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Bradford Hill criteria

  1. Strength (effect size): A small association does not mean that there is not a causal effect, though the larger the association, the more likely that it is causal.
  2. Consistency (reproducibility): Consistent findings observed by different persons in different places with different samples strengthens the likelihood of an effect.
  3. Specificity: Causation is likely if there is a very specific population at a specific site and disease with no other likely explanation. The more specific an association between a factor and an effect is, the bigger the probability of a causal relationship.[1]
  4. Temporality: The effect has to occur after the cause (and if there is an expected delay between the cause and expected effect, then the effect must occur after that delay).
  5. Intervention gradient (dose-response relationship): Greater exposure should generally lead to greater incidence of the effect. However, in some cases, the mere presence of the factor can trigger the effect. In other cases, an inverse proportion is observed: greater exposure leads to lower incidence.[1]
  6. Plausibility: A plausible mechanism between cause and effect is helpful (but Hill noted that knowledge of the mechanism is limited by current knowledge).
  7. Coherence: Coherence between epidemiological and laboratory findings increases the likelihood of an effect. However, Hill noted that "... lack of such [laboratory] evidence cannot nullify the epidemiological effect on associations".
  8. Experiment: "Occasionally it is possible to appeal to experimental evidence".
  9. Analogy: The use of analogies or similarities between the observed association and any other associations.
  10. Some authors consider, also, Reversibility: If the cause is deleted then the effect should disappear as well.


  • Odds ration with face covering use 0.30 on a US navy ship. Strongest measure of those looked at.[2]
  • "Face mask use could result in a large reduction in risk of infection (n=2647; aOR 0·15, 95% CI 0·07 to 0·34, RD −14·3%, −15·9 to −10·7; low certainty)"[3]
  • "after a mask mandate was put in place, there was an average weekly reduction of 25-to-31 per cent in new COVID-19 cases"[4]


  • "In countries with cultural norms or government policies supporting public mask-wearing, per-capita coronavirus mortality increased on average by just 15.8% each week, as compared with 62.1% each week in remaining countries."[5]
  • In a systematic review of 29 trials of face mask use for viral infections by coronaviruses 28 showed a benefit or trend to benefit with one trial showing no difference. Upon meta analysis overall there was clear benefit.[3]


  • Perfect use of face masks in a very high risk situation resulted in very good prevention. "Among 139 clients exposed to two symptomatic hair stylists with confirmed COVID-19 while both the stylists and the clients wore face masks, no symptomatic secondary cases were reported; among 67 clients tested for SARS-CoV-2, all test results were negative."[6]


  • Instituting face mask requirements in public is temporally associated with a decrease in number of cases. "Mandating face mask use in public is associated with a decline in the daily COVID-19 growth rate by 0.9, 1.1, 1.4, 1.7, and 2.0 percentage points in 1–5, 6–10, 11–15, 16–20, and 21 or more days after state face mask orders were signed, respectively."[7]
  • "We find that, in the first few weeks after implementation, mask mandates are associated with a reduction of 25 percent in the weekly number of new COVID-19 cases."[8]
  • "The number of COVID-19 cases in Arizona stabilized and then decreased after sustained implementation and enforcement of statewide and locally enhanced mitigation measures, beginning approximately 2 weeks after implementation and enforcement of mask mandates and enhanced sanitations practices"[9]

Intervention gradient

  • Greater mask use results in greater population benefit "In the Hong Kong Special Administrative Region (HSKAR) where 96.6% of the general public wore face masks, the COVID-19 rate in HSKAR was significantly lower than in other countries with lower compliance of face mask usages."[10]


  • Makes physiological sense.[11]
  • "Surgical face masks significantly reduced detection of influenza virus RNA in respiratory droplets and coronavirus RNA in aerosols, with a trend toward reduced detection of coronavirus RNA in respiratory droplets."[12]
  • "COVID-19 transmits mostly via airborne respiratory droplets (someone inhales the virus when the infected person exhales)"[13]
  • "N95 masks, medical masks, and homemade masks made of four‐layer kitchen paper and one‐layer cloth could block 99.98%, 97.14%, and 95.15% of the virus in aerosols."[13]


  • Experimentation including that done by the lay public,[14] support the epidemiological benefit.


  • Cohort study "Face mask use by the primary case and family contacts before the primary case developed symptoms was 79% effective in reducing transmission (OR=0.21, 95% CI 0.06 to 0.79). Daily use of chlorine or ethanol based disinfectant in households was 77% effective (OR=0.23, 95% CI 0.07 to 0.84). Wearing a mask after illness onset of the primary case was not significantly protective."[15]


  • Evidence supports the use of masks for other respiratory viruses such as influenza.[16][17]
  • "Conclusion cloth face masks are a preventive measure with moderate efficacy in preventing the dissemination of respiratory infections caused by particles with the same size or smaller than those of SARS-CoV-2."[18]
  • "Indeed, surgical masks substantially reduce emissions of influenza and (common cold) coronaviruses in exhaled breath,7 and in one controlled experiment masks decreased transmission of tuberculosis from humans to guinea pigs by 50%."[19]


  • "Coronavirus cases rise in states with relaxed face mask policies"[20]

See also


  1. 1.0 1.1 Hill, Austin Bradford (1965). "The Environment and Disease: Association or Causation?". Proceedings of the Royal Society of Medicine. 58 (5): 295–300. doi:10.1177/003591576505800503. PMC 1898525. PMID 14283879.
  2. Payne, Daniel C. (2020). "SARS-CoV-2 Infections and Serologic Responses from a Sample of U.S. Navy Service Members — USS Theodore Roosevelt, April 2020". MMWR. Morbidity and Mortality Weekly Report. 69. doi:10.15585/mmwr.mm6923e4. ISSN 0149-2195.
  3. 3.0 3.1 Chu, Derek K; Akl, Elie A; Duda, Stephanie; Solo, Karla; Yaacoub, Sally; Schünemann, Holger J; Chu, Derek K; Akl, Elie A; El-harakeh, Amena; Bognanni, Antonio; Lotfi, Tamara; Loeb, Mark; Hajizadeh, Anisa; Bak, Anna; Izcovich, Ariel; Cuello-Garcia, Carlos A; Chen, Chen; Harris, David J; Borowiack, Ewa; Chamseddine, Fatimah; Schünemann, Finn; Morgano, Gian Paolo; Muti Schünemann, Giovanna E U; Chen, Guang; Zhao, Hong; Neumann, Ignacio; Chan, Jeffrey; Khabsa, Joanne; Hneiny, Layal; Harrison, Leila; Smith, Maureen; Rizk, Nesrine; Giorgi Rossi, Paolo; AbiHanna, Pierre; El-khoury, Rayane; Stalteri, Rosa; Baldeh, Tejan; Piggott, Thomas; Zhang, Yuan; Saad, Zahra; Khamis, Assem; Reinap, Marge; Duda, Stephanie; Solo, Karla; Yaacoub, Sally; Schünemann, Holger J (June 2020). "Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis". The Lancet. 395 (10242): 1973–1987. doi:10.1016/S0140-6736(20)31142-9.
  4. (PDF) Retrieved 3 November 2020. {{cite web}}: Missing or empty |title= (help)
  5. "Association of country-wide coronavirus mortality with demographics, testing, lockdowns, and public wearing of masks. Update August 4, 2020". doi:10.1101/2020.05.22.20109231v5. {{cite journal}}: Cite journal requires |journal= (help); |access-date= requires |url= (help)
  6. Hendrix, M. Joshua (2020). "Absence of Apparent Transmission of SARS-CoV-2 from Two Stylists After Exposure at a Hair Salon with a Universal Face Covering Policy — Springfield, Missouri, May 2020". MMWR. Morbidity and Mortality Weekly Report. 69. doi:10.15585/mmwr.mm6928e2. ISSN 0149-2195.
  7. Lyu, Wei; Wehby, George L. (1 August 2020). "Community Use Of Face Masks And COVID-19: Evidence From A Natural Experiment Of State Mandates In The US: Study examines impact on COVID-19 growth rates associated with state government mandates requiring face mask use in public". Health Affairs. 39 (8): 1419–1425. doi:10.1377/hlthaff.2020.00818.
  8. (PDF) Retrieved 3 November 2020. {{cite web}}: Missing or empty |title= (help)
  9. Gallaway, M. Shayne (2020). "Trends in COVID-19 Incidence After Implementation of Mitigation Measures — Arizona, January 22–August 7, 2020". MMWR. Morbidity and Mortality Weekly Report. 69. doi:10.15585/mmwr.mm6940e3. ISSN 0149-2195.
  10. Cheng, VC; Wong, SC; Chuang, VW; So, SY; Chen, JH; Sridhar, S; To, KK; Chan, JF; Hung, IF; Ho, PL; Yuen, KY (July 2020). "The role of community-wide wearing of face mask for control of coronavirus disease 2019 (COVID-19) epidemic due to SARS-CoV-2". The Journal of infection. 81 (1): 107–114. doi:10.1016/j.jinf.2020.04.024. PMID 32335167.
  11. [1]
  12. Leung, Nancy H. L.; Chu, Daniel K. W.; Shiu, Eunice Y. C.; Chan, Kwok-Hung; McDevitt, James J.; Hau, Benien J. P.; Yen, Hui-Ling; Li, Yuguo; Ip, Dennis K. M.; Peiris, J. S. Malik; Seto, Wing-Hong; Leung, Gabriel M.; Milton, Donald K.; Cowling, Benjamin J. (May 2020). "Respiratory virus shedding in exhaled breath and efficacy of face masks". Nature Medicine. 26 (5): 676–680. doi:10.1038/s41591-020-0843-2. ISSN 1546-170X.
  13. 13.0 13.1 "COVID-19: How effective are masks?". Retrieved 31 August 2020.
  14. [2]
  15. Wang, Y; Tian, H; Zhang, L; Zhang, M; Guo, D; Wu, W; Zhang, X; Kan, GL; Jia, L; Huo, D; Liu, B; Wang, X; Sun, Y; Wang, Q; Yang, P; MacIntyre, CR (May 2020). "Reduction of secondary transmission of SARS-CoV-2 in households by face mask use, disinfection and social distancing: a cohort study in Beijing, China". BMJ global health. 5 (5). doi:10.1136/bmjgh-2020-002794. PMID 32467353.
  16. Cowling, BJ; Zhou, Y; Ip, DK; Leung, GM; Aiello, AE (April 2010). "Face masks to prevent transmission of influenza virus: a systematic review". Epidemiology and infection. 138 (4): 449–56. doi:10.1017/S0950268809991658. PMID 20092668.
  17. MacIntyre, CR; Cauchemez, S; Dwyer, DE; Seale, H; Cheung, P; Browne, G; Fasher, M; Wood, J; Gao, Z; Booy, R; Ferguson, N (February 2009). "Face mask use and control of respiratory virus transmission in households". Emerging infectious diseases. 15 (2): 233–41. doi:10.3201/eid1502.081167. PMID 19193267.
  18. Lima, MMS; Cavalcante, FML; Macêdo, TS; Galindo Neto, NM; Caetano, JÁ; Barros, LM (2020). "Cloth face masks to prevent Covid-19 and other respiratory infections". Revista latino-americana de enfermagem. 28: e3353. doi:10.1590/1518-8345.4537.3353. PMID 32785565.
  19. Javid, B; Weekes, MP; Matheson, NJ (9 April 2020). "Covid-19: should the public wear face masks?". BMJ (Clinical research ed.). 369: m1442. doi:10.1136/bmj.m1442. PMID 32273278.
  20. "Coronavirus cases rise in states with relaxed face mask policies". Retrieved 31 August 2020. {{cite news}}: External link in |work= (help)

Further reading

  • Clinical Wiki[3]