Review
Version 2
Preserved in Portico This version is not peer-reviewed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)
Version 4 : Received: 30 October 2020 / Approved: 2 November 2020 / Online: 2 November 2020 (10:18:00 CET)
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)
Version 4 : Received: 30 October 2020 / Approved: 2 November 2020 / Online: 2 November 2020 (10:18:00 CET)
A peer-reviewed article of this Preprint also exists.
Journal reference: PNAS 2020
DOI: 10.1073/pnas.2014564118
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 develop an analytical framework to examine an overlooked aspect of mask usage: masks as source-control targeting egress from the wearer with benefits at the population-level, rather than as PPE used for ingress control for health-care workers with focus on individual outcomes. We consider and 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 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 other measures, and second, reduce the transmission probability per contact. 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 reducing 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. Given the current shortages of medical masks we recommend the adoption of public cloth mask wearing, as an effective form of source control for now, 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.
Keywords
COVID-19; SARS-CoV-2; masks; pandemic
Subject
MEDICINE & PHARMACOLOGY, General Medical Research
Copyright: This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Comments (17)
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Commenter: Jeremy Howard
Commenter's Conflict of Interests: Author
Commenter: JN
The commenter has declared there is no conflict of interests.
But what happens after an hour or 2 hours of using a cloth mask by an infected person? Haven't we now introduced a new vector for transmission, that is probably spraying more virus into the air, possibly even smaller virus particles that have evaporated and are now traveling further? In addition, you only need to spend 5 minutes watching how people are using masks to realize that they are actually touching this surface more than they might normally even touch their face.
This paper also fails to mention the outcome of the Seongman Bae study, that ALL of the masks showed more contamination on the OUTSIDE of the masks. And further, concluded that "both surgical and cotton masks seem to be ineffective in preventing the dissemination of SARS–CoV-2 from the coughs of patients with COVID-19 to the environment and external mask surface".
Commenter: Stephen Lowell
The commenter has declared there is no conflict of interests.
Commenter: Zac G
The commenter has declared there is no conflict of interests.
Unlike a bacteria, a virus is unable to multiply outside the host. The viral load on the outside of the mask is low compared to the viral load in saliva that is expelled from the mouth during speaking (especially susceptible are fricatives 'f', 's', 'v' and 'z' as well as sounds like 'p' and 't'). During a conversation at close distance and/or in noisy environments, some saliva may be transferred between persons as it is expelled from a speaking person and enters the mouth of a second person.
It seems reasonable to assume that fresh saliva being passed from an infected persons' mouth to a non-infected persons' mouth must have a much higher chance of spreading the virus than any mechanism by which viruses on the outside of a face mask can spread. It is difficult to imagine how a viable viral load would be able to transfer from the outside of a mask and cause a new infection. During every transfer from one surface to the next, the number of viruses on a surface decreases exponentially at minimum due to dilution alone. Additional decay of viruses that are damage during the transfer from one surface to the next increases the rate of decay even further.
Given the (speculated) low chances of the virus spreading through face mask or through surfaces in general, it seems awkward that the WHO is putting so much emphasis on face mask handling. Although this is appropriate for a hospital setting where health-care workers are potentially exposed to dangerous and resistant bacteria, this handling seems unnecessary and even counterproductive for face mask wearing, as it is used to argue against the wearing of face masks by the general public by some national health organizations.
Commenter: Sampo Smolander
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Commenter:
The commenter has declared there is no conflict of interests.
The only real-life scenario studied so far by C Raina MacIntyre et al. in a Vietnamese hospital showed that cloth masks were significantly less effective than controls (normal practice which involves surgical masks) in preventing infection. The models all systematic reviews published so far indicate a lack of significance for wearing masks in public. When you have over 11 studies (B. J. COWLING et al. 2010) show lack of significance and suddenly one says there is significance for wearing a cloth mask I would get suspicious. The only significant difference found in the above mentioned systematic review was for a controlled experimental setup where volunteers where asked to cough towards a petri dish and showed that nothing was on the petri dish compared to volunteers not wearing masks. The study did not investigate the leaked particles from the sides of the mask which in a real-life scenario might be even more scary than those coming directly out of the subject.
There are many conditions that need to be taken into account when considering cloth masks, like humidity, the concentration of viral droplets in the environment, whether the purpose is to protect others or protect from others, whether the subject is touching the mask, whether there are viruses on the surface around the subject. If the protection is less than 5% better than a control, then it should not be adopted in my opinion.
Commenter: Paul V Sullivan
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The authors state that masks are effective, but in households the compliance was less than 50%. Doesn't help if you don't wear it.
The above article is freely downloadable.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2662657/
The commenter has declared there is no conflict of interests.
The commenter has declared there is no conflict of interests.
The paragraph then ends listing countries with mask policies that had a “successful” response. There’s no indication as to why masks were a net positive (and risk compensation was not an offset), other than a statement from the government in those countries. What’s the reasoning for even including that argument there? It doesn’t seem to be well grounded.
Commenter: FvhG
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Given an R_0 of 2.4, a reproduction number R_e of 1 requires a 58% decrease of the reproduction number (1 - (1/2.4)). The graph however shows that a ~40% adherence rate combined with a 100% mask efficacy reduces the R_e to 1. This seems incorrect given an R_0 of 2.4. Absent any other factors, it is expected that an R_0 requires an adherence rate of 58% in order to obtain an R_e equal to 1.
Commenter: Manfred Schopp
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With 100% mask efficacy, 35.5% usage reduces infections due to reduced emission by 35.5% and again by 35.5% due to reduced intake.
Hence formula to get from 2.4 to 1 is x= 1 - sqrt(1/2.4)
Commenter: Bruce L. Daniel
The commenter has declared there is no conflict of interests.
The commenter has declared there is no conflict of interests.
The study is available in two forms, a summarized form from the CDC and a full article from the WHO:
The SARS-CoV-2 virus that causes the respiratory disease COVID-19 is for all intents and purposes an Influenza respiratory disease in that it functions and behaves as previous influenza respiratory diseases (with relevant specific deviations and strains which influenza's exhibit). The prevalent detrimental portion of COVID-19, as with other respiratory diseases, being the pneumonia after the virus has run its course. See:
Commenter: FvhG
The commenter has declared there is no conflict of interests.
The studies that measure effectiveness of face mask wearing which are included in their meta-analysis are mainly in household settings. The study notes that "Some studies reported that low compliance in face mask use could reduce their effectiveness." (page 26). The low or unknown mask wearing compliance in these studies prevent any conclusion to be drawn. The study therefore concludes that "there was no evidence that face masks are effective in reducing transmission of laboratory-confirmed influenza."
The study recommends face mask wearing under circumstances currently present as the result of SARS-CoV-2 on the basis of mechanistic plausibility:
"Face masks worn by asymptomatic people are conditionally recommended in severe epidemics or pandemics, to reduce transmission in the community. Although there is no evidence that this is effective in reducing transmission, there is mechanistic plausibility for the potential effectiveness of this measure.
A disposable surgical mask is recommended to be worn at all times by symptomatic individuals when in contact with other individuals. Although there is no evidence that this is effective in reducing transmission, there is mechanistic plausibility for the potential effectiveness of this measure." (page 14).
The WHO-published study is therefore not a counterpoint to the conclusion of this studies, because:
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The point I was making was that this article uses much of the studies that the study the WHO and CDC article, listed above in my original comment, would have also reviewed, and found them either to be overly bias (page 11 of WHO publication) or did not provide actual evidence during the Randomized Controlled Trials (RCT), that masks actually had the desired effect. Here they recognized the RCT's as the best actual evidence, as opposed to observational (which was lower than RCT but higher than computational model) or computer models - since these last two can be more prone to assumptions and bias that can lead to improper conclusions. These studies reviewed by the meta-study that underlies the WHO and CDC articles were from the dates 1946–July 27, 2018. While this pre-print article does include some newer publications, it appears it would be strengthened at minimum, if it also applied the same criteria to it's sources which are newer (addressing possible bias and evidence in the studies used), and to address the areas of older sources which would also be part of the underlying study in the WHO and CDC publications listed above, but were found to not actually support RCT evidence that masks had a significant impact on influenza.
I believe this is particularly important, since the point of this pre-print is explicitly geared toward influencing public policy. It also relies on studies that could easily be attributed to other, although probably less likely, conclusions. For instance, it talks about a case study of airline passengers coming from China. Yet, the underlying articles does not lead to the actual study. For instance, since the flight is coming from China, and some areas of China having been exposed to SARS-Cov-2 since Aug 2019, could it be that most of the passengers and crew were from that region and already had antibodies (had herd immunity been reached), or where was the COVID-19 passenger in their infectious stage, etc.. The second case study actually dealt with Influenza A, but made the statement "We recommend a more comprehensive intervention study to accurately estimate this effect." In regards to mask effectiveness as a recent meta-study, I can find similar articles, from authoritative publication sources, that point out opposite conclusions (that masks are somewhat effective in certain aspects, but have no clinical evidence that they actually have a significant impact on influenza), such as, as an example (although it is classified commentary, it does much of the meta-study review with citations):
This underscores the need to actually deal with any study bias or assumptions, and using a similar approach that the study that underlies the CDC and WHO publicans used, could help alleviate similar studies that have come to opposite conclusions (or at least not as strong of a conclusion as to the effectiveness of masks).
In addition, my other underlying point to this pre-print, since it explicitly is geared toward influencing public policy, is to ask the fundamental question: Is suppression of the virus actually the best course of action for those who are not fragile, or who are not protecting someone at home, or in a nursing home, who is fragile and has comorbidities? Up until now, the reason given for suppression and flattening the curve have relied upon a number of key assumptions, mainly that a vaccine was needed, and secondly that people would flawlessly perform until the vaccine was completed. Neither of these are reasonable in reality - humans are social and there will be contacts (particularly within households), and vaccines are also difficult to develop successfully for coronaviruses (not to mention anything of the actual approval process). There are other factors that should be considered - what about cross immunity as a form of immunization? What about natural herd immunity? What about the fact that COVID-19 as a virus is not really the detrimental part, it is the pneumonia that follows it (which with quick identification can be combated with antibiotics just as it would be with any influenza respiratory disease)? As Dr. Knut M. Wittkowski has pointed out in the links I provided above, suppression only keeps the respiratory disease in the population longer, widens the actual impact, and prevents it from being destroyed through natural herd immunity (or possibly by using a cross-immunity with SARS-Cov[-1]). While this is outside of the scope of the pre-print at first glance, since this pre-print is explicitly is geared toward influencing public policy, I believe it would be much stronger if it actually approached the potential assumption and bias topics of the underlying studies used and even delved into why it believes suppression, rather than quickly fostering natural herd immunity while attempting to protect the vulnerable, is the desired goal.
In regards to the bullet points:
Commenter: Justin Nathaniel White
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There are many examples of the general public fidgiting with masks and not properly sealing masks allowing any potential benefit to be lost. Additionally, it creates a 'magic glove' syndrome where they may wear the masks for too long and not replace these.
The report handwaves these as insignificant and claims these concerns will not affect the finding.
While public information campaigns could prove useful, the uptake on similar public awareness re:seatbelts and drinking and driving bear looking at to see if these measures could be adopted in time to have any significant impact.
Commenter: Noam Peleg
The commenter has declared there is no conflict of interests.