Effectiveness of Surgical and Cotton Masks in Blocking SARS–CoV-2: A Controlled Comparison in 4 Patients
Authors: Seongman Bae, Min-Chul Kim, Ji Yeun Kim, Hye-Hee Cha, Joon Seo Lim, Jiwon Jung, Min-Jae Kim, Dong Kyu Oh, Mi-Kyung Lee, Seong-Ho Choi, Minki Sung, Sang-Bum Hong, Jin-Won Chung, Sung-Han Kim
Journal: Annals of Internal Medicine (2020)

Letters6 April 2020Effectiveness of Surgical and Cotton Masks in Blocking SARS–CoV-2: A Controlled Comparison in 4 PatientsFREEThis article has been retracted. See retraction:RetractionsJul 2020Notice of Retraction: Effectiveness of Surgical and Cotton Masks in Blocking SARS-CoV-2FREESeongman Bae, MD, Min-Chul Kim, MD, Ji Yeun Kim, PhD, Hye-Hee Cha, BS, Joon Seo Lim, PhDSeongman Bae, MDSearch for more papers by this author, Min-Chul Kim, MDSearch for more papers by this author, Ji Yeun Kim, PhDSearch for more papers by this author, Hye-Hee Cha, BSSearch for more papers by this author, Joon Seo Lim, PhDSearch for more papers by this authorSeongman Bae, MD, Min-Chul Kim, MD, Ji Yeun Kim, PhD, Hye-Hee Cha, BS, Joon Seo Lim, PhD, Jiwon Jung, MD, Min-Jae Kim, MD, Dong Kyu Oh, MD, Mi-Kyung Lee, MD, Seong-Ho Choi, MD, Minki Sung, PhD, Sang-Bum Hong, MD, Jin-Won Chung, MD, and Sung-Han Kim, MDSeongman Bae, MDAsan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea (S.B., J.Y.K., H.C., J.J., M.K., D.K.O., S.H., S.K.)Search for more papers by this author, Min-Chul Kim, MDChung-Ang University Hospital, Seoul, South Korea (M.K., M.L., S.C., J.C.)Search for more papers by this author, Ji Yeun Kim, PhDAsan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea (S.B., J.Y.K., H.C., J.J., M.K., D.K.O., S.H., S.K.)Search for more papers by this author, Hye-Hee Cha, BSAsan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea (S.B., J.Y.K., H.C., J.J., M.K., D.K.O., S.H., S.K.)Search for more papers by this author, Joon Seo Lim, PhDClinical Research Center, Asan Institute for Life Sciences, Asan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea (J.S.L.)Search for more papers by this author, Jiwon Jung, MDAsan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea (S.B., J.Y.K., H.C., J.J., M.K., D.K.O., S.H., S.K.)Search for more papers by this author, Min-Jae Kim, MDAsan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea (S.B., J.Y.K., H.C., J.J., M.K., D.K.O., S.H., S.K.)Search for more papers by this author, Dong Kyu Oh, MDAsan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea (S.B., J.Y.K., H.C., J.J., M.K., D.K.O., S.H., S.K.)Search for more papers by this author, Mi-Kyung Lee, MDChung-Ang University Hospital, Seoul, South Korea (M.K., M.L., S.C., J.C.)Search for more papers by this author, Seong-Ho Choi, MDChung-Ang University Hospital, Seoul, South Korea (M.K., M.L., S.C., J.C.)Search for more papers by this author, Minki Sung, PhDSejong University, Seoul, South Korea (M.S.)Search for more papers by this author, Sang-Bum Hong, MDAsan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea (S.B., J.Y.K., H.C., J.J., M.K., D.K.O., S.H., S.K.)Search for more papers by this author, Jin-Won Chung, MDChung-Ang University Hospital, Seoul, South Korea (M.K., M.L., S.C., J.C.)Search for more papers by this author, and Sung-Han Kim, MDAsan Medical Center, University of Ulsan College of Medicine, Seoul, South Korea (S.B., J.Y.K., H.C., J.J., M.K., D.K.O., S.H., S.K.)Search for more papers by this authorAuthor, Article, and Disclosure Informationhttps://doi.org/10.7326/M20-1342 SectionsAboutVisual AbstractPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinkedInRedditEmail This article has been retracted. See Notice of Retraction.Background: During respiratory viral infection, face masks are thought to prevent transmission (1). Whether face masks worn by patients with coronavirus disease 2019 (COVID-19) prevent contamination of the environment is uncertain (2, 3). A previous study reported that surgical masks and N95 masks were equally effective in preventing the dissemination of influenza virus (4), so surgical masks might help prevent transmission of severe acute respiratory syndrome–coronavirus 2 (SARS–CoV-2). However, the SARS–CoV-2 pandemic has contributed to shortages of both N95 and surgical masks, and cotton masks have gained interest as a substitute.Objective: To evaluate the effectiveness of surgical and cotton masks in filtering SARS–CoV-2.Methods and Findings: The institutional review boards of 2 hospitals in Seoul, South Korea, approved the protocol, and we invited patients with COVID-19 to participate. After providing informed consent, patients were admitted to negative pressure isolation rooms. We compared disposable surgical masks (180 mm × 90 mm, 3 layers [inner surface mixed with polypropylene and polyethylene, polypropylene filter, and polypropylene outer surface], pleated, bulk packaged in cardboard; KM Dental Mask, KM Healthcare Corp) with reusable 100% cotton masks (160 mm × 135 mm, 2 layers, individually packaged in plastic; Seoulsa).A petri dish (90 mm × 15 mm) containing 1 mL of viral transport media (sterile phosphate-buffered saline with bovine serum albumin, 0.1%; penicillin, 10 000 U/mL; streptomycin, 10 mg; and amphotericin B, 25 µg) was placed approximately 20 cm from the patients' mouths. Patients were instructed to cough 5 times each onto a petri dish while wearing the following sequence of masks: no mask, surgical mask, cotton mask, and again with no mask. A separate petri dish was used for each of the 5 coughing episodes. Mask surfaces were swabbed with aseptic Dacron swabs in the following sequence: outer surface of surgical mask, inner surface of surgical mask, outer surface of cotton mask, and inner surface of cotton mask.The median viral loads of nasopharyngeal and saliva samples from the 4 participants were 5.66 log copies/mL and 4.00 log copies/mL, respectively. 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. All swabs from the outer mask surfaces of the masks were positive for SARS–CoV-2, whereas most swabs from the inner mask surfaces were negative (Table).Table. SARS–CoV-2 Viral Load in Patient Samples, Petri Dishes, and Mask SurfacesDiscussion: Neither surgical nor cotton masks effectively filtered SARS–CoV-2 during coughs by infected patients. Prior evidence that surgical masks effectively filtered influenza virus (1) informed recommendations that patients with confirmed or suspected COVID-19 should wear face masks to prevent transmission (2). However, the size and concentrations of SARS–CoV-2 in aerosols generated during coughing are unknown. Oberg and Brousseau (3) demonstrated that surgical masks did not exhibit adequate filter performance against aerosols measuring 0.9, 2.0, and 3.1 μm in diameter. Lee and colleagues (4) showed that particles 0.04 to 0.2 μm can penetrate surgical masks. The size of the SARS–CoV particle from the 2002–2004 outbreak was estimated as 0.08 to 0.14 μm (5); assuming that SARS-CoV-2 has a similar size, surgical masks are unlikely to effectively filter this virus.Of note, we found greater contamination on the outer than the inner mask surfaces. Although it is possible that virus particles may cross from the inner to the outer surface because of the physical pressure of swabbing, we swabbed the outer surface before the inner surface. The consistent finding of virus on the outer mask surface is unlikely to have been caused by experimental error or artifact. The mask's aerodynamic features may explain this finding. A turbulent jet due to air leakage around the mask edge could contaminate the outer surface. Alternatively, the small aerosols of SARS–CoV-2 generated during a high-velocity cough might penetrate the masks. However, this hypothesis may only be valid if the coughing patients did not exhale any large-sized particles, which would be expected to be deposited on the inner surface despite high velocity. These observations support the importance of hand hygiene after touching the outer surface of masks.This experiment did not include N95 masks and does not reflect the actual transmission of infection from patients with COVID-19 wearing different types of masks. We do not know whether masks shorten the travel distance of droplets during coughing. Further study is needed to recommend whether face masks decrease transmission of virus from asymptomatic individuals or those with suspected COVID-19 who are not coughing.In conclusion, 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.References1. Johnson DF, Druce JD, Birch C, et al. A quantitative assessment of the efficacy of surgical and N95 masks to filter influenza virus in patients with acute influenza infection. Clin Infect Dis. 2009;49:275-277. [PMID: 19522650] doi:10.1086/600041 Google Scholar2. Feng S, Shen C, Xia N, et al. Rational use of face masks in the COVID-19 pandemic. Lancet Respir Med. 20 March 2020. [Epub ahead of print]. [PMID: 32203710] doi:10.1016/S2213-2600(20)30134-X Google Scholar3. Oberg T, Brosseau LM. Surgical mask filter and fit performance. Am J Infect Control. 2008;36:276-282. [PMID: 18455048] doi:10.1016/j.ajic.2007.07.008 Google Scholar4. Lee SA, Grinshpun SA, Reponen T. Respiratory performance offered by N95 respirators and surgical masks: human subject evaluation with NaCl aerosol representing bacterial and viral particle size range. Ann Occup Hyg. 2008;52:177-185. [PMID: 18326870] doi:10.1093/annhyg/men005 Google Scholar5. Ksiazek TG, Erdman D, Goldsmith CS, et al; SARS Working Group. A novel coronavirus associated with severe acute respiratory syndrome. N Engl J Med. 2003;348:1953-1966. [PMID: 12690092] Google Scholar Comments 0 Comments Sign In to Submit A Comment Sung-Han Kim, MD.Asan Medical Center28 April 2020 Author's response We totally agree with Dr. Glele and colleagues’ comment on the high variability of coughing intensity within subjects. Furthermore, it is worth to note that one of eight coughing experiments without mask in patients with COVID-19 revealed a negative SARS-CoV-2 PCR result (Table 1). The heterogeneity of transmission of coronavirus including SARS-CoV, MERS-CoV, and SARS-CoV-2 may explain this observation. The recent study reported that none 41 healthcare workers with most surgical masks and minor N95 masks who were exposed to the aerosol-generating procedures in eventually diagnosed COVID-19 patients developed symptoms, and all PCR tests for SARS-CoV-2 were negative (COVID-19 and the risk to health care workers: a case report. Ann Intern Med 2020 March 16). Given that viral expectoration from coughing COVID-19 patients was not uniform based on our experiment, cautious interpretation for unusual transmission events is always needed. Dr. Glele and colleagues also commented that no detection of SARS-CoV-2 RNA from inner surface except one patient precludes any reliable conclusions. We assume that multiple factors may affect swab sampling from the outer and inner surfaces of the masks. Although environmental sampling from hard surfaces such as plastic or metal has been widely studied, there are limited studies on sampling from fabric materials. Elution of punched layers of face masks may provide more valuable information about the surface contamination of the masks. Further studies are needed on the viral contamination of mask surfaces. In this context, this variability of viral shedding from coughing within the subject and the nature of fabric swab sampling should be bear in mind for the interpretation of our small experimental data. As Dr. Glele and colleagues’ comment, Leung et al. reported the efficacy of surgical masks in reducing coronavirus detection and viral load from 17 patients (Nat Med 2020 Apr 3). The big difference between Leung’s study and ours is the method of collecting human coronavirus particles from the patients. Leung’s study collected virus particles by a closed system such as G-II bioaerosol collecting device which consists of a large cone connected with a closed duct. In contrast, we collected virus particle of SARS-CoV-2 directly from coughing COVID-19 patients with an open air system in a negative pressure room. Furthermore, the results of the efficacy of surgical masks on influenza virus from Leung’s study (Nat Med 2020 Apr 3) are different from those by the previous study (Clin Infect Dis 2009; 49:275-7). The different methodology of sample collection may explain this discrepancy. Dr. Purens and colleagues pointed the statistical issue. Our complete case analysis (CCA) may overestimate the true value. In contrast, if we included “not detectable” as “zero”, the calculation may underestimate the true value. So, an alternative calculation such as single imputation or Dr. Purens’ calculation may result in the value between these two. Thank you for suggesting one of good sensitivity analysis. We appreciated Dr. Yeung’s good balanced view of our study results. We agree with Dr. Yeung’s opinion on that our small study (n=4) is a pilot study. We have recently completed additional mask tests in 7 COVID-19 patients to compare the use of surgical masks to the use of N95-equivalent respirators. We believe that these data will provide more information on this issue. Furthermore, other independent groups should evaluate the outward and inward protective effectiveness of various masks against SARS-CoV-2 with more well-designed protocols in which the issues raised in this pilot study by many experts can be settled. Therefore, we totally agree with Dr. Yeung’s view on this pilot study like the glass half full or empty. Christopher T. Leffler, MD, MPH.1 Edsel Ing MD, MPH, CPH, MIAD.2 Joseph D. Lykins V, MD.1 Craig A. McKeown, MD3. Andrzej Grzybowski, MD.41. Virginia Commonwealth University 2. University of Toronto 3. University of Miami 4. University of Warmia and Mazury30 April 2020 Prevention of the spread of coronavirus using masks. We read the work which concluded “both surgical and cotton masks seem to be ineffective in preventing the dissemination of SARS–CoV-2…”1 In fact, compared with the control condition, the petri dish viral load was less with a cloth mask for all patients, and in half, was not detectable.1 Such reductions do help at the population level.2,3 We retrieved mortality and testing data for 169 countries from a publicly available source on April 22, 2020.4 On average, the time from infection to symptoms is 5.1 days, and that from infection to death is 23 days.2 Therefore, the date of each country’s initial infection was estimated as the earlier of: 5 days before the first reported infection, or 23 days before the first death.4,5 As deaths by April 22, 2020 would typically reflect infections beginning 23 days previously (by March 30), both the time from the first infection, and from the time the public began wearing masks, until March 30 were determined. Countries in which mask usage has been widespread include Hong Kong, South Korea, Malaysia, Taiwan, Japan, and Mongolia.2 Mandates for wearing of masks in public had been issued by March 30 in Thailand (March 12), Vietnam (March 16), Czechia (March 19), and Slovakia (March 25).2 The exponential growth associated with the spread of an epidemic appears linear on a logarithmic scale.2 By multivariable linear regression, significant predictors of the logarithm of each country’s per-capita coronavirus mortality included: duration of infection in the country, duration of wearing masks, population size, and per-capita testing (all p<0.001, Table 1). In a population not wearing masks, the per-capita mortality tended to increase each week by a factor of 10^0.156 = 1.43, or 43%. On the other hand, in a population wearing masks, the per-capita mortality tended to increase by a factor of 10^(0.156-0.144) = 1.028, or just 2.8%. The positive association with testing probably reflects the greater recognition of coronavirus-related mortality with more testing, as well as the increased incentive countries have to test when they suffer a more intense outbreak. These results support the universal wearing of masks by the public to suppress the spread of the coronavirus. Mask-wearing should be adopted immediately, based on the precautionary principle.2,3 References.1. Bae S, Kim MC, Kim JY, Cha HH, Lim JS, Jung J, Kim MJ, Oh DK, Lee MK, Choi SH, Sung M. Effectiveness of surgical and cotton masks in blocking SARS–CoV-2: a controlled comparison in 4 patients. Annals of Internal Medicine. 2020 Apr 6.2. Leffler CT, Ing E, McKeown CA, Pratt D, Grzybowski A. Final Country-wide Mortality from the Novel Coronavirus (COVID-19) Pandemic and Notes Regarding Mask Usage by the Public. April 4, 2020. Available from: https://www.researchgate.net/publication/340438732_Country-wide_Mortality_from_the_Novel_Coronavirus_COVID-19_Pandemic_and_Notes_Regarding_Mask_Usage_by_the_Public DOI: 10.13140/RG.2.2.36006.27200 3. Howard J, Huang A, Li Z, Tufekci Z, et al. Face masks against COVID-19: an evidence review. Preprints 2020; published online April 12. DOI:10.20944/preprints202004.0203.v1 (preprint).4. Worldometers. COVID-19 Coronavirus Pandemic. Available from: https://www.worldometers.info/coronavirus/?utm_campaign=homeAdUOA?Si Accessed April 22, 2020.5. European Centre for Disease Prevention and Control. COVID-19 Coronavirus data. Available from: https://data.europa.eu/euodp/en/data/dataset/covid-19-coronavirus-data Accessed April 16, 2020.None of the authors has any conflicts of interest to disclose. Table 1. Predictors of (log) Country-wide Per-capita Coronavirus Mortality by Multivariable Linear Regression in 169 Countries. Coefficient (SE) 95% CI P value.Duration in country (weeks) 0.156 (SE 0.034) (95% CI 0.089 to 0.223) p<0.001.Time wearing masks (weeks) -0.144 (SE 0.033) (95% CI -0.209 to -0.079) p<0.001.Population (log) -0.297 (SE 0.079) (95% CI -0.453 to -0.141) p<0.001.Tests per capita (log) 0.612 (SE 0.085) (95% CI 0.445 to 0.779) p <0.001.Constant -2.571 (SE 0.368) (95% CI -3.299 to -1.844) p<0.001. Eugene Y.H. YeungFaculty of Medicine, University of Ottawa; Eastern Ontario Regional Laboratory Association (EORLA)27 April 2020 Effectiveness of Masks in Blocking SARS-CoV-2: Depends on Whether You See the Glass Half Full or Empty It is difficult to draw a solid conclusion from a study of 4 participants, which clearly lacked statistical power to detect difference between control and intervention groups. This is a pilot study at best, but our interpretation depends on whether we see the glass half full or empty. Optimistic researchers would notice a trend of decrease in SARS-CoV-2 viral load when each participant had face mask on. Although the study found contamination on the outer surface of face masks, there was no evidence that the viral particles bypassed the mask and entered the wearers’ mucosa. Three of the 4 participants had undetectable viral load in inner surface of masks. These findings suggested potential role of masks as barriers against entrance of viral particles. Optimistic researchers would be satisfied with these preliminary findings, and thereby conduct a larger study with sufficient statistical power. On the contrary, pessimistic researchers would see this study as a failure and conclude masks are ineffective in preventing the dissemination of SARS–CoV-2. As Sir Winston Churchill stated, “A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty.” Disclosures: I have been paid for working in primary and secondary care settings, but not for writing this letter. Opinions expressed are solely my own and do not express the views of my employer. Ludwig Serge Aho Glele, Sara romano-Bertrand, Jean-Francois Gehanno, Didier LepelletierEpidemiology, infection control, evaluation. Dijon, Montpellier, Rouen, Nantes. France and public health 27 April 2020 General response to Bae et al. We read with interest the article by Seongman Bae et al. (1) estimating the blocking power of surgical mask and cotton mask against SARS-CoV-2.Patients with known viral loads had to cough five times in a petri dish following the sequence: no mask, surgical mask, cotton mask then no mask again. Different petri dishes were used for each of the five cough episodes and we assume that each patient coughed 5 times on each petri dish for each step of the sequence, as there were only four steps by sequence.Authors implicitly consider that the intensity of coughing does not vary between subjects and during the course of the experiment, which is not in line with the high variability within subjects (2).Outcomes criteria were the contamination of petri dishes, and of external and internal surfaces of masks. No air samples were collected close to patients along with the experiment but it would be informative on SARS-CoV-2 shedding through ineffective masks.Outer surfaces of masks were more contaminated than inner surfaces, but this was in fact assessed only for one patient (patient 3), since inner surface contamination was not detected for the three other patients. This precludes any statistical test and therefore any reliable conclusion.Authors based the statement that neither surgical nor cotton masks effectively filtered SARS–CoV-2 during coughs on only two patients (1 and 3) without any statistical test. The median viral loads (log copies/mL) in nasopharynx and saliva from the four participants were respectively of 5.66 and 4.00, but varied from 3.51 to 7.68. Furthermore viral loads, when detected, were often very close to the RT-PCR detection limit. This can induce bias but is not discussed by authors. We therefore consider that their statement cannot be considered reliable. A study on 17 patients demonstrated the efficacy of surgical masks in reducing coronavirus detection and viral loads in both large respiratory droplets and aerosols (3).Non-parametric tests can be performed even with very small samples (4). Potential confounding factors, particularly viral loads, were collected but were not statistically analysed. Larger sample size would have allowed the development of an experimental design that could consider: initial viral load level and correlation of the data (difference in viral load between outer and inner surfaces, initial level in the oropharynx and mask contamination, contamination of petri dishes and surfaces...). Such a more complex experimental design (5) would allow more reliable conclusions.References1. Bae S, Kim M-C, Kim JY, Cha H-H, Lim JS, Jung J, et al. Effectiveness of Surgical and Cotton Masks in Blocking SARS-CoV-2: A Controlled Comparison in 4 Patients. Ann Intern Med. 2020 Apr 6; 2. Duguid JP. The size and the duration of air-carriage of respiratory droplets and droplet-nuclei. J Hyg (Lond). 1946 Sep;44(6):471–9. 3. Leung NHL, Chu DKW, Shiu EYC, Chan K-H, McDevitt JJ, Hau BJP, et al. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nat Med [Internet]. 2020 Apr 3 [cited 2020 Apr 6]; Available from: http://www.nature.com/articles/s41591-020-0843-24. Dwivedi AK, Mallawaarachchi I, Alvarado LA. Analysis of small sample size studies using nonparametric bootstrap test with pooled resampling method. Statistics in Medicine. 2017;36(14):2187–205. 5. Baker TB, Smith SS, Bolt DM, Loh W-Y, Mermelstein R, Fiore MC, et al. Implementing Clinical Research Using Factorial Designs: A Primer. Behav Ther. 2017;48(4):567–80. Ken LimCyberMedia Convergence consulting20 April 2020 Principal Investigator Extremely flawed experiment got published with n=4!!! OFC the virus went thru! the masks aren’t waterproof. OFC it went thru, force of cough pushed it thru. The test should be how many viral droplets appeared on another person or surface 2-3m away! The test should be hi-speed video showing # of particles expelled w & w/o mask! Unbelievably poor experiment! Kristopher Purens, PhD, Abigail Purens, DVM/MPH candidateDescartes Labs, Inc., University of Minnesota College of Veterinary Medicine17 April 2020 Statistical analysis shows decreased airborne SARS-CoV-2 transmission with the use of masks in line with previous studies To test the efficacy of masks to reduce respiratory transmission of SARS-CoV-2, Bae et al.(1) replicated methods previously published by Johnson et al. (2009)(2) in an important early comparative study. A precautionary approach to new public health threats such as the COVID-19 pandemic is to use the best available models as analogues, make conservative recommendations, and update as new data become available. This necessitates careful null hypothesis selection and an information-gained approach to new data and ongoing analysis. A precautionary null hypothesis to COVID-19 is to test whether new evidence is strong enough to reject prior recommendations, such as widespread mask use. Johnson found that masks reduced respiratory transmission of influenza virus, a disease commonly used as a model for SARS-CoV-2.(3) In this context, Bae’s null hypothesis that masks do not reduce viral load transmission was inappropriate. Combined with Bae’s small sample size, this led to reporting of mask wearing causing no significant reduction in SARS-CoV-2 viral load transmission, in contrast to Johnson’s findings for influenza.Additionally, statistical analyses for non-normally distributed data and small sample size are appropriate in this context, to prevent being misled by violating the assumptions of common statistical methods. Two such appropriate analyses are probability based methods, and permutation tests. Analytical power can be increased by treating each pair of masked/non-masked attempts as a trial, and correcting for differences in base viral load for each individual.(4) We assumed no detection (ND) just below the lowest detected threshold reported, with differences calculated from that highest-reasonable viral load that would result in ND. To this end we performed two tests: 1) non-parametric probabilistic approach testing whether Bae’s results indicate masks caused no reduction in respiratory SARS-CoV-2 transmission and 2) permutation resampling testing of whether Bae’s results were significantly different than Johnson’s influenza virus transmission results.(4) Our analysis found that masks provide >0 reduction in viral load transmission (p=0.0078) and that Bae’s results for SARS-CoV-2 were not significantly different from Johnson’s results for influenza in reducing respiratory viral load transmission (p = 0.158). Our results support the continued use of influenza as a model for public health decisions regarding SARS-CoV-2. Importantly for public health, our analysis supports current recommendations for widespread mask wearing during the COVID-19 pandemic.(5) The combined data set assembled, Bae et al. and Johnson et al., and analysis is available at https://github.com/purens/sars_cov2_masks to allow further study.References1. Bae S, Kim M-C, Kim JY, Cha H-H, Lim JS, Jung J, et al. Effectiveness of Surgical and Cotton Masks in Blocking SARS–CoV-2: A Controlled Comparison in 4 Patients. Annals of Internal Medicine [Internet]. 2020 Apr 6 [cited 2020 Apr 7]; Available from: https://doi.org/10.7326/M20-13422. Johnson DF, Druce JD, Birch C, Grayson ML. A Quantitative Assessment of the Efficacy of Surgical and N95 Masks to Filter Influenza Virus in Patients with Acute Influenza Infection. Clin Infect Dis. 2009 Jul 15;49(2):275–7.3. Cowling BJ, Ali ST, Ng TWY, Tsang TK, Li JCM, Fong MW, et al. Impact assessment of non-pharmaceutical interventions against COVID-19 and influenza in Hong Kong: an observational study. medRxiv. 2020 Mar 16;2020.03.12.20034660.4. Sokal RR, Rohlf FJ. Biometry : the principles and practice of statistics in biological research. 4th ed. New York: WH Freeman and Company; 2013. 937 p. 5. CDC. Recommendation Regarding the Use of Cloth Face Coverings, Especially in Areas of Significant Community-Based Transmission [Internet]. Centers for Disease Control and Prevention. 2020 [cited 2020 Apr 5]. Available from: https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/cloth-face-cover.html Sung-Han Kim, MD.Asan Medical Center15 April 2020 Author's response to the comments Thank you for your thoughtful comments on the concept of the strong ability of airborne transmission of SARS-CoV-2 (Dr. Shu Yuan). So, they commented the possibility of the environmental air contamination before the patients wore the masks. I also agree with that environmental air contamination may result in outer surface contamination of masks and petri dish in front of the patients. Initially, we did not concern about air contamination by coughing without mask, like the previous study (Clin Infect Dis 2009; 49:275-7). So, we performed testing sequence as coughing without mask first. But, it is possible that initial coughing without masks might contaminate the surrounding air, given that NEJM paper demonstrated air stability of SARS-CoV-2. However, negative pressure room where the patients stayed had more than 12 air change per hour, so theoretically 99% of particles is cleared within 23 min. In addition, we used small petri dish, so it is unlikely that aerosol landed on this area of small petri dish during the testing with subsequent mask changes. Actually, we performed air sampling before this experiment to investigate the aerosol transmission in the patients’ room. We had collected about 1,000 L air for 20 min by air sampler (Sartorius) like our previous study in MERS infected patients’ room (Clin Infect Dis 2016; 63:363-9). We can found a few positive PCR results from air sampling, although we collected air sampling without active coughing (unpublished data). Instead, we assume that fine aerosols leaked from the masks may contaminated the outer surface of the masks. In addition, we hypothesized the spit without virus particle might be deposited in the inner surface of the mask like Dr. Hoehn’