under their assumptions, face masks work, but don't actually prove that their assumptions match reality - or did I miss something?

Indeed this is the money paragraph: "The effectiveness of masks, however, is still under debate. Compared to N95/FFP2 respirators which have very low particle penetration rates (around ~5%), surgical and similar masks exhibit higher and more variable penetration rates (around ~30-70%) (2, 3). Given the large number of particles emitted upon respiration and especially upon sneezing or coughing (4), the number of respi…

What I don't understand... the UK was already doing challenge studies (intentionally infecting consenting volunteers).

Why didn't they make one with masks? Really can't be that hard to take a known infected person, sit them on a chair, arrange 100 people around them for a few hours with masks applied in a checkerboard pattern, and observe the results.

This is usually out of question for ethical reasons, but they already decided that that wasn't an obstacle, so why not (also) do a study on this?

(Or rather, arrange 10x 10 people, in 5 rounds putting a mask on the infected person.)

Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#129

Post by tgsovlerkhgsel »

carlmr wrote: post #121
wrycoder wrote: post #115

Earlier quoted context omitted.

I suspect that in locations with a higher prevalence of masking the population is generally more vigilant: more hand washing, more social distance, stay at home when ill, avoid obviously ill people, and so on. May be a case of correlation, not causation. It would be interesting to have a trial where everyone was wearing N95s, but we're not likely to see that.

>It would be interesting to have a trial where everyone was wearing N95s, but we're not likely to see that. Bavaria and Austria had FFP2/KN95 only while the rest of Germany had the choice of surgical masks or FFP2, where almost everybody picked surgical masks (cheaper, not as annoying to wear). Now the rest of Germany switched to only FFP2 in public transport. So we do have some population studies. It didn't seem to…

Ventilation could also be a factor. You're more likely to open the window when the air outside is pleasant.

Re: Face masks effectively limit the probability of SARS-CoV-2 transmission

#130

Post by _Nat_ »

rsj_hn wrote: post #75
tgsovlerkhgsel wrote: post #39

This entire study seems to be centered around a theoretical, mathematical model, and I didn't see any attempt to actually validate that model. Basically, the authors seem to assume that the virus behaves according to their formulas, and show that under their assumptions, face masks work, but don't actually prove that their assumptions match reality - or did I miss something?

Indeed this is the money paragraph: "The effectiveness of masks, however, is still under debate. Compared to N95/FFP2 respirators which have very low particle penetration rates (around ~5%), surgical and similar masks exhibit higher and more variable penetration rates (around ~30-70%) (2, 3). Given the large number of particles emitted upon respiration and especially upon sneezing or coughing (4), the number of respi…

tl;dr- There wasn't really a conundrum to explain related to mask-effectiveness at different levels. Rather, it's more like a lack of good data, as prior studies being cited were ridiculously weak.

---

This seems like a largely helpful characterization.

However:

> So they are left with a conundrum that places which use surgical masks seem to be better off, whereas randomized control studies of surgical masks show little to no benefit. This is what they try to explain with a mathematical model.

There wasn't really a conundrum. When the study writes:

> Moreover, randomized clinical trials show inconsistent or inconclusive results, with some studies reporting only a marginal benefit or no effect of mask use (5, 6).

, the two studies they cite with inconclusive data are ridiculously weak.

The first one (their "5") is [https://academic.oup.com/jid/article/201/4/491/861190]. This was published in 2010, with data from 2006-to-2007, for "influenza-like illness" (not SARS-CoV-2, nor even influenza (explained below)). Apparently they asked students living in the dorms to do certain behaviors for weeks at a time. They concluded:

> Conclusions: These findings suggest that face masks and hand hygiene may reduce respiratory illnesses in shared living settings and mitigate the impact of the influenza A(H1N1) pandemic.

And if that sounds inconclusive, yeah.. their study was ridiculously under-powered. In fact:

> This study has several limitations. First, influenza incidence was low, so it is likely that most ILI cases were not associated with influenza infection, even though the study was conducted during the influenza season.

So not only were they not writing about COVID, apparently the authors argue that most of the cases probably weren't even influenza. There're other huge problems with this study too.

The second one (their "6") is [https://pubmed.ncbi.nlm.nih.gov/33205991/]. This study assigned subjects a recommendation to wear masks -- presumably some who were recommended to wear masks didn't, while some who weren't recommended to wear masks did.

Even then, their statistical analysis was very noisy:

> Although the difference observed was not statistically significant, the 95% CIs are compatible with a 46% reduction to a 23% increase in infection.

And their stated limitations:

> Limitation: Inconclusive results, missing data, variable adherence, patient-reported findings on home tests, no blinding, and no assessment of whether masks could decrease disease transmission from mask wearers to others.

Point being that there's not really a conundrum related to things not fitting together so much as a simple lack of good data.

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