Preprint Review Version 1 This version is not peer-reviewed

Face Masks Against COVID-19: An Evidence Review

Version 1 : Received: 10 April 2020 / Approved: 12 April 2020 / Online: 12 April 2020 (17:41:10 CEST)
Version 2 : Received: 12 May 2020 / Approved: 13 May 2020 / Online: 13 May 2020 (13:16:04 CEST)
Version 3 : Received: 10 July 2020 / Approved: 12 July 2020 / Online: 12 July 2020 (16:11:50 CEST)

How to cite: Howard, J.; Huang, A.; Li, Z.; Tufekci, Z.; Zdimal, V.; van der Westhuizen, H.; von Delft, A.; Price, A.; Fridman, L.; Tang, L.; Tang, V.; Watson, G.L.; Bax, C.E.; Shaikh, R.; Questier, F.; Hernandez, D.; Chu, L.F.; Ramirez, C.M.; Rimoin, A.W. Face Masks Against COVID-19: An Evidence Review. Preprints 2020, 2020040203 (doi: 10.20944/preprints202004.0203.v1). Howard, J.; Huang, A.; Li, Z.; Tufekci, Z.; Zdimal, V.; van der Westhuizen, H.; von Delft, A.; Price, A.; Fridman, L.; Tang, L.; Tang, V.; Watson, G.L.; Bax, C.E.; Shaikh, R.; Questier, F.; Hernandez, D.; Chu, L.F.; Ramirez, C.M.; Rimoin, A.W. Face Masks Against COVID-19: An Evidence Review. Preprints 2020, 2020040203 (doi: 10.20944/preprints202004.0203.v1).

Abstract

The science around the use of masks by the general public to impede COVID-19 transmission is advancing rapidly. Policymakers need guidance on how masks should be used by the general population to combat the COVID-19 pandemic. Here, we synthesize the relevant literature to inform multiple areas: 1) transmission characteristics of COVID-19, 2) filtering characteristics and efficacy of masks, 3) estimated population impacts of widespread community mask use, and 4) sociological considerations for policies concerning mask-wearing. A primary route of transmission of COVID-19 is likely via small respiratory droplets, and is known to be transmissible from presymptomatic and asymptomatic individuals. Reducing disease spread requires two things: first, limit contacts of infected individuals via physical distancing and contact tracing with appropriate quarantine, and second, reduce the transmission probability per contact by wearing masks in public, among other measures. The preponderance of evidence indicates that mask wearing reduces the transmissibility per contact by reducing transmission of infected droplets in both laboratory and clinical contexts. Public mask wearing is most effective at stopping spread of the virus when compliance is high. The decreased transmissibility could substantially reduce the death toll and economic impact while the cost of the intervention is low. Thus we recommend the adoption of public cloth mask wearing, as an effective form of source control, in conjunction with existing hygiene, distancing, and contact tracing strategies. We recommend that public officials and governments strongly encourage the use of widespread face masks in public, including the use of appropriate regulation.

Subject Areas

COVID-19; SARS-CoV-2; masks; pandemic

Comments (73)

Comment 1
Received: 13 April 2020
Commenter: David Curtis
The commenter has declared there is no conflict of interests.
Comment: This article completely fails to consider the costs of mask-wearing. There's a reason why we don't do this. It impairs our quality of life. We like to feel fresh air on our faces. We like to go out and see people. We like to be able to talk to people unencumbered. We like to be able to see people's expressions, their smiles.

Sure, make a judgement about the relative benefits and harms of mask-wearing. But don't purport to be writing some kind of rational appraisal while completely ignoring the obvious loss of quality of life which universal, permanent mask-wearing would entail.
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Response 1 to Comment 1
Received: 15 April 2020
Commenter: Raquel Fernandez de Bobadilla
The commenter has declared there is no conflict of interests.
Comment: How can anyone consider the temporal disconfort of wearing a mask including the loss of mimical comunication in a mortal pandemia that is transforming the mimical expressions of millions of people in so long lasting suffering, desperate and loneliness expressions. How can anyone be so superficial claiming to that point in this devastating plague?
Response 2 to Comment 1
Received: 15 April 2020
Commenter: Juan Lee
The commenter has declared there is no conflict of interests.
Comment: Yes, seeing the smiles of the loved ones is important for life. But the loved ones need to be ALIVE first.
For my understanding, what's been discussed in this review is not permanent mask-wearing, but a temporal in response to the epidemic, and how long it needs to last depends on how well the control policies are implemented, including locked-down, quarantine, social distancing, and yes, wearing masks.
You may think the COVID19 is not such a big threat and not worth sacrificing your right of free-breath for several months. But it is indeed life-threatening for the elderly, with a death rate higher than 15% even with adequate medical care. According to this article, wearing masks is not only to protect the wearer, but more importantly, to protect the others, especially the old ones. They are grandpas and grandmoms of our children. They've been hard-working and contributed a lot to their families and their nations. They are not expendables. It's worth everything to protect them -- our mother and fathers and childhood teachers who protected us when we are young and fragile. For a kid, nothing can replenish the unconditional love from grandparents, their kisses and hugs and cakes and smiles.
I'm very much willing to accept the inconvenience of wearing-masks, and much more, so my toddler can grow up with their grandpas and grandmoms be around, holding their hands and teaching them how to walk.
Response 3 to Comment 1
Received: 18 April 2020
Commenter: Jonathan Shaw
The commenter has declared there is no conflict of interests.
Comment: Possibly true. But not as detrimental to quality of life as catching COVID-19. And, I’d suggest, not as detrimental as your finding out, through contact tracing, that you’ve personally been spreading the virus when you could easily have avoided doing so.

The long-term impacts of COVID-19 are also unknown at this point, and might well further impact on quality of life.
Response 4 to Comment 1
Received: 20 April 2020
Commenter: Dr. Barry Pless
The commenter has declared there is no conflict of interests.
Comment: I think the article is solid. As for the comments from D Curtis re quality of life, what he appears to have overlooked is that none of the lovely experience he lists are likely to be experienced by someone who is dead or stuck in hospital on a ventilator for weeks on end. Nor does he appear to realize that masks are to protect others. I guess Curtis lives in a country where only 'me' counts. I wonder where that might be?
Response 5 to Comment 1
Received: 22 April 2020
Commenter: Anil Panigrahi
The commenter has declared there is no conflict of interests.
Comment: David Curtis fails to understand the difference between "saving a life" and "savoring life". You savor something when you have no worries. When you stand in the middle of a pandemic, you either hide or run for your life - neither of which is a matter of comfort or enjoyment. A face mask saves you from others, and others from you! Have you heard of cancer? The patients undergo unimaginable discomfort during the treatments because the goal is to save their lives. Per your whim, it would be wise to ban all radiation/chemo therapies, I guess?
Response 6 to Comment 1
Received: 23 April 2020
Commenter: Emmett Keyser
The commenter has declared there is no conflict of interests.
Comment: If face coverings impair the transmission of the virus why not advocate some sort of hazmat suit? And since the medical community would need access before general population then why not advocate for homemade cloth bio hazmat suits? The answer gets directly to the comment which is that there is more to simply "stopping" the spread of the virus however much merit there is to that in the first place. It has to be weighed against everything else including quality of life.

In other words, what advocates for face coverings are saying is "we do not know if it helps or the evidence is anecdotal at best but since it only costs a few bucks we should all do it anyway regardless of any downsides" which is just another way of advocating for something that hasn't been proven and discarding reason and common sense.
Response 7 to Comment 1
Received: 23 April 2020
The commenter has declared there is no conflict of interests.
Comment: An interesting study and the authors can be commended for the analysis. I think there are three aspects that are worth pointing out:
1. The overall quality of the evidence is low. A few examples (not exhaustive -- I think the authors realize these limitations):
a. Studies that measure the effectiveness of mask wearing AND enhanced or increased hand washing are inherently confounded. It is very, very difficult to untangle the individual components of the effectiveness.
b. Similarly, studies looking at cloth mask blocking are mechanistic and provide no direct infection result.
c. Clinical effectiveness studies use medical masks, not cloth masks.
d. Population and country studies are highly confounded.
e. Etc.

2. The study omitted many potential harms. Compensatory behavior as a potential har was addressed via analogy (which is excellent, but insufficient). Other typically observed and potentially hazardous behaviors of the general population were not. These include: increased touching of face/eyes with the consequent creation of more infected surfaces; creation of potentially very infectious cloth masks that must be handled appropriately, etc.

3. Cultural differences that may impact the overall effectiveness of a population scale implementation.

In the end we must accept (and I think the authors state as much) that the evidence is weak in both directions. We do not have good quality evidence supporting the use of masks in the general population and we do not have quality evidence against. Policymakers must decide on the basis of indicia and reasonable hypotheses and there is no definitely wrong nor definitely right answer. However, providing conflicting directives in distinct (yet comparable) geographies may provide the basis for analysis that (still short of an RCT) could illuminate future decisions.

Additionally, it is imperative that the public be informed of the limitations of the science. When policy recommendations are issued with the cloak of scientific certainty (which in this case it is lacking) any subsequent shift in policy (possibly grounded in better evidence) unfairly builds public distrust in science. Such a disillusionment with science and science-based policy can turn more deadly than the issue the policy was meant to address (I give the anti-vaccine movement for evidence).
Response 8 to Comment 1
Received: 12 May 2020
Commenter: Chris Albone
The commenter has declared there is no conflict of interests.
Comment: Sure... I do like to feel the beaultiful Autumn breeze on my face...

But given a choice of either being locked up for months more, not being able to go to the theatre, see bands or the like - or worse yet, becoming infected - I think that the cost is minor at best.

We're now starting to investigate a phased appraoch to moving back to the office - and we are going to do this in a way that minimises the possibility of infection. I can see that the mandatory use of masks would be one way to achieve this. Again - given the choice of being able to work from the office at least some of the time, vs being stuck at home for weeks on end, I'll choose the mask.
Response 9 to Comment 1
Received: 12 May 2020
The commenter has declared there is no conflict of interests.
Comment: The commenter to which I’m responding, seems to misunderstand the role of the paper submitted. The question of how to balance the general quality of life against epidemiological concerns is outside the scope of the article. In this instance, politicians, not doctors or academicians will have the unenviable job of making those uncertain calls.
Response 10 to Comment 1
Received: 12 May 2020
Commenter: Andri Gretarsson
The commenter has declared there is no conflict of interests.
Comment: The commenter to which I’m responding, seems to misunderstand the role of the paper submitted. The question of how to balance the general quality of life against epidemiological concerns is outside the scope of the article. In this instance, politicians, not doctors or academicians will have the unenviable job of making those uncertain calls.
Response 11 to Comment 1
Received: 12 May 2020
Commenter: Kyle D San Giovanni
The commenter has declared there is no conflict of interests.
Comment: I find it strange not to understand the difference between life versus quality of life
Simply put there is no measure of quality if one is dead
The evidence in favor of everyone wearing a simple mouth/nose covering to protect both or all parties from getting ill, passing on a disease or dying is overwhelming
Time to wake up, be honest and stop lying to yourself and others...
Response 12 to Comment 1
Received: 12 May 2020
The commenter has declared there is no conflict of interests.
Comment: There are many things that this study did not measure. In fact it did not set out to measure any quality of life changes associated with masks. So it has done what it has set out to do. This is how science works.

It also did not measure the loss of vitamin d associated with covering your face while possibly outside. It did not measure the increased drag that one might experience when covering their face with cloth there for slowing down people's travel and possibly having an economic impact because people are moving more slowly. It did not measure strain on vertebrae associated with elastic neck coverings. It did not measure the time lost and economic impact that would be felt by the amount of ridiculous comments people would make replying to this study.

Not measuring these things does not make it any less valid. It just simply does not measure them.
Response 13 to Comment 1
Received: 12 May 2020
Commenter: Bengt Johansson
The commenter has declared there is no conflict of interests.
Comment: My respons to the first persosns comment. I think he is pulling our legs and what he writes is a satire not something he really means. If he really mean it than he need to educate himself in a number of fields to get an understanding of logical reasoning.
Response 14 to Comment 1
Received: 12 May 2020
Commenter: francis capet
The commenter has declared there is no conflict of interests.
Comment: any person not wearing a facemask is a potential danger for his/her surroundings.

there is no way to protect oneself from this danger (except professional PPE)

people do have a right to be legally protected from dangers like tobacco smoke, vehicles exceeding speed limits, unsafe food, dangerous animals and people, etc... these dangers are forbidden by law

so should be unmasked persons in the public space - in an epidemic context, declared such by a legal authority

even at a distance from other persons, he/she deposits droplets of infective material on surfaces that will be touched by othe persons
even if he/she thinks to be immune : this is not visible, not wearing a mask sends a message this is the way to do
Response 15 to Comment 1
Received: 23 May 2020
Commenter: Robert Kernodle
The commenter has declared there is no conflict of interests.
Comment: I completely agree that the article fails to adequately consider the humane factor in any mandatory face mask requirement, for an alleged disease that has not proven as deadly as popular accounts of it would have us believe.

Furthermore, the article fails to address the undeniable reality of ... lack of proper use ... in many people who wear the masks. Everywhere I go, where people are wearing masks, I always see people using them incorrectly -- touching them repeatedly, wearing them below their nose half the time, wearing them around their neck half the time, pulling them on or off to engage in their normal reflexive behaviors, like answering their mobile phone, scratching their nose, or simply removing them, so that their words can be understood in a normal conversation (which masks disrupt).

Also, my reading of research does not establish the level of certainty that the authors of the article convey, as I point out in this article I wrote:

hubpages.com/health/COVID-19-Face-Mask-Protection-What-Leaders-Fail-to-Understand
The incredibly small size of a virus, coupled with the disruption of normal reflexes by a mask, coupled with lack of human discipline and willingness to use masks correctly, make the widespread expectation of mandatory mask wearing completely unreasonable.
Response 16 to Comment 1
Received: 26 May 2020
Commenter: Laurel Robison
The commenter has declared there is no conflict of interests.
Comment: There is a difference between peer reviewed science and the opinions and personal preferences expressed here in the comments. Facebook might be a better and more influential outlet for expressing the desire to feel the wind in your face, see facial expressions, or let people know your hard and fast pre-formed opinion wasn't changed by this well-cited article which discusses some very well-established principles of epidemiology. Let's be clear, science is not here for you to believe in, it is here to understand. Your "opinion" is meaningless without thoughtful research and citations for every assertion. All the non-technical pieces floating around on the internet about ANY subject are really meaningless in the context of juried published research.
Response 17 to Comment 1
Received: 28 May 2020
The commenter has declared there is no conflict of interests.
Comment: @ David C. No one ever said you can’t enjoy the fresh air outside or that you can’t speak to your family household members without a mask! You can still go outside without wearing a mask, as long as you are practicing social distancing. I do that every day.

@ Robert K. If you are concerned about other people’s improper mask use, this is why you wearing one for yourself serves as back up protection, along with all other protective and preventative measures discussed.

The mask is just common sense, has been utilized for years and is inexpensive. The main purpose is to reduce overall risk, prevent a hospital surge and medical supply shortages, and to buy us time until a long term solution is developed (like the vaccine) and was never presented as 100% fool proof. It’s not that complicated but has been explained so many times both by medical professionals across the country but on Trumps own team as well.

It is very clear that this anti-mask movement was never about the mask but the political fears and paranoia held by people who primarily enjoy mocking those of us that support masks.
Response 18 to Comment 1
Received: 18 June 2020
The commenter has declared there is no conflict of interests.
Response 19 to Comment 1
Received: 5 July 2020
Commenter: Nat Fauci
The commenter has declared there is no conflict of interests.
Comment: You are more than right. The CDC makes the following clear; asymptomatics don't spread covid (supported by WHO), symptomatics spread it thru infected droplets, not mere air, masks do not protect the wearer and covid is not a fatal disease. There is much more, but, as with any proof, one counterexample is sufficient and here I have provided four.

Responding in fear decreases the ability of the immune system to do its job. Fear not, eat low calorie, high nutrition food, love your neighbor!
Comment 2
Received: 15 April 2020
The commenter has declared there is no conflict of interests.
Comment: I'd be interested to hear the author's comments on the efficacy of home made masks when they are moist - to my knowledge, the papers cited are for ideal i.e. dry masks, and there is evidence (referred to in the studies) that moist masks may allow stripping of wet particles, and subsequent transmission of dry particles.

Some other notes - it think it would be clearer to refer to "seasonal coronavirus" rather than "coronavirus" to distinguish more from COVID19. Also, the Czech Repulic is now Czechia.
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Response 1 to Comment 2
Received: 12 May 2020
Commenter: Peter Porco
The commenter has declared there is no conflict of interests.
Comment: "Czech Republic" is still the name, the formal name, of the country. Czechia is informal, in the same way America is the informal name of the United States of America.
Comment 3
Received: 16 April 2020
Commenter: Wu Zunyou
The commenter has declared there is no conflict of interests.
Comment: This review have some valid points, but for some points, there is lacking evidence, for example, claim:

"The available evidence suggests that near-universal adoption of non-medical masks when out in public, in combination
with complementary public health measures could successfully reduce effective-R to below 1.0, thereby stopping community spread."

Where is the evidence for that?

In this review, it is stated:

"...mask wearing when going out increasing to 97.5%, without mandatory requirements."

So if almost 100% of people are wearing masks in some countries, where is the evidence that R0 will be reduced to below 1?

Some of the statements made me look at the authors, and the very first author is Jeremy Howard.

With all due respect, let's see who is Jeremy Howard.

"Jeremy Howard is an entrepreneur, business strategist, developer, and educator."

https://www.fast.ai/about/#fast-ai
So, no medical background there, and further investigation leads us to his twitter page, called:

"Jeremy #Masks4All Howard"

And website:

https://masks4all.co/
Where it is stated:

"Request Mandatory Mask Laws"

On his Twitter page, amongst other things, it is stated.

The Czech Republic went from zero mask usage to 100% in 10 days, and in the process they halted the growth of new covid-19 cases.

https://twitter.com/jeremyphoward/status/1242894378441506816

Czech Republic made face masks mandatory on 18. march 2020

https://www.upol.cz/nc/en/covid-19/news/clanek/new-measures-in-force-in-the-czech-republic-face-masks-mandatory-and-limited-shop-operations/
Here are the official statistics of the cases in Czech Republic

https://onemocneni-aktualne.mzcr.cz/covid-19

Where is the evidence that mask wearing halted the growth of new covid-19 cases?

With all due respect, after reading this review, and afterfurther investigation, there are some valid points, but this review seems to be very biased.
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Response 1 to Comment 3
Received: 17 April 2020
Commenter: Wu Zunyou
The commenter has declared there is no conflict of interests.
Comment: Response to the comment from anonymous,

Czech Republic made face masks mandatory on 18. March 2020

https://www.upol.cz/nc/en/covid-19/news/clanek/new-measures-in-force-in-the-czech-republic-face-masks-mandatory-and-limited-shop-operations/
Statement from Jeremy #Masks4All Howard on 25. March 2020

The Czech Republic went from zero mask usage to 100% in 10 days, and in the process they halted the growth of new covid-19 cases.

https://twitter.com/jeremyphoward/status/1242894378441506816

Official statistics of the cases in Czech Republic

https://onemocneni-aktualne.mzcr.cz/covid-19

So again, where is the evidence for this statement?

"The Czech Republic went from zero mask usage to 100% in 10 days, and in the process they halted the growth of new covid-19 cases."

This for Czech Republic is just one thing amongst other thing that are stated. For example, someone in some country could say that not wearing masks have halted the growth of new covid-19 cases, but the question remains the same, where is the evidence for that claim? Or someone in for exaple Democratic Republic of the Congo, population 101,780,263, or Tajikistan, population 9,275,827, both countries with no confirmed cases, could state that there is no novel coronavirus in their country, and the funny thing is, that information would be more accurate than statement from Jeremy #Masks4All Howard.

Comparison with just numbers, Sweden population 10,333,456, no lockdown, 1,400 deaths, Belgium, population 11,515,793, in lockown, 5,163 deaths, Switzerland, population 8,570,146, 1,288 deaths, so what just plain numbers tells us, Sweden does not have lockdown, there are no strict measures, so that must be better than measures in Belgium and Switzerland? There are multiple factors that are affecting on numbers in different countries, and just numbers and statistics does not tell the whole story.
Response 2 to Comment 3
Received: 17 April 2020
Commenter: Avery Cargo
The commenter has declared there is no conflict of interests.
Comment: With all do respect, when you complain about curiculum, essentially debating scientific facts by trying to discredit whoever doesn't share your views, instead of the research, constructively, with experiments and observations, you're not applying scientific methds, you're playing petty politics, and shouldn't be here in the first place. This is not only insulting to the research you mention, this is insulting to anybody reading it.

Also there's a very high chance of COI, why else would anybody waste resources complaining in absolutely unconstructive ways about research? Again this is politics, not science, that man is clearly a disinformation agent.
Response 3 to Comment 3
Received: 17 April 2020
Commenter: Wu Zunyou
The commenter has declared there is no conflict of interests.
Comment: Response to Avery Cargo, I don't know to whom you are referring to, but that is exactly the point, where are scientific facts and evidence to confirm some of the statements?
Response 4 to Comment 3
Received: 17 April 2020
Commenter: DJ
The commenter has declared there is no conflict of interests.
Comment: I'm not sure why there is resistance to the notion of mask wearing. There seems to a sizable body of supportive evidence, but there will always be an argument to he had againsed any notion. Regardless to the balance of where the truth lies, wearing a mask won't create harm, so its appears to be a practice that potentially gives benefits, that vastly outweigh any perceived negatives.
Mask wearing may help, may not, depending on a person's opinion, but there's no downside. In those circumstances, anything that may be of benefit, is worthwhile.
Trying to discredit the notion, by pointing to the background of supporting commentators feels disingenuous and and odd stance, given the current circumstances.
There are creditable, medically qualified exponents of mask wearing. To target any that are from differing backgrounds may be construed as pedantic.
Response 5 to Comment 3
Received: 17 April 2020
Commenter: Jeremy Howard
The commenter has declared there is no conflict of interests.
Comment: Generally, we don't argue about the validity of a paper based on who wrote it. As I researched this topic I became so convinced that it can help people, that I dropped everything and, for no financial reward whatsoever, have dedicated weeks of my life to communicating this to scientific and lay audiences. Apparently one person has a view that doing so means that my scientific input is less valuable. So be it. For those that are convinced by credentials rather than science, here are the other authors on this paper:

• Anne W. Rimoin: Professor, Department of Epidemiology; Director, UCLA Center for Global and Immigrant Health; Director, UCLA-DRC Health Research and Training Program
• Christina Ramirez: Professor of Biostatistics, UCLA Fielding School of Public Health; Specializes in infectious disease and machine learning; worked on modeling the effects of masks
• Larry Chu: Professor of Anesthesiology, Perioperative and Pain Medicine and Director of the Stanford Anesthesia Informatics and Media (AIM) Lab. Executive Director of Stanford Medicine X, the world's most-discussed academic program on emerging technology and medicine.
• Zdimal Vladimir: Head of Department of Aerosols Chemistry and Physics; Institute of Chemical Process Fundamentals of the CAS
• Zeynep Tufekci: Associate Professor, UNC School of Information and Library Science; Adjunct Professor, Department of Sociology; Writer: The Atlantic and NY Times), including Why Telling People They Don’t Need Masks Backfired
• Lex Fridman: research scientist at MIT, works on autonomous vehicles & applications of deep learning in human robot interaction.
• Amy Price: Senior Research Scientist, Stanford AIM Lab; Stanford Covid-19 Evidence Service; Editor: British Medical Journal (BMJ)
• Lei-Han Tang: Professor, Hong Kong Baptist University, Beijing Computational Science Research Center
• Zhiyuan Li: Professor, Center for Quantitative Biology, Peking University; research in quantitative modeling of biological networks and bioinformatics; worked on the toy model of the mask-wearing effect.
• Helene-Mari van der Westhuizen: Researcher at Oxford University, Department of Primary Health Care Sciences, studies strategies for TB infection control in high burden settings; co-founder of TB Proof, which focuses on preventing Tuberculosis transmission in healthcare facilities and reducing TB stigma.
• Austin Huang: lead an applied ML team at Fidelity. PhD from Harvard-MIT Division of Health Sciences and Technology, researched HIV at Brown and worked on drug development at Pfizer R&D. Developing the Hasktorch library to advance functional programming for machine learning.
• Danny Hernandez: research scientist at OpenAI. Works on measuring and forecasting AI progress and has expertise on making judgement based forecasts given weak evidence.
• Gregory Watson: Researcher at UCLA Center for Health Policy Research
• Arne von Delft: University of Cape Town; Co-founder of TB Proof
• Frederik Questier: Professor at Vrije Universiteit Brussel; Founder of Open Patent Office
• Reshama Shaikh: Statistician; worked on clinical trials at pharmaceutical companies including Pfizer, Merck and Teva. Founder: Data Umbrella
• Christina Bax: University of Pennsylvania
• Viola Tang: Hong Kong University of Science and Technology
Response 6 to Comment 3
Received: 4 May 2020
Commenter: Wu Zunyou
The commenter has declared there is no conflict of interests.
Comment: Reply to Jeremy Howard, when someone who calls himself Jeremy #Masks4All Howard, when someone with strong advocacy towards something with cherry picking and false data, or better yet, with no data and evidence for some claims, advocate for something to be mandatory, paper itself and those who wrote it must be thoroughly checked and questioned.
Response 7 to Comment 3
Received: 11 May 2020
The commenter has declared there is no conflict of interests.
Comment: Replying to Wu Zunyou, Martin Goodson and Jeremy Howard,

Agree with the need for skepticism until this paper is peer reviewed. Some numbers are wrong. Maybe should not be enough to cast doubt on the final conclusion, but...

... I admit to a wry smile that one of the authors has "expertise on making judgement based forecasts given weak evidence"
Response 8 to Comment 3
Received: 27 June 2020
Commenter: Philip Macanick (Click to see Publons profile: )
The commenter has declared there is no conflict of interests.
Comment: Wu Zunyou compares Belgium and Sweden. Belgium has counted every suspected case even not tested as a Covid-19 death. And Sweden’s numbers are far from under control. Their chief epidemiologists now admits their approach was a mistake, and it has killed far more people than in most directly comparable countries.

In any case I don’t know what that has to do with evidence for masks as the paper is not about lockdowns.
Comment 4
Received: 17 April 2020
Commenter: Wu Zunyou
The commenter has declared there is no conflict of interests.
Comment: Response to Avery Cargo, I don't know to whom you are referring to, but that is exactly the point, where are scientific facts and evidence to confirm some of the statements?
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Comment 5
Received: 17 April 2020
Commenter: martin goodson
The commenter has declared there is no conflict of interests.
Comment: The article appears to mis-state the findings of the Bae et al 2020 paper:

Another relevant (but under-powered, with n=4) study (31) found that a cotton mask blocked 96% (reported as 1.5 log units or about a 36-fold decrease) of viral load on average, at eight inches away from a cough from a patient infected with COVID-19. If this is replicated in larger studies it would be an important result, because it has been shown (32) that "ev- ery 10-fold increase in viral load results in 26% more patient deaths" from "acute infections caused by highly pathogenic
viruses".


The relevant sections in Bae et al is as follows:
" The median viral loads after coughs without a mask, with a surgical mask, and with a cotton mask were 2.56 log copies/mL, 2.42 log copies/mL, and 1.85 log copies/mL, respectively. "

The difference in log units between cotton mask and no mask is 2.56-1.85, or 0.71. This is a 5.1-fold decrease, not a 36-fold decrease.
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Response 1 to Comment 5
Received: 27 April 2020
Commenter: Andrew Walsh
The commenter has declared there is no conflict of interests.
Comment: Interesting article but I agree with Martin Goodson.
Authors seem to have confused the relationships between log scales and % differences somehow.
Makes me wonder how many other similar errors there are - I am not about to check every reference to find out.
Suggest Jeremy double check all his numbers.
Comment 6
Received: 17 April 2020
Commenter: DJ
The commenter has declared there is no conflict of interests.
Comment: John from Boston , you are indeed correct to point out the psychological harm from mask wearing. I didn't think it enought to warrant a mention in this context, maybe an oversight. but I can understand the rationale. Duly acknowledged. Its certainly not a lack of empathy, that's an assumption. David Curtis no doubt places importance on the freedom of not having constraints of mask wearing placed on him, that I also understand. The unfortunate situation is that all these psychological issues pale besides the monumental adversity that the entire human race is experiencing. The stakes are higher than anything in living memory. People cite the second world war as a comparison. This was a time when people were forced in to food rationing, blitz bombing, blackouts, poverty and bereavement. At present, people are facing the prospect of considerably less social and psychological stresses in order to improve the prospects of everyone. Its not just the medical impact, but an unprecedented economical catastrophe. The psychological effects of wearing a mask for a duration, is , in the grand scheme, not a big sacrifice, to help people stay alive. I really shouldn't have to gain trust of someone who should be able to gauge the seriousness of the situation for themselves. Social distancing is for many, a form of torture. This I can fully understand. Mask wearing is a measure that could potentially allow for an easing of social distancing. Given the option, stay inside, or go outside with a mask and in the process prevent contamination leading to potential death. Most people could see the preferable option. It's also not a difficult thing to understand that short term suffering will lead to a better long term outcome.
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Comment 7
Received: 17 April 2020
Commenter: Lars De Vleeschauwer
The commenter has declared there is no conflict of interests.
Comment: I'd be interested to see the autors wearing cotton masks for 8 hours straight especially in very warm conditions.

I'd also be interested for them to comment why WHO says:

Cloth (e.g. cotton or gauze) masks are not recommended under any circumstance.

https://www.who.int/docs/default-source/documents/advice-on-the-use-of-masks-2019-ncov.pdf
ECDC also:

'There is conflicting evidence on the protective effect for the wearer of medical face masks for influenza-like
illness (ILI) and laboratory-confirmed influenza in household settings
' [5,15,18,19]. 'Based on the lack of
evidence, it has so far not been recommended that people who are not ill or who are not providing care to a
patient should wear a mask to reduce influenza or COVID-19 transmission.
' However, WHO’s guidance on ‘Nonpharmaceutical public health measures for mitigating the risk and impact of epidemic and pandemic influenza’,
conditionally recommends face mask use in the community for asymptomatic individuals in severe
epidemics or pandemics in order to reduce transmission in the community; this is based on mechanistic
plausibility for the potential effectiveness of this measure [20]. It should be noted that all relevant evidence
comes from studies on influenza and other coronaviruses and may not be directly applicable to COVID-19.
'There is no evidence that non-medical face masks or other face covers are an effective means of respiratory
protection for the wearer of the mask. Overall, various non-medical face masks were shown to have very
low filter efficiency (2–38%) [21]. In one study, cotton surgical masks were associated with a higher risk of
penetration of microorganisms and ILI compared to no masks
'[5].
'There is limited indirect evidence showing that non-medical face masks made from various materials may
decrease the release to the environment of respiratory droplets produced by coughing, but available evidence
suggests that non-medical face masks are less effective than medical masks as a means for source control
'
[22]. There are no established standards for self-made non-medical face masks. One of the advantages of nonmedical face masks made of cloth or other textiles is that they can be made easily and can be washed and
reused.

https://www.ecdc.europa.eu/sites/default/files/documents/COVID-19-use-face-masks-community.pdf
Cotton face masks if used must be washed after every use and sterilized unlike two ply disposable masks which can be binned. Both need to be changed after use of two hours so as to ensure effective filtration of bacteria.

https://www.ijic.info/article/download/10788/7862/
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Response 1 to Comment 7
Received: 29 April 2020
Commenter: Frank Cathey
The commenter has declared there is no conflict of interests.
Comment: No where in the linked article or in your copy/paste from the article does the WHO say "Cloth (e.g. cotton or gauze) masks are not recommended under any circumstance."
Response 2 to Comment 7
Received: 30 April 2020
The commenter has declared there is no conflict of interests.
Comment: Response to Frank Cathey's comment

I believe you need to look at the final sentence in https://www.who.int/docs/default-source/documents/advice-on-the-use-of-masks-2019-ncov.pdf
Response 3 to Comment 7
Received: 5 May 2020
The commenter has declared there is no conflict of interests.
Comment: There is new WHO guidance that does NOT say that it is not recommended. That WHO document is from January. There is a new one from April 6 2020: https://apps.who.int/iris/rest/bitstreams/1274280/retrieve

"The use of masks made of other materials (e.g., cotton fabric), also known as nonmedical masks, in the community setting has not been well evaluated. There is no current evidence to make a recommendation for or against their use in this setting."

They also include all details surrounding the use and when/where/how they are suggesting they could have considerations. This is more in line with what the CDC is recommending.
Comment 8
Received: 19 April 2020
Commenter: craig medred
Commenter's Conflict of Interests: I have a bias toward reasoned thought.
Comment: This article provides a nice illustration of the importance of peer review. There are a lot of claims made. Little or no evidence is offered to support those claims.

The studies prime conclusion - "The available evidence suggests that near-universal adoption of non-medical masks when out in public, in combination with complementary public health measures could successfully reduce effective-R to below 1.0, thereby stopping community spread - is a hypothesis confounded by public-health measures known to work, ie. quarantine, isolation and distancing.

The study is in this context mislabeled. It is not an objective review of what is and isn't known about the real-world performance of masks in reducing the transmission of infectious disease. It is advocacy for masks.

Sadly, the UNESCO standard it cites could be used to advocate for almost any sort of safety equipment anyone thought beneficial. There would be far fewer brain injuries if people were required to wear helmets at all times, far fewer motor vehicle fatalities if motor vehicles were banned, far fewer deaths from obesity and heart disease if governments rationed food to control how much people eat, and more.

One can go a long way down that road. And along the way it is much easier to find evidence - say for motor vehicles - to identify restrictions that "would
save lives. Motor-vehicle traffic deaths can be documented.

There is no documentation here, and what little evidence is provided leads only to the conclusion that no one knows if masks works or not in the complicated real world where laymen not only put them on but take them off, put them back on, handle them, clean them, store them, and possibly even share them.

Given all these variables, it's not inconceivable that in a non-medical environment the masks might actually make things worse. Think about the filters in a home HVAC system. That system is how your home breathes, and those filters regularly need to be changed because they fill up with gunk.

Now think about the person who puts on the same mask day after day, wanders around a supermarket sucking up air, repeatedly takes the mask off, regularly touches his hands to the front of the mask where all that he has inhaled is accumulating, and then rubs his eyes.

Is this mask really going to reduce his chances of infection?
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Response 1 to Comment 8
Received: 21 April 2020
The commenter has declared there is no conflict of interests.
Comment: no this would probably help an individual engaging in every possible way in the wrong behavior, but then again why would someone do that, I mean wear a mask. Your description seems to apply to the kind of person who would not care enough to wear a mask in the first place.

But let's keep in mind that mask wearing is not about individuals, take the same individual engaging the exact same behavior, first in a supermarket where he is the only person wearing a mask vs a supermarket where everybody wears a mask. Which situation do you think would better protect him and everyone ?

This is the logic behind wearing masks.

I'll quote the conclusion of Professional and Home-Made Face Masks Reduce Exposure to Respiratory Infections among the General Population:
Any type of general mask use is likely to decrease viral exposure and infection risk on a population level, in spite of imperfect fit and imperfect adherence, personal respirators providing most protection. Masks worn by patients may not offer as great a degree of protection against aerosol transmission.

you might want to also read covid-19: why we should all wear masks — there is new scientific rationale
Response 2 to Comment 8
Received: 24 April 2020
Commenter: Ed Brown
Commenter's Conflict of Interests: I also share Mr. Medred's conflict of interest: I have a bias toward reasoned thought and tested facts.
Comment: Mr. Medred and Wu Zunyou,
I agree with your synopses and just wanted to point out as a scientist, as far as I can tell from the available literature on the mask subject, most authors who are actually doing lab science and publishing refereed articles in science journals rather than doing administration jobs or running businesses for a living and writing opinion pieces, contend that masks don't do much, if anything, to limit the inhalation of virus-containing particles from the air into someone wearing a mask. I think most scientists who have read any lab studies from refereed journals would agree on that.
Unfortunately, public policy makers just want to keep public panic at bay by any means they can-- even by giving people a "here, feel good doing this" type of nonsensical solution to help them feel like they can be in control of things that can kill them or their loved ones.
So, half the reason for wearing a mask has no obvious purpose. If you wear a mask and inhale a source of the virus from the air, the mask won't stop the virus from attacking the mask wearer's lung tissue. It will get through the mask.
Therefore, the issue is more about wearing a mask to protect others from what is in a wearer's breath or that emanates from a wearer's mouth. Fair enough. Unfortunately, most of the exhaled breath of a typical wearer (each breath is about .75-2 liters of air) does not go through a face mask, it goes around the edges of the mask near the face and out where the pressure/ cloth resistance is lower. The aerosolized, tiny micron-sized particles of virus-containing droplets of sputum/water/surfactant/phlegm from the breath do not get captured by the cloth, they are forced around or through it. So, a mask doesn't help there, either. Mask studies tend to involve measuring a decrease of particles from masks that are sealed around the edges to a flat surface so that all the contaminated air is forced through the fiber under pressure; or measuring only the particulates that come through the mask fibers and ignoring anything else that goes around the mask edges.
Now, I agree that if a person is coughing and sneezing up a storm while shopping at the local grocery store for dinner, a face mask might keep some of the larger droplets of spit and phlegm in the 0.1mm to 2mm range from getting all over everyone else; but for regular breathing? No. The droplets are too small to catch with the fiber weave. They get blown around the mask edges.
However, in the latter coughing/sneezing situation, a more responsible and sensible action for a reasonably-intelligent person to take when they are experiencing those types of coughing and sneezing problems with or without a fever is to stay home all-together and have someone else do whatever needs to be done out side of the home for them. Because a mask is a poor substitute for staying at home when you are sick.
Of course, this does not fix any issues with morons who see no problems exercising their God-given right to sneeze, sniffle and cough wherever they want to. But that is a separate issue. The best that can happen is for sick people to realize that they should stay home.
Response 3 to Comment 8
Received: 27 April 2020
Commenter: Andrew Walsh
The commenter has declared there is no conflict of interests.
Comment: To Ed Brown

As a scientist who has a bias towards "tested facts", can you please provide references to your statement,

"as far as I can tell from the available literature on the mask subject, most authors who are actually doing lab science and publishing refereed articles in science journals rather than doing administration jobs or running businesses for a living and writing opinion pieces, contend that masks don't do much, if anything, to limit the inhalation of virus-containing particles from the air into someone wearing a mask"

In absence of any references I would contend your reply is an "opinion piece".
Response 4 to Comment 8
Received: 18 May 2020
Commenter: Dr. Joseph Alan Bauer
The commenter has declared there is no conflict of interests.
Comment: This evidence-based analysis makes a very valid conclusion which is by wearing face masks the transmission of the virus will be lessened.

Based on your last statement which illustrates the improper use of a face mask, does increase the chances that the individual you describe has a greater chance to get SARS-COV-2, but the mistake in science that you and all the dissenters of this study are making, in the drama of someone telling you to wear a mask, is that face masks, especially cloth and basic surgical masks, are designed to protect others, NOT THE USER, which is why they are worn by surgeons everyday.

The N95 does more to protect the user but should not be needed if the public is taught how to wear a mask.

The bottom line is the conclusion which is espoused by this study is valid: if everyone wears some sort of face protection the transmission of the virus will be less than if no one wears a mask. BE RESPOBSIBLE. BE A ROLE MODEL. WEAR FACE PROTECTION!
Comment 9
Received: 19 April 2020
Commenter: Peter Hutten-Czapski
The commenter has declared there is no conflict of interests.
Comment: There are knowledgeable physicians of good intention with reasonable grounds who point to a lack of data showing effectiveness of masks for COVID 19 and point to a variety of potential negative effects of masking.
1) Procedural / "Surgical" / (K)N95(100) FFP2 / P2 / P3 masks are needed by Health Care Workers
2) As the lay public is not easily trained in proper mask use and as a result will touch and adjust masks potentially increasing infection (and some influenza mask trials make this concern not easy to refute out of hand)

Further more Asian examples, if related to universal mask use and not superior testing and contact tracing, relate in a large part to surgical mask wearing which are demonstrably superior to cloth.

I personally find the balance of evidence pointing to using cloth masks as a reasonable public health strategy, likely to reduce transmission rates (particularly from asymptomatic and presymptomatic patients). However I freely admit that I may be entirely wrong as good quality patient orientated evidence of such an intervention in COVID 19 is frankly lacking.

Unfortunately a randomized controlled trial to definitively explore the risks and benefits of a universal masking policy during the COVID 19 pandemic is not going to happen (see below). We thus have to do the best we can with data as mentioned in this study, and with mixed messaging in North America undermining the number of people masking, and hence any potential benefit.

if we consider New York State
population 19.45 million (half of that New York City)
with 223,699 cases to April 17th
thats 1 COVID case for each 87 population
although suspiciously only 550,579 New Yorkers have been tested, its likely a significant under-count
incidence in official cases is increasing by 10,000 patients a day

Lets set up a theoretical study
Incidence, group A 0.0071% for a two week trial the "placebo group" is what is happening now
Incidence, group B 0.0065% assuming a 10% reduction of cases in the "masked group"

how big a study would you need to show if masks have a 10% reduction or a larger benefit?
Lets use standard rules of scientific statistical validity
Alpha 0.05 Beta 0.2 Power 0.8
we would need a study of size 59,298,612 patients
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Comment 10
Received: 21 April 2020
Commenter: Gerard Krause
The commenter has declared there is no conflict of interests.
Comment: We know that the virus can enter your body through the mucous membranes eyes, nose and mouth, therefore, in PPE equipment it is mandatory to use protective googles, which could also be a good measure to slow the spread of the virus in certain situations described in the RKI recommendations.

Also, all PPE equipment must be properly fitted, but even in the hospitals we have situations where hospitals appear to be in breach of Health and Safety Executive guidelines:

https://www.theguardian.com/world/2020/apr/14/nhs-hospitals-accused-of-risking-staff-lives-by-abandoning-ppe-fit-tests-coronavirus
Regarding masks and RKI recommendations;

RKI recommends wearing a mouth-nose cover (textile barrier in the sense of an MNS) in certain situations in public space. Wearing the mouth and nose covering can be an additional component to reduce the rate of spread of COVID-19 in the population - but only if distance (at least 1.5 meters) from other people, cough and sneeze rules and good hand hygiene is observed.

Wearing a mouth-to-nose cover in public life can help slow the spread of COVID-19 in the population and protect high-risk groups from infections. This applies in particular to situations in which several people meet in closed rooms and stay there longer (e.g. work place) or the distance of at least 1.5 m to other people cannot be maintained (e.g. in shops, on public transport). The prerequisite for this is that enough people wear a mouth-nose cover and handle the mouth-nose cover correctly: the cover must be worn close-fitting over the mouth and nose and changed when wet; it must not be plucked (not even unconsciously) while it is being worn, nor should it be worn around the neck.

Such a protective effect has not yet been scientifically proven, but it does appear plausible. On the other hand, there is no evidence for self-protection.

https://www.rki.de/SharedDocs/FAQ/NCOV2019/FAQ_Mund_Nasen_Schutz.html
Recommendation for wearing cotton masks everywhere all of the time is not a good idea. Study mentioned in this pre print, a cluster randomised trial of cloth masks compared with medical masks in healthcare workers in conclusion says:

This study is the first RCT of cloth masks, and the results caution against the use of cloth masks. This is an important finding to inform occupational health and safety. Moisture retention, reuse of cloth masks and poor filtration may result in increased risk of infection. Further research is needed to inform the widespread use of cloth masks globally. However, as a precautionary measure, cloth masks should not be recommended for HCWs, particularly in high-risk situations, and guidelines need to be updated.

https://bmjopen.bmj.com/content/5/4/e006577
One more recent papers says:

While social distancing and good hand hygiene are the most important methods to prevent virus transmission, new guidelines state that healthy individuals can consider wearing masks in public settings, particularly when physical distancing is difficult (like in grocery stores or pharmacies). Primary benefits of wearing a mask include limiting the spread of the virus from someone who knows or does not know they have an infection to others. Masks also remind others to continue practicing physical distancing. However, nonmedical masks may not be effective in preventing infection for the person wearing them.

https://jamanetwork.com/journals/jama/fullarticle/2764955
The evidence is not sufficiently strong to support widespread use of facemasks as a protective measure against COVID-19. However, there is enough evidence to support the use of facemasks for short periods of time by particularly vulnerable individuals when in transient higher risk situations. Further high quality trials are needed to assess when wearing a facemask in the community is most likely to be protective.

https://www.medrxiv.org/content/10.1101/2020.04.01.20049528v1
Even the best respirators are not 100% efficient:

https://multimedia.3m.com/mws/media/409903O/respiratory-protection-against-biohazards.pdf?mod=article_inline
Furthermore:

Potential Risks When Some Special People Wear Masks

Zheng Zhaoshi, PH.D. M.D. | No. 1 Department of Neurology, The Third Hospital of Jilin University
As more asymptomatic patients with SARS-CoV-2 infection are found in the world, wearing N95 masks is undoubtedly an effective respiratory protective measure (1). However, wearing an N95 mask for a long time may bring potential risks and even complications that are difficult to eliminate.

Oxygen concentration inhaled by healthy subjects wearing a surgical mask covering an N95 respirator decreases to about 17%, and the concentration of carbon dioxide increases to about 1.2% - 3% in a short period of light work (2-3). Although participants did not show any obvious changes in physical function and did not have any discomfort ratings, the average carbon dioxide concentration inhaled was far higher than the limit of 0.1% of indoor carbon dioxide concentration in many countries. With prolonged mask wearing, untoward reactions may gradually appear. In another long-term study, after wearing an N95 mask for 12 hours the CO2 concentration of subjects increased to 41.0 mmHg, far higher than the baseline value of 32.4mm Hg at the beginning of the test (4). The subjects mainly reported headache, dizziness, feeling tired and communication obstacles. In real life, the situations and time of wearing masks are much longer than the above experimental research settings.

Therefore, when taking strong measures to prevent and control COVID-19 we should also pay attention to implementation of individualized strategies. Pregnant women and patients with basic respiratory diseases are the most vulnerable groups. These high-risk groups should try to choose a way of isolation away from others. Many kindergartens and middle schools in many provinces of China have begun to resume classes or plan to resume classes in the near future. Most of them require students to wear masks during the school period. It is questionable that students wear masks for a long time, especially for the children in low school years.

We declare no conflict of interest.

Reference

1. Offeddu V, Yung C F, Low M S, et al.Effectiveness of Masks and Respirators Against Respiratory Infections in Healthcare Workers: A Systematic Review and Meta-Analysis.Clinical Infectious Diseases, 2017, 65(11): 1934-1942. doi: 10.1093/cid/cix681

2. Roberge R J, Coca A, Williams W J, et al. Surgical mask placement over N95 filtering facepiece respirators: physiological effects on healthcare workers[J]. Respirology, 2010, 15(3): 516-521. doi:10.1111/j.1440-1843.2010.01713.x

3. Sinkule E J, Powell J B, Goss F L, et al.Evaluation of N95 Respirator Use with a Surgical Mask Cover: Effects on Breathing Resistance and Inhaled Carbon Dioxide.Annals of Occupational Hygiene, 2013, 57(3): 384-398. doi:10.1093/annhyg/mes068

4. Rebmann T, Carrico R, Wang J, et al.Physiologic and other effects and compliance with long-term respirator use among medical intensive care unit nurses. American Journal of Infection Control, 2013, 41(12): 1218-1223. doi:10.1016/j.ajic.2013.02.017.
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Response 1 to Comment 10
Received: 21 April 2020
Commenter: Caterina Zanetti
The commenter has declared there is no conflict of interests.
Comment: I think it would be highly beneficial not jumping between Health Care Workers and the general public, between self-protection and protection of other people. It seems to me that this was the case in your references.
FFP2 or FFP3 (N95 or N99) respirators are designed to protect Health Care Workers (or other persons), from breathing in particles sized 0,2-2 micron or more. Their main purpose is the protection of the wearer. Respirators fall under Personal Protective Regulation, European standard EN 149, which specifies performance requirements and test methods to assure the required filtration efficacy.
A medical mask is intended to prevent anyone wearing it from spreading droplets. The main intended use of medical face masks is to protect the patient from infective agents spread by Health Care Workers into the operating theatre. The medical face masks fall under the Medical Device Regulation, European standard EN 14683, which specifies requirements and test methods to limit the transmission of infective agents.
Medical face masks may also be intended to be worn by patients to reduce the risk of spreading infections transmitted by droplets.
Jeremy Howards and Colleagues argue that medical face masks can also be effective in reducing the emission of droplets from the nose and mouth of an asymptomatic carrier of COVID-19.
In certain circumstances, medical masks may be intended to protect the wearer against splashes of potentially contaminated liquids or aerosols, but they are not tested, nor certified, to assure their capability to do that.
Mixing up things doesn't help.
Response 2 to Comment 10
Received: 21 April 2020
Commenter: Gerard Krause
The commenter has declared there is no conflict of interests.
Comment: Caterina Zanetti, this is advocacy that all people must wear masks all the time, therefore we could by that presume worst case scenario that all could be infected, therefore, working conditions from HWC's is used for this scenario where all people must wear masks all the time. In your comment, you are refering to respirators, and medical masks, and this is advocacy for all masks, and most of the people will wear cotton masks, which from various sources is not recommended.

Hendrik Streeck said no to masks

https://www.prisma.de/news/Markus-Lanz-Virologe-Hendrik-Streeck-ist-gegen-Mundschutz-Pflicht,25983653
In their study which is in development, they saw that the virus is transmitted only in close contact, and he said Sars-CoV-2 is a droplet infection and not an airborne infection, and we have measures like social distancing.

https://www.zeit.de/wissen/gesundheit/2020-04/hendrik-streeck-covid-19-heinsberg-symptome-infektionsschutz-massnahmen-studie/komplettansicht
Even more, for asymptomatic carriers, Anders Tegnell tells us: "there is a possibility that asymptomatics might be contagious, and some recent studies indicate that. But the amount of spread is probably fairly small compared to people who show symptoms. In the normal distribution of a bell curve asymptomatics sit at the margin, whereas most of the curve is occupied by symptomatics, the ones that we really need to stop."

https://www.nature.com/articles/d41586-020-01098-x
To summarize, there is not enough evidence for advocacy that all people must wear masks all the time, and it is proven that cotton masks can do more damage than good.
Comment 11
Received: 22 April 2020
Commenter: Emmett Keyser
The commenter has declared there is no conflict of interests.
Comment: This paper can be debunked simply due to the fact that in China, HK, South Korea, Taiwan, etc the % of face coverings most likely exceeds 80% and yet still they experienced exponential growth in infections even if eventually it has dropped off. Not to mention that filtration performance of their face masks probably exceeds those that cloth provides.

Listen, even the paper states something like "3%-60% for various cloth materials". That is some statistical spread. So you're telling me that 80% participation rate with the absolute best case material (60%) for filtration is all that it takes to completely stop this virus? Hmm, something tells me that's not right. This virus isn't going away no matter how many people participate in face coverings. Some things you can't stop no matter how much wishful thinking effort you put in.

The complete picture is obviously complex but there are some basic realities that need to be stated. This is a novel virus, no matter what we do people are going to get sick, lifestyle choice comorbidities are stressing the medical system (lung disease, heart disease, obesity, diabetes, etc), a certain % will die no matter what, and no face covering in the world no matter how much you want to dress it up as "medical or scientific" is going to be effective at filtering .1 micron sized pathogens, not to mention that other infection vectors probably have at least some contributions as well.

Dressing up (no pun intended) "expert" input as scientific is what dilutes the people's trust in science and I'm afraid that this is an example of this. That Atlantic did a disservice to their reputation by printing an article purporting to be of a scientific nature based on this paper.
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Response 1 to Comment 11
Received: 26 April 2020
Commenter: Richard Collins
The commenter has declared there is no conflict of interests.
Comment: Do you have a reference or an explanation for the claim that Taiwan has "experienced exponential growth in infections"?
Taiwan has, to date, detected only 55 cases of local infection (excluding the cases on a navy ship).
Response 2 to Comment 11
Received: 9 May 2020
Commenter: Dario Padovan
The commenter has declared there is no conflict of interests.
Comment: I think that there is a point missed by most comments, and Mr. Keyser's comment is one of those. Someone says that the masks (being them surgical or cloth one) cannot stop a 0.1 µm viral particle: that's true, but that's not the intended aim. Covid-19, according to all evidence so far (for instance you can read: https://www.who.int/news-room/commentaries/detail/modes-of-transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations ), is not an airborne disease. The evidence so far is that the transmission is through respiratory droplets, ranging between 5 and 10 µm, and not through droplet nuclei, that are below 5 µm in diameter. So the masks have to stop just droplets >5µm (that are the ones that makes a 1m-1.5m social distancing useful). There is evidence that a face covering (even a damp cloth) can highly reduce droplet emissions in the environment (Anfinrud, P., Stadnytskyi, V., Bax, C. E., & Bax, A. (2020). Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering. New England Journal of Medicine, NEJMc2007800).
Response 3 to Comment 11
Received: 11 May 2020
Commenter: Ben
The commenter has declared there is no conflict of interests.
Comment: The contributor's argument on the numbers is flawed: clearly masks cannot stop all spread in a population, but they can reduce the rate of spread, and the numbers in Asia bear this out.

To suggest that a large number (say 80%) of people in Hong Kong (etc) wearing face masks should prevent exponential growth is based on
(1) an incorrect assumption that face masks are 100% effective, and
(2) an apparent misunderstanding of the significance of “exponential growth”; this term means only that numbers were at one point increasing proportionately (as will inevitably occur at some point with an outbreak of almost any size).

In terms of the mathematics, the fact that the death rate is 130 times worse in Europe than Asia seems by far the most important fact to focus on (see Worldometer figures 8th May 2020 for China, Japan, Vietnam, Thailand and South Korea where the average is 3 deaths per million, compared with an average of 398 per million in Germany, UK, France, Italy and Spain ). Quite why such astonishing statistics are missed not just by the media but in discussions like this is surprising, to say the least.

To explore the causes of these statistics it would be helpful to have some mathematical modelling for how face masks can impact on the infection rate (intuitively it seems obvious that face masks are the single biggest explanatory factor for the global variation in the spread of Covid-19, but some mathematical analysis would be appreciated from anyone out there). In contrast to these overwhelming statistics, the discussions I see here on precise bio-mechanics of how the virus interacts with a face mask appear like angels dancing on the head of a pin.
Comment 12
Received: 24 April 2020
Commenter: Lisa M Brosseau
The commenter has declared there is no conflict of interests.
Comment: Many of the investigations included in this review did not perform filter or fit tests appropriately. For example, the article by Davies et al. did not challenge the filters with small particles. Particles generated by a Collison nebulizer are typically quite large (greater than 3 microns); even after evaporation it is very unlikely that the particles were the size of an individual organism. An all-glass impinger has very low sampling efficiency for small particles; thus, even if there were smaller particles present in the challenge aerosol they would not have been sampled using this device. They found relatively high filter efficiency because they were actually measuring fairly large particles and using a low flow rate. There are similar problems with the fit testing conducted by van Doremalen et al. -- while there may have been some small particles in the air, the most common particle sizes are fairly large (greater than 1 micron), which are captured with relatively high efficiency. These are not the particle sizes of greatest concern for SARS-CoV-2 deep lung infections. Cloth masks may prevent emission of larger droplets, which may be responsible for some of the milder upper respiratory symptoms, but the more severe disease is clearly caused by deep lung infections.
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Comment 13
Received: 24 April 2020
Commenter: Michael Brenner
The commenter has declared there is no conflict of interests.
Comment: "Not peer reviewed." The judge is using non-peer reviewed material to back his policy...
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Response 1 to Comment 13
Received: 12 May 2020
The commenter has declared there is no conflict of interests.
Comment: Meaningful, compelling, and correct arguments often appear in forums that are not peer reviewed. Peer review - on average - probably reduces, but by no means does it eliminate, the publication of nonsense. On average. Most peer reviewed publications - even in the highest impact journals - fail the test of time: they are fiction and fantasy. Quite a bit of published and peer reviewed literature on that in recent years. It was called an "irreproducibility crisis," until it was observed that it is hardly a crisis, but instead just business as usual.

Perhaps evidence could be assessed on its merits? It's hard, I know. Certainly, it would be a misrepresentation to claim that a manuscript was peer reviewed, when it was in fact not peer reviewed. But comment 13 reeks of snark.
Comment 14
Received: 27 April 2020
Commenter: Andrew Walsh
The commenter has declared there is no conflict of interests.
Comment: I would suggest the authors include references to masks with valves.
I see many pictures of people (especially police but also some healthcare workers) wearing masks with valves.
Such masks are intended to prevent infection of the wearer. The valve serves only to make expiration easier.. with less effort.
The valve does absolutely nothing to prevent transmission from infected wearers to other nearby individuals.
I believe it is imperative to include this fact if your main argument is that masks prevent transmission from infected people to others.
Many may otherwise wear such masks (eg. P2V) with the mistaken belief they are helping others when in fact they would be putting them at greater risk.
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Response 1 to Comment 14
Received: 17 May 2020
Commenter: Deb McDonald
The commenter has declared there is no conflict of interests.
Comment: I have heard much about the valve increasing risk to those near the person wearing a mask with a valve. But I have seen no scientific evidence that this is an accurate claim. If anyone in this community has seen such evidence, please share it here.
Response 2 to Comment 14
Received: 8 July 2020
Commenter: Vinay
The commenter has declared there is no conflict of interests.
Comment: Masks with Valve should be strictly prohibited in medical settings especially during a pandemic.
These articles burnish this fact
Bollinger NJ (2004) NIOSH respirator selection logic: US Department of Health and Human Services, Public Health Service,Centers. [Google Scholar]
Zhou, S. S., Lukula, S., Chiossone, C., Nims, R. W., Suchmann, D. B., & Ijaz, M. K. (2018). Assessment of a respiratory face mask for capturing air pollutants and pathogens including human influenza and rhinoviruses. Journal of thoracic disease, 10(3), 2059–2069. https://doi.org/10.21037/jtd.2018.03.103
Comment 15
Received: 11 May 2020
The commenter has declared there is no conflict of interests.
Comment: I applaud the use of masks for public safety. Wearing them is a common practice in Japan and China, where it has been established for many years that they reduce the spread of viruses.
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Response 1 to Comment 15
Received: 14 May 2020
Commenter: Gee Pik
The commenter has declared there is no conflict of interests.
Comment: Worked well, didn't it?
Comment 16
Received: 12 May 2020
The commenter has declared there is no conflict of interests.
Comment: A reasonable review of the literature on the effectiveness of wearing masks in preventing the spread of the Corona virus (or other similar virus) would provide an account of the methodology used to conduct the search and screen potential articles. It would include the relevant PICO, a detailed list of terms used in the search strategy, a list of databases searched, inclusion and exclusion criteria for selecting studies to be included and an accounting of the number of references retrieved, the number excluded (with reasons for exclusion) and the final tally of included articles. A useful review would, in addition, included some type of grading of the evidence in included articles with respect to risk of bias. This review fails on both accounts.
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Comment 17
Received: 12 May 2020
Commenter: Donna Gilmore
The commenter has declared there is no conflict of interests.
Comment: This paper does a disservice to science and the public. This is a life and death situation, so recommendations should be based on science, not on unsubstatiated hope, assumptions, intuition, or cherry picked data.

Only N95 respirators are proven effective against viruses and even those have their limitations. Essential workers and the public should be told the truth of how they are risking their health when people wear only cloth or surgical masks. Now is not the time for placebos. People are only safe in quarantine until this virus is contained, but that doesn't fit the economic narrative. This paper should be recalled. It does more harm than good.

Two reports below with facts that everyone needs to know:

Aerosol studies on SARS-CoV-2 coronavirus indicate it is very hardy and still infectious after 16 hours, significantly worse than previously expected. "this virus is remarkably resilient in aerosol form".
Long-term persistence of SARS-CoV-2 and ability to maintain infectivity when suspended in aerosols for up to 16 hours.

Comparative dynamic aerosol efficiencies of three emergent coronaviruses and the unusual persistence of SARS-CoV-2 in aerosol suspensions, April 18, 2020
https://www.medrxiv.org/content/10.1101/2020.04.13.20063784v1 doi: https://doi.org/10.1101/2020.04.13.20063784

Alyssa C Fears, William B Klimstra, Paul Duprex, Amy Hartman, Scott C. Weaver, Ken S. Plante, Divya Mirchandani, Jessica Plante, Patricia V. Aguilar, Diana Fernandez, Aysegul Nalca, Allison Totura, David Dyer, Brian Kearney, Matthew Lackemeyer, J. Kyle Bohannon, Reed Johnson, Robert F Garry, Doug S Reed, Chad J Roy

Masks-for-all for COVID-19 not based on sound data, 4/1/2020, University of Illinois at Chicago, Lisa M Brosseau, ScD, and Margaret Sietsema, PhD
https://www.cidrap.umn.edu/news-perspective/2020/04/commentary-masks-all-covid-19-not-based-sound-data
Conclusions
While this is not an exhaustive review of masks and respirators as source control and PPE, we made our best effort to locate and review the most relevant studies of laboratory and real-world performance to inform our recommendations. Results from laboratory studies of filter and fit performance inform and support the findings in real-world settings.
• Cloth masks are ineffective as source control and PPE,
• Surgical masks have some role to play in preventing emissions from infected patients, and 
• Respirators [N95] are the best choice for protecting healthcare and other frontline workers, but not recommended for source control. 
• These recommendations apply to pandemic and non-pandemic situations.

Leaving aside the fact that they are ineffective, telling the public to wear cloth or surgical masks could be interpreted by some to mean that people are safe to stop isolating at home. It's too late now for anything but stopping as much person-to-person interaction as possible.

Masks may confuse that message and give people a false sense of security. If masks had been the solution in Asia, shouldn't they have stopped the pandemic before it spread elsewhere?

Dr. Lisa M Brosseau is a national expert on respiratory protection and infectious diseases and professor (retired), University of Illinois at Chicago http://lisabrosseau.com

Dr. Margaret Sietsema is also an expert on respiratory protection and an assistant professor at the University of Illinois at Chicago
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Response 1 to Comment 17
Received: 15 May 2020
Commenter: Howell Lewis
The commenter has declared there is no conflict of interests.
Comment: Look at the real world : In terms of the mathematics, the fact that the death rate is 130 times worse in Europe than Asia seems by far the most important fact to focus on (see Worldometer figures 8th May 2020 for China, Japan, Vietnam, Thailand and South Korea where the average is 3 deaths per million, compared with an average of 398 per million in Germany, UK, France, Italy and Spain ). Quite why such astonishing statistics are missed not just by the media but in discussions like this is surprising, to say the least.

Please do not argue that it is not possible compare countries because..blah blah...death rates are the bottom line.
Response 2 to Comment 17
Received: 18 May 2020
Commenter: Dr. Joseph Alan Bauer
The commenter has declared there is no conflict of interests.
Comment: Your comments actually do more harm to the public than this article. Namely, you add to the confusion surrounding the best way to protect yourself and your loved ones against this deadly virus. The study does not say that if you wear a face mask you will not get SARS-COV-2. What you don't take into account is that most simple masks were designed to protect others, no the mask wearer.

Many people are asymptomatic carriers probably as high as 30-40% although this is has not been established as the testing is not reliable at this time. So by wearing even a simple mask, homemade or basic surgical, other people will be better protected than a symptomatic person irresponsibly, and unknowingly infecting others just by talking.


The basic principle of everyone wearing a mask will help to decrease SARS-COV-2 transmission and combat COVID19.
But the message must be a positive one to ensure compliance. Wearing a face mask is a BADGE OF HONOR and it shows you appreciate the healthcare professionals on the front-line risking their lives for you everyday; to do otherwise is a real insult.

But a wild west approach serves no one. BE RESPONSIBLE! WEAR FACE PROTECTION!
Comment 18
Received: 13 May 2020
The commenter has declared there is no conflict of interests.
Comment: Emphasis on contact tracing is senseless without the implementation of widespread, effective testing. The lack of urgency or regard for testing in the United States is reflective of the chaotic mismanagement of the federal government, and the willful ignorance of its deteriorating society.
Contact tracing will prove to serve only as a conduit to punishment and incarceration in the United States.
Testing should be prioritized.
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Comment 19
Received: 13 May 2020
Commenter: Jon Gulley
The commenter has declared there is no conflict of interests.
Comment: I am writing to give the view of a lay reader concerning the paper and some of the comments. Specifically, I would like to share my main questions and the answers I identified.

The paper surveys literature concerning the effect of wearing face-masks on the transmission of Covid-19. It is therefore likely to be of value to policy-makers as an information resource, as a summary of key points from the literature and in identifying experts in the field as authors of the paper itself and its references. Along with others making comments, I would like to thank the large and distinguished team of authors from a range of institutions for working together to produce such an informative and interesting paper on this important topic.

The paper argues forcefully for the wearing of face-masks as a measure to mitigate the spread of Covid-19. As noted by Comment 3, at least one leading author, Jeremy Howard, is a key participant in the wider public debate in the area. In Response 5 to Comment 3, he argues that his views do not invalidate the study or the methodology and highlights the size and strength of the authorship team.

In reading the paper, my questions and the answers I identified were as follows:

1) Has a taxonomy of face-mask characteristics emerged?

- This question is addressed in Section 2. The three main classes mentioned are respirators, surgical face-masks and masks made domestically. The domestically produced masks feature a range of household materials. Comment 14 highlights the dimension of some masks having valves.

2) What are the figures of merit by which face-mask quality can be measured?

- This is also addressed in Section 2, specifically for household materials, with the filtration rate for 0.02 micrometer particles being cited as a metric. If there is a degree of consensus in the literature around the measurement and its usefulness, measurements from the literature for various materials might usefully be summarised so that recommendations on materials can be made available to the public.

3) How can the effect on disease transmission be described or measured?

- In Section 5, the multiplier on R0, the transmission rate, is cited as a figure of merit, specifically in respect of modelling by the HKBU COVID-19 Modelling Group, along with a formula for the multiplier. My remaining question is whether the formula and the values used are the subject of consensus or whether others might use a different metric, different formula or different estimates.

4) Under what conditions have the effects of face-masks on disease transmission been studied?

- This question is addressed at the end of Section 5, citing the differences between countries in respect of face-mask wearing customs and the large differences in outcomes. Causal attribution is not deemed appropriate by the authors. Notwithstanding, I would like to understand the extent to which the existence of correlation has been established.

5) What arguments have been made in the scientific literature for and against using face-masks?

- The paper presents a body of argument in the literature in favour of wearing face-masks. It would be useful to know whether scientific arguments have been made against wearing face-masks or against use of compulsion. I noted Comment 6 and Comment 10 as relevant to this, specifically concerning psychological harms and excess carbon dioxide intake respectively.

6) Have there been studies of how any policy has been or could be implemented?

- Section 6 discusses this point, in the context of recommending government intervention in favour of face-masks. I understand that there are more details in the literature cited, specifically refs 91, 92 & 93.

7) What further research work is needed in this area?

- I felt it would have been good to summarise, if possible and appropriate, any important gaps in the research space and key weaknesses of evidence which have been identified as a result of the study.


In conclusion, I would hope that the paper will be made available to those influencing policy on the basis that they will take the opportunity to engage with the authors to test the strength of evidence and examine the benefits and costs surrounding face-masks.
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Comment 20
Received: 16 May 2020
Commenter: Graeme tait
The commenter has declared there is no conflict of interests.
Comment: This article shows a graph and formula which I believe significantly misrepresents the practical effect of masks. Namely that the reduction in reproduction number from masking is given by (1-e*pm)^2.

Based on my own analysis, I believe this formula was derived assuming a homogeneous population of infectious people - that is, that the same proportion of infectious people exists in both the masked and unmasked groups.

In reality, people who wear masks ate likely to wear them consistently, and people who do not wear masks are likely to not wear them consistently,

Thus, in an initially homogeneous population which is not masked, when you mask a certain fraction, the fraction of people who are infectious in the unmasked group will increase relative to that in the masked group.

The following simple case serves to point this out. Consider masks with 100% effectiveness, worn by say a given cohort which is 70% of the population under study. All transmission then occurs from unmasked people to unmasked people. All infection will die out in the masked group. In the unmasked group, assume a Ro value of 2.4. Thus, each infectious, unmasked person tries to infect 2.4 others. But only 30% of the population is unmasked. So, they will each actually infect 2.4 * 0.3 people. From this, it should be clear that the effective reduction in R is by a factor or 0.3, whereas the quoted formula would predict a reduction of (1-0.7)^2 = 0.09.
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Comment 21
Received: 18 May 2020
Commenter: Dr. Joseph Alan Bauer
The commenter has declared there is no conflict of interests.
Comment: I commend the authors for this timely, evidence-based analysis.

I have read several comments on this site, many from those that have no experience in science and medicine whatsoever, which is a real disservice to those trying to make a decision as to the best way to protect others, themselves and their loved ones.

The science is clear, if everyone wears a mask in public the transmission of the virus will be lessened. Yes, a face mask is not 100% effective, even a N-95 as the effectiveness depends on the user; the user should wash their hands before donning a mask, the user should not touch their face or mask while wearing; the user should wash their hands prior to removing.

The bottom line is that public health service announcements need to be broadcast by all media outlets to better train the general public on the "do's" and "don'ts" of wearing a face mask. Wearing a face mask is a BADGE OF HONOR and a symbol of concern for others and most of all is a display of PATRIOTISM as everyone wants the USA and the world to get back to normal. BE RESPONSIBLE. WEAR A FACE MASK!
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Response 1 to Comment 21
Received: 13 July 2020
Commenter: Claus Roeder
The commenter has declared there is no conflict of interests.
Comment: "...the user should wash their hands before donning a mask, the user should not touch their face or mask while wearing; the user should wash their hands prior to removing."

Yeah we dont live in a perfect world, sorry to say but nobody does this. I live in Germany where we have to wear a mask in trains, stores, even at work I wear one 8h...
I and many others touch the mask much more often than the face, because it doesn't fit, it isn't comfortable, hot etc.... Just watch people in a grocery store. Also I never wash my hands before donning a mask and I never saw one doing this, also prior to removing I never do this and (also watching people coming out of a train/store) nobody does it.
Furthermore many people reuse the 1xmask or dont wash the cloth mask after usage, I read in Hongkong 80% reuse the mask,which is only 1xusable.
Although I think there are sure benefits of wearing a mask, the efficacy of it is WAYYYY to overrated, and there are surely a lot of confounder effects that help reducing the infections (didnt an author say that wearing masks without social distancing won't work?).
Also I read that the daily infections in Japan (mid of July), are increasing rapidly,although everybody wears a mask?
Switzerland,where mask are not mandatory, have almost no new infections.
Comment 22
Received: 25 May 2020
Commenter: Jody B
The commenter has declared there is no conflict of interests.
Comment: This article didn’t cite one experiment or clinical trial. Since when does writing an essay qualify as science?
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Comment 23
Received: 28 May 2020
The commenter has declared there is no conflict of interests.
Comment: In reply to "The preponderance of evidence indicates that mask wearing reduces the transmissibility per contact by reducing transmission of infected droplets in both laboratory and clinical contexts. "

Where are the laboratory and clinical findings to support your claim of "preponderance of evidence"?
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Comment 24
Received: 1 July 2020
Commenter: Brian M
The commenter has declared there is no conflict of interests.
Comment: You use the term "exponential growth" numerous (3) times and indeed also include some kind of calculation where R0 = 1.3 vs R- = 2.4 leads to "31,280 cases by the month’s end (R0 = 2.4) vs. only 584 cases (R0 = 1.35)."

Two comments
1) You have apparently never used even a rudimentary SIR model. Growth is not exponential - it is logistic and only *looks* exponential for a short time at the start of the outbreak . This exponential phase, w/ respect to COVID-19, is mostly an artifact of testing rates going from 0 to N. When back-calculating, R0 is accurately determined by the slope of the LINEAR portion of the logistic

2)R0 alone for a population is effective for backwards comparisons of two epidemics - but it is not a forecasting tool.

R0 inherently has a distribution with variance and mean; the variance is just as, if not more, important than the mean for forecasting purposes. Even if only a fraction of infected population has R0 > 1 infections continue to grow depending on the standard deviation.
R0 variance and logistic growth curves are fundamentals of epidemics and no broad conclusion should be made from forecasts that do not consider them.
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Comment 25
Received: 2 July 2020
Commenter: Richard Gillard
The commenter has declared there is no conflict of interests.
Comment: I would like to comment on some of the responses of those who are pro mask wearing. The patronising, high handed, self righteous and pompous nature of some of these comments makes me wonder if a certain degree of Shilling is going on. I understand people are sometimes paid to support a certain point of view during internet discussions.

The responses to David Curtess's initial response, in particular, seem to be somewhat shrill. Mr Curtess's point was a perfectly reasonable one.
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