Face masks for the public during the covid-19 crisis
BMJ 2020; 369 doi: https://doi.org/10.1136/bmj.m1435 (Published 09 April 2020) Cite this as: BMJ 2020;369:m1435Read our latest coverage of the coronavirus pandemic
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Dear Editor,
It is necessary to add more caution to the application of the precautionary principle in the case of face masks (1).
This principle should be applied to all proposed interventions systematically, as principles of evidence based medicine teach us.
Example: Major of Moscow ordered Moscow citizens to wear not only face masks in public places, but also the gloves. Being systematic, one should apply the precautionary principle to the recommendation to wear gloves.
Any intervention has its costs. One of the costs is that once accepted on the base of the insufficient evidence they may harm directly or indirectly (2). The costs of the face masks is not dangerous, but other dangers are possible.
In general the systematic use of the precautionary principle may involve enormous costs and harms more than the harm from the real or imagined problem they are designed to prevent (3).
1. Greenhalgh T, Schmid MB, Cypiononka t, et al. Face masks for the public during the COVID-19 crisis. BMJ 2020;369:m1435 doi: 10.1136/bmj.m1435
2. Ioannidis, J. P. A. (2020). "Coronavirus disease 2019: The harms of exaggerated information and non-evidence-based measures." Eur J Clin Invest 50(4): e13222. doi: 10.1111/eci.13222
Vlassov, V. V. (2017). "Precautionary bias." Eur J Public Health 27(3): 389-389. doi: 10.1093/eurpub/ckx013
Competing interests: No competing interests
Dear Editor
Making use of the precautionary principle, Greenhalgh et al. made a compelling argument that the current evidence is sufficient to promote the use of face masks in public to prevent the spread of COVID-19 [1]. I would like to draw attention to one key argument they omitted. This is the ecological-level evidence that countries where face masking was widespread have generally experienced far milder epidemics than countries where this practice was scarce [2, 3]. Surveys from Hong Kong have shown that in response to COVID-19, 98.8% of individuals now report wearing masks when going out [4]. The wearing of face masks in public has also been widespread in China, Japan, Taiwan, Macau, South Korea and Vietnam [2, 3]. As of 27/05/2020, the median COVID-19 mortality in these countries was 3 deaths/million inhabitants (IQR 0.3-5; (data from https://www.worldometers.info/coronavirus/). This is approximately 100-fold lower than COVID-19 mortality in Belgium, Italy, France, Spain, United Kingdom and the United States (median 545 deaths/million, IQR 437-580) where, until recently, the use of face masks in public was not encouraged [2, 3]. Ecological studies have found a negative country-level association between the widespread use of face masks in public and reduced COVID-19 incidence and mortality [2, 3].
It may be difficult for medical professionals to accept this type of ecological evidence [5]. In part this results from the prominence we give to the ecological fallacy – when an inference is made about an individual based on aggregate data for a group. Less attention is given to the individualistic fallacy – where individual-level data are assumed to be sufficient to explain population level phenomena [5]. However, population level processes can play a crucial role in the spread of infectious diseases and the effectiveness of specific interventions may be best appreciated if considered at a population level [5, 6]. Most of the evidence evaluated by Greenhalgh et al., related to the effect of masks in reducing risk of acquiring COVID-19 by wearers. Whilst this is important, various lines of evidence suggest that the major effect of masks is in reducing the risk of transmission [7-9]. A recent study, for example, found that face masks were effective in reducing the transmission of corona viruses during normal breathing [9]. At a population level these reductions in transmission could have a profound effect. For example, two modelling studies have found that once approximately 80% of persons use masks in public the SAR-CoV-2 effective reproductive number drops rapidly below one [7, 8]. However, if only 50% use face masks the effect on incidence is minimal [7].
It is thus the combination of the individual and population level data that provides the best argument for the promotion of masking in public. Like Greenhalgh et al., I acknowledge that the evidence-base for this recommendation is not perfect and that masks are certainly not a panacea for COVID-19 prevention. In particular, Asian countries with widespread masking appear to have been more effective in testing, tracing and isolating than other areas [10, 11]. When the head of the Chinese Center for Disease Control and Prevention was asked where Europe and the United States were erring in their COVID-19 responses, he answered that it was their failure to promote the widespread usage of face masks in public [12]. In the middle of a pandemic it is not crucial to know whether the masking or the testing/isolation/distancing strategies are more important. In these circumstances erring on the side of caution seems prudent and thus countries should implement all the COVID-19 control measures shown to work in countries with effective responses. Once convincing evidence is produced that one of these components is ineffective this component could be dropped. If we had followed this interpretation of the precautionary principle, we would have been promoting public face masking from February/March 2020 when various Asian countries turned their epidemics around [10].
References
1. Greenhalgh T, Schmid MB, Czypionka T, Bassler D, Gruer L. Face masks for the public during the covid-19 crisis. BMJ. 2020;369:m1435. Epub 2020/04/11. doi: 10.1136/bmj.m1435. PubMed PMID: 32273267.
2. Cheng VC, Wong SC, Chuang VW, So SY, Chen JH, Sridhar S, et al. The role of community-wide wearing of face mask for control of coronavirus disease 2019 (COVID-19) epidemic due to SARS-CoV-2. J Infect. 2020. Epub 2020/04/27. doi: 10.1016/j.jinf.2020.04.024. PubMed PMID: 32335167; PubMed Central PMCID: PMCPMC7177146.
3. Kenyon C. Widespread use of face masks in public may slow the spread of SARS CoV-2: an ecological study. medRxiv 2020;doi: https://doi.org/10.1101/2020.03.31.20048652.
4. Cowling BJ, Ali ST, Ng TWY, Tsang TK, Li JCM, Fong MW, et al. Impact assessment of non-pharmaceutical interventions against coronavirus disease 2019 and influenza in Hong Kong: an observational study. Lancet Public Health. 2020;5(5):e279-e88. Epub 2020/04/21. doi: 10.1016/S2468-2667(20)30090-6. PubMed PMID: 32311320; PubMed Central PMCID: PMCPMC7164922.
5. Subramanian SV, Jones K, Kaddour A, Krieger N. Revisiting Robinson: the perils of individualistic and ecologic fallacy. Int J Epidemiol. 2009;38(2):342-60; author reply 70-3. Epub 2009/01/31. doi: 10.1093/ije/dyn359. PubMed PMID: 19179348; PubMed Central PMCID: PMCPMC2663721.
6. Morris M, Goodreau S, Moody J. Sexual networks, concurrency and STD/HIV. In: Holmes KK, editor. Sex Transm Dis. 4th ed. New York: McGraw-Hill Medical; 2008. p. xxv, 2166 p.
7. Kai D, Goldstein G-P, Morgunov A, Nangalia V, Rotkirch A. Universal masking is urgent in the COVID-19 pandemic: SEIR and agent based models, empirical validation, policy recommendations. arXiv preprint arXiv:200413553. 2020.
8. Kot AD. Critical levels of mask efficiency and of mask adoption that theoretically extinguish respiratory virus epidemics. medRxiv. 2020.
9. Leung NHL, Chu DKW, Shiu EYC, Chan KH, McDevitt JJ, Hau BJP, et al. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nat Med. 2020;26(5):676-80. Epub 2020/05/07. doi: 10.1038/s41591-020-0843-2. PubMed PMID: 32371934.
10. Iwasaki A, Grubaugh N. Why does Japan have so few cases of COVID19? . EMBO Mol Med. 2020. doi: 10.15252/emmm.202012481.
11. Cohen J, Kupferschmidt K. Countries test tactics in ‘war’ against COVID-19. Science. 2020;367(6484):1287-8.
12. Cohen J. Not wearing masks to protect against coronavirus is a ‘big mistake,’ top Chinese scientist says. Science Magazine; 27 March 2020.
Competing interests: No competing interests
Dear Editor,
I align with Drs Anand and Sholapurkar that the public deserve a choice on face covering, especially when the more vulnerable members of society venture out more and shops are busier. The older & vulnerable groups, who may have difficulty tying masks and need to see expressions and lip read, are in my view ideal for face shields (visors), which also cover eyes, nose and mouth and prevent face touching.
I asked 12 local friends aged 48-83 to test visor use at the shops and fill in and comment on a survey questionnaire using a Likert scale (very poor, poor, dont know, good and very good) inspired by Sapoval et al (1) concerning use, vision, tolerability, safety and choice. The Friends Test used a polypropylene visor (face shield) which cost below £5 and is produced by a local company supplying health care facilities although not medically certified.
Where responses were filled in:
One felt at high risk shopping, 3 at significant risk and 6 at small risk.
9/12 would use the visor repeatedly, one for work. Washing the visor put no-one off, 0/12 touched their face, 10/12 would use a visor next time shopping, 2 a mask at times and 2 wouldn't wear anything. Six of the 7 who had experience of using a mask preferred a visor.
Visor vision was rated good or very good for 12/12 and comfort good or very good for 12/12. Asked which appeared safest to them, 10/12 stated a visor, 2 a mask and one didn't respond. Comments were that it wasn't as easily portable and possibly easier to damage than a mask, 2 felt self-conscious, one felt claustrophobic. One who usually touches face a lot said this didn't happen in the visor; one commented that it was easier to talk, though muffled, and another that vision isn't perfect for reading labels but is good.
I am now asking a group of 60-70 year old patients to participate in evaluation. Face shields (visors) appear a useful alternative. Worrying about if they prevent droplets entering the person if someone coughs nearby is like asking me if I can walk through a closed door, although I would like to see more science on efficacy.
Best wishes
Dr Jane Wilcock (GP)
1. Sapoval M, Gaultier A, Del Giudice C, Pellerin O, Kassis-Chikhani N, Lemarteleur V, Fouquet V, Tapie L, Morenton P, Tavitian B, Attal J, 3D-printed face protective shield in interventional radiology: evaluation of an immediate solution in the era of COVID-19 pandemic, Diagnostic and Interventional Imaging (2020), doi: https://doi.org/10.1016/j.diii.2020.04.004
Competing interests: No competing interests
Dear Editor
On 1 Nov 1918, the “Stars and Stripes” (the official newspaper of the American Expeditionary Forces) proudly announced: “Gauze masks for men on transport keep flu at bay.” [1] Five thousand men crossed the North Atlantic wearing masks fashioned from squares of gauze “saturated with a one percent solution of iodine” and fastened with strips of adhesive. There were “no missing men when the rolls were called.” They announced that the backbone of the epidemic was “broken” among army ranks. Their celebrations were premature; more enlisted men were yet to die from influenza than from enemy fire. [2]
But this view of masks during the 1917 influenza pandemic was not unanimous. Neurologist, Dr James Chricton-Browne, interviewed in The Observer, was sceptical. “The fact that the influenza organism is so infinitely minute that it can make its way through porcelain throws doubt on the value of the mask. …[I]ts use in the streets with the addition of goggles as has been proposed would, I believe, be futile, and would probably, if resorted to on a large scale, produce panic, which has always contributed to the spread of epidemic disease.” [3]
As we bicker today over whether masks should be worn to prevent COVID-19, we can see that this is a long-standing debate. Longstanding as well are the points upon which these two historical perspectives converge. Chricton-Browne highlighted the “avoidance of places of public assembly” while the army doubled the allotted space-per-man in tents and ocean transport in order to provide “more room to move”. Both emphasised control of droplets during coughing and sneezing, by “…covering of the mouth and nose … with the subsequent destruction of the disinfection of the handkerchief….” [3] or having “[iodized] gauze at hand, ready for use.” [1]
The more things change, the more they stay the same.
References
1. Gauze Masks for Men on Transport keep Flu at Bay. (1918, Nov 1). The Stars and Stripes. The Stars and Stripes. 1918 1 November.
2. Byerly C. The U.S. Military and the Influenza Pandemic of 1918-1919. Public Health Reports. 2010;125(Supplement 3):82-91.
3. The Fight against the ‘flu: Hints and hopes. Interview with Sir J. Chricton-Browne The Observer. 1919 2 March.
Dear Editor
Competing interests: No competing interests
Dear Editor
In line with Greenhalgh and colleagues’ recommendation, health authorities around the globe are recommending cloth masks and face coverings in public as COVID-19 exit strategies (1). In the meantime concerns are being expressed that these are desperate measures, that anything less than the inward protection of N95/FFP2 respirators, should be of little benefit. Surgical masks were developed for outward protection of the operating field against droplet spread. Should everybody be protected if everyone would wear a mask, as in "I protect you, you protect me"? Using Schlieren imaging, Tang and colleagues previously demonstrated substantial airflows through gasps between face and mask when coughing, which theoretically could allow small particles to escape (2).
While crucial for inward protection, Richard Riley, the Johns Hopkins professor who proved the airborne transmission of tuberculosis, was not concerned about these kinds of leaks: “Covering your cough should be reasonably effective since infectious droplets forcefully expelled from the mouth have not yet evaporated to smaller airborne droplet nuclei and should stick to an obstructing surface on impact.” One could argue that he based his theory on Newton’s first law of motion - the inertia law.
Five years ago, we asked cystic fibrosis patients to crawl inside large cylindrical tanks from which the air could be extracted after cough experiments (3). Surgical mask blocked the generation of 88% of airborne droplet nuclei containing viable Pseudomonas (95% CI 81–96%), a result in line with Riley’s theory. Our result could later be reproduced, very recently even for seasonal coronaviruses (4-5).
While talking, cloth masks used by surgeons in 1962 reduced 97% of droplet nuclei (6).
To address remaining concerns about SARS-CoV-2 passing through saturated cloth when talking, sneezing or coughing, termed “bubble tube effect” in 1975, an extra barrier of densely woven fabric or even a filter between the layers of fabric, is sometimes being recommended (7). Till proven that this is unnecessary, we believe this makes sense as another precautionary principle.
Finally, to translate experimental efficacy in real-life effectiveness, one should be vigilant about not only hand hygiene, but also mask hygiene.
References:
1. Greenhalgh T, Schmid MB, Czypionka T, Bassler D, Gruer L. Face masks for the public during the covid-19 crisis. BMJ2020;369:m1435. doi:10.1136/bmj.m1435 pmid:32273267
2. Tang JW, Liebner TJ, Craven BA, Settles GS. A schlieren optical study of the human cough with and without wearing masks for aerosol infection control. J R Soc Interface. 2009 Dec 6;6 Suppl 6:S727-36. doi: 10.1098/rsif.2009.0295.focus. Epub 2009 Oct 8.
3. Vanden Driessche K, Hens N, Tilley P, Quon BS, Chilvers MA, de Groot R, Cotton MF, Marais BJ, Speert DP, Zlosnik JEA. Surgical masks reduce airborne spread of Pseudomonas aeruginosa in colonized patients with cystic fibrosis. Am J Respir Crit Care Med. 2015 Oct 1;192(7):897-9. doi: 10.1164/rccm.201503-0481LE.
4. Wood ME, Stockwell RE, Johnson GR, Ramsay KA, Sherrard LJ, Jabbour N, Ballard E, O'Rourke P, Kidd TJ, Wainwright CE, Knibbs LD, Sly PD, Morawska L, Bell SC. Face masks and cough etiquette reduce the cough aerosol concentration of Pseudomonas aeruginosa in people with cystic fibrosis. Am J Respir Crit Care Med 2018;197:348–355.
5. Leung NHL, Chu DKW, Shiu EYC, Chan K, McDevitt JJ, Hau BJP, Yen H, Li Y, Ip DKM, Malik Peiris JS, Seto W, Leung GM, Milton DK, Cowling BJ. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nat Med(2020). https://doi.org/10.1038/s41591-020-0843-2
6. Greene VW and Vesley D. Method for evaluating effectiveness of surgical masks. J. Bacteriol. 83, 663-7 (1962)
7. Quesnel LB. The efficiency of surgical masks of varying design and composition. Br. J. Surg. 62, 936-40 (1975)
Competing interests: No competing interests
Dear Editor,
Greenhalgh et al's paper (1) played a significant role in the UK debate around the use of face coverings during the Covid-19 pandemic. Subsequent to this publication, the UK Government has recommended cloth face coverings in enclosed public spaces (2). The precautionary principle leads to risk mitigation depending on the strength of the scientific evidence and this can include the non-mandated use of face coverings. The non-mandated use of face coverings leads to a need for a strong public health message to promote the wearing of face coverings given the importance of prevention in the absence of a vaccine. This means that the medical community has a role in shaping the current public health message about face coverings and interfacing with industry to promote their use.
Yours sincerely
Dr Justin Marley
1. Greenhalgh T, Schmid MB, Czypionka T, Bassler D, Gruer L. Face masks for the public during the covid-19 crisis. BMJ [Internet]. British Medical Journal Publishing Group; 2020 [cited 2020 May 19]; 369. Available from: https://www.bmj.com/content/369/bmj.m1435
2. UK Government. Our plan to rebuild: The UK Government's COVID-19 recovery strategy. 2020 . CP 239 [internet]. [cited 2020 May 19]. Available from https://www.gov.uk/government/publications/our-plan-to-rebuild-the-uk-go...
Competing interests: No competing interests
Dear Editor
A simple matter of Commonsense
1. Anyone who has ever worked in an operating theatre remembers that you are required to wear a face mask. Nobody ever did randomised controlled trials.
2. I want to protect others from MY corona virus if I harbour it. I have been in splendid isolation since about three weeks before the Prime Minister thought of it.
Yes, I go out for a walk, when most people are asleep. Today I saw, at a distance of several meters, two men, two dogs, one child, one lady on a bike.
We gave one another a wide berth.
3. When I have to go to the GP surgery for an injection for cancer of the prostate, I will wear a privately purchased (£9:99) mask, which I hope will save me from being infected by the staff at the surgery, others at the surgery; some of them might be innocent carriers.
4. I would welcome criticisms of my brand of commonsense. It transcends the wisdom of the WHO, the Prime Minister, NHS England.
Competing interests: Susceptible, I think, to Covid-19
Trisha Greenhalgh et al [1] make a very long and laborious case for using face masks (coverings) in public. There have been many similar suggestions previously [2,3]. However, their long winded approach full of research jargon [1] has not been understood or heeded by the English politicians. Hence, the official policy of Public Health England (PHE) and Department of Health (DOH) continues not to positively recommend use of face-masks in public. They seem to keep on repeating the mantra of their experts that ‘there is no definitive evidence of benefit of face-masks to reduce transmission of covid-19’. The DOH seems to have an uncanny habit of delaying several different beneficial measures based on misguidance.
Politicians should note the well accepted wise adage that ‘the lack of evidence of effectiveness does not mean the evidence of lack of effectiveness’. In general, most medical guidelines have more than 50% of their practice recommendations based on level 3 or 4 evidence (i.e. expert consensus mainly but no definitive evidence). Many studies show that face-masks are effective but equal numbers arrive at the opposite conclusion. Does it mean that face-masks are effective and ineffective at the same time? Or does it mean that face-masks are effective 50% of the time (that would be a great advantage)?
It does not mean either. All it means is that the clinical studies have been affected by several uncontrollable confounding factors and methodological variations. Thus, the resulting ‘lack of evidence’ has everything to do with the limitations of clinical studies in this particular situation and far less to do with the face-masks themselves. Medicine is a soft science and clinical studies are quite different from experiments in physics and chemistry.
Hence, in this particular situation, the so called ‘evidence or lack of it’ should not be the determinant of the public policy at all. We have to draw inferences from several other observations from these studies (like testing of barrier function) and observed pattern of the covid-19 spread and come to pragmatic decisions [2,3]. Pros and cons of expert warnings of the ‘dangers’ of face-masks need to be examined critically.
Some experts are alarmed that the public will touch the virus-contaminated surface of the masks and put fingers into their mouths or eyes. Is it better to inhale the virus in the first place then? The suggestion that the lay public cannot be taught simple safety precautions while using face-masks seems very patronising. Similar is another unjustified fear that the public will be falsely reassured and forget or ignore other measures like hand-washing or physical distancing. In fact it is more likely that wearing face-masks will be a constant reminder that we are living in unusual times and need to take all precautions.
It is high time that PHE and DOH refrain from the mantra of ‘no definitive evidence’ and make pragmatic recommendation of wide use of washable cloth face-masks in public spaces which would be important to release the lock-down safely.
Mr S. L. Sholapurkar, MD, DNB, MRCOG
Bath, UK, 19th May 2020.
References:
1. Trisha Greenhalgh and colleagues. We should advise the public to wear face masks. BMJ 2020, 369:m1435
2. Sholapurkar SL. Important practical and pragmatic safety measures for health care workers during covid-19 pandemic: The role and practicality of surgical face-masks. https://www.bmj.com/content/368/bmj.m1316/rr
3. Sholapurkar SL. Facemasks: The Evidence and Myths amongst experts and public officials regarding wearing of protective Face-masks during the covid-19 pandemic. https://www.bmj.com/content/369/bmj.m1434/rr-17
Competing interests: No competing interests
Dear Editor,
The Covid-19 pandemic, which started in Wuhan, China, in December 2019, has swept the world in less than three months with no end in sight. As of May 19, 2020 at 7 am, 188 countries and regions had reported 4,822,430 confirmed cases and 318,851 deaths, corresponding to a mortality rate of 6.6% (https://coronavirus.jhu.edu/map.html), posing a formidable challenge to the health of citizens throughout the world.
The public health imperative is to interrupt viral transmission in order to prevent spread of the disease. Many strategies have been deployed. However, essential measures are straightforward: engaging in social distancing, wearing masks, and washing hands. Interestingly, these measures have been embraced to varying degrees in different parts of the world. Although there is now broad agreement that social distancing and handwashing can effectively interrupt viral transmission, there is a large discrepancy across countries and cultures in the perception of the effectiveness of wearing a mask. In many Asian countries, wearing a mask publicly is a common occurrence, even though there is no legal requirement to do so, as mask-wearing has become cultural acceptable. By contrast, in the United States and many European countries, the majority of people do not routinely wear a mask, except perhaps when they are ill, and even this is uncommon. This approach is consistent with the recommendations issued by the World Health Organization (https://www.who.int/emergencies/diseases/novel-coronavirus-2019/advice-f...).
Why do people's perceptions of mask-wearing differ so widely across cultures? The answer is clearly complicated and depends on one’s perspective.
One school of thought, which is common in many western countries, tends to rely on evidence to inform decision-making and eschews practices that cannot be demonstrated to be true. As there is no direct evidence to support the hypothesis that universal mask-wearing amid a pandemic limits the spread of the virus, and given that masks are a relatively scarce resource during this pandemic, only selective mask-wearing was recommended in the early phase of the pandemic. However, whether or not it is pragmatic, it is not clear that this strategy is optimal for combating SARS-CoV-2. And there is unlikely to be robust evidence produced in real-time demonstrating the comparative effectiveness of universal mask-wearing as compared with selective mask-wearing.
Another school of thought, seen with greater frequency in Asian countries, is rooted in a different logic, less dependent on formal evidence than on observation. In this case, patients need to wear masks because they can spread the virus and masking prevents transmission of human respiratory viruses.1 Because asymptomatic carriers can also spread the virus,2-4 they should wear a mask. And because we cannot be entirely sure whether or not we are asymptomatic carriers; according to this logic, everyone should wear a mask. Wearing a mask reduces not only airborne transmission but also contact transmission because, in theory, it reduces the precipitation of the airway droplets on the surface of an object, a source for contact transmission. Importantly, wearing a mask reduces the number of times our hands come in contact with our face, a common route of self-inoculation and transmission. Research has found that humans touch their faces for seemingly no reason every 2.5 minutes—a behavior that is common among mammals.5 This hard-wired behavior starts in utero. It is impossible for us to instantly stop doing something that we have been subconsciously doing our entire lives. Therefore, wearing a mask protects us from the most significant threat of self-inoculation; i.e., our own hands. People may argue that there is always a risk of cross-contamination between the mask and hands. Yes, that is possible; however, it is clearly a lower risk than that associated with touching our eyes, nose, or mouth. Furthermore, wearing a mask does not affect handwashing in any way. Finally, it is essential to note that wearing a mask stops airway droplets from accessing the portion of the face that is covered.
Additional considerations need to be taken into account when deciding whether or not to wear a mask, including the available resources and the cultural significance of wearing a mask.6 In the absence of adequate supplies, prioritizing patients, healthcare workers, and vulnerable populations is the right thing to do; however, doing so based on limited resources is not the same as asserting that masks should be selectively worn.
Some cultures may regard wearing a mask as embarrassing or a symbol of repression. Some people become anxious or alarmed at the sight of an individual wearing a mask, especially amid this pandemic. These are cultural and educational issues. The solution is to explain the benefits of mask-wearing and help people understand that doing so could slow the spread of this vicious disease.
So which strategy is superior? Universal or selective mask-wearing? With the lack of direct scientific evidence, we should use common sense. The pandemic is currently under control in Asian countries, including China and South Korea. There are many reasons for this. It is highly likely that society's acceptance and perception of mask-wearing is one among these potential explanations. Our posture echoes the recent publication in the BMJ which concluded that “in the face of a pandemic the search for perfect evidence may be the enemy of good policy.”7
References
1. Leung NHL, Chu DKW, Shiu EYC, et al. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nature Medicine 2020 doi: 10.1038/s41591-020-0843-2
2. Bai Y, Yao L, Wei T, et al. Presumed Asymptomatic Carrier Transmission of COVID-19. Jama 2020 doi: 10.1001/jama.2020.2565 [published Online First: 2020/02/23]
3. Rothe C, Schunk M, Sothmann P, et al. Transmission of 2019-nCoV infection from an asymptomatic contact in Germany. New England Journal of Medicine 2020
4. Li R, Pei S, Chen B, et al. Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2). Science 2020
5. Kwok YL, Gralton J, McLaws ML. Face touching: a frequent habit that has implications for hand hygiene. Am J Infect Control 2015;43(2):112-4. doi: 10.1016/j.ajic.2014.10.015 [published Online First: 2015/02/01]
6. Feng S, Shen C, Xia N, et al. Rational use of face masks in the COVID-19 pandemic. The Lancet Respiratory Medicine 2020
7. Greenhalgh T, Schmid MB, Czypionka T, et al. Face masks for the public during the covid-19 crisis. BMJ 2020;369:m1435. doi: 10.1136/bmj.m1435
Competing interests: No competing interests
Laying straw men to rest
Dear Editor
The many responses to our article on face masks [1], including recent letters in the print issue, include a number of arguments of the general format "mask wearing could [hypothetically] cause the following harm". In the seven weeks since our original BMJ article was published, many empirical studies have been published which support the efficacy or confirm the lack of serious harms associated with masking by the lay public. To my knowledge, no robust study has been published which bears out the claim that the harms of masking in the midst of this pandemic outweigh the benefits. I have addressed these issues in a recently published peer-reviewed paper entitled 'Laying Straw Men to Rest'.[2]
[1] Greenhalgh T, Schmid MB, Czypionka T, et al. Face masks for the public during the covid-19 crisis. Bmj 2020;369:m1435. doi: 10.1136/bmj.m1435.
[2] Greenhalgh T. Face coverings for the lay public: laying straw men to rest. J Eval Clin Practice 2020, e13415.
https://onlinelibrary.wiley.com/doi/10.1111/jep.13415
Competing interests: I'm joint lead author of the original article