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Face masks for the public during the covid-19 crisis

BMJ 2020; 369 doi: (Published 09 April 2020) Cite this as: BMJ 2020;369:m1435

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Rapid Response:

Re: Face masks for the public during the covid-19 crisis

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 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].

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:
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

28 May 2020
Chris Kenyon
Infectious Diseases Physician
Institute of Tropical Medicine, Antwerp
Nationale straat 155