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How effective are masks?

Many Alaskans have expressed an interest in reading the scientific studies that support of the use masks to reduce transmission of COVID-19. Here are some of the key studies: 

Leffler, C.T., Edsel, I., Lykins, J.D., Hogan, M.C., McKneown, C.A., & Grzybowski, A. “Association of country-wide coronavirus mortality with demographics, testing, lockdowns, and public wearing of masks (Update July 2, 2020).” 

Key Findings:

  • An analysis across all of the continents was conducted reviewing the number of days after onset of the preliminary infections in countries occurred, and reported mask initiation by the public occurred. Additionally, the study reviews trends per days since onset of reported infection in the country, taking into account government mandates and/or recommendations regarding masks. The authors then ran models in consideration of the different risk factors in each country.·
  • Countries that had early mask usage (less than 30 days from the onset of an outbreak in that country) experienced per-capita mortality several orders of magnitude less than countries that did not.
  • The mortality was almost undetectable in countries that recommended masks within 15 days of the onset of an outbreak in their country.
  • For each week of the outbreak without masks, a 38.5% increase in per-capita mortality occurred. Mortality decreased by 4.6% for every week the country wore masks. This was based on evaluating per-capita mortality based on five days before the first case (average time it takes for someone to demonstrate COVID-19 symptoms).
  • In non-mask recommending countries, for every week that went by, per-capita mortality increased by 55%. In contrast, for mask recommending countries, per-capita mortality just increased by 7.2% per week. These numbers were based on running a model to compare country mask usage versus country non-mask usage and time, while taking into account other statistically significant risk factors in countries such as “time in lockdown”, “urbanization”, “obesity prevalence”, and population older than 60 years old.
  • In their models, the authors found that what generally explained higher mortality rates were: duration of the outbreak, duration of mask wear, proportion or the population age 60 or over, urbanization, and obesity prevalence. Countries that recommended mask wearing had a remarkably lower rate of per-capita mortality in their models than those that did not have mask wearing recommendations.

Chi-Chung Cheng, V. & Yuen, K. (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.” Journal of Infection.

Key findings:

  • 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.
  • Community-wide mask wearing may contribute to the control of COVID-19, particularly by controlling droplet spread from individuals with subclinical or mild COVID-19.

Lyu, W. & Wehby, G.L. (June 2020). “Community use of face masks and COVID-19: evidence from a natural experiment of state mandates in the US.” Health Affairs.

Key findings:

  • Between April 8 and May 15, 15 U.S. states plus the District of Columbia (DC) mandated face masks; 20 additional states have employee-only mandates requiring some employees wear a face mask at work. States that mandate their residents to wear facial cloth coverings experience statistically significant declines in their case rates for every five days the mandate is in place. 
  • States that only mandated employees wear masks did not experience statistically significant differences in case rates compared to those that did not have any mask mandates.

Leung, N.H.L., Chu, D.K.W., Shiu, E.Y.C. et al. (April 2020). Respiratory virus shedding in exhaled breath and efficacy of face masks. Nature Medicine.
Key Findings:

  • 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.
  • Surgical face masks could prevent transmission of human coronaviruses and influenza viruses from symptomatic individuals.

Davies A, Thompson KA, Giri K, Kafatos G, Walker J, Bennett A. (August 2013) Testing the efficacy of homemade masks: would they protect in an influenza pandemic? Disaster Med Public Health Prep.

Key Findings:

  • The median-fit factor of the homemade masks was one-half that of the surgical masks. Both masks significantly reduced the number of microorganisms expelled by volunteers, although the surgical mask was 3 times more effective in blocking transmission than the homemade mask.
  • A homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection.

Green CF, Davidson CS, Panlilio AL, et al. (May 2021) Effectiveness of selected surgical masks in arresting vegetative cells and endospores when worn by simulated contagious patients. Infection Control & Hospital Epidemiology
Key Findings:

National Academies of Sciences, Engineering, and Medicine. (April 2020). Rapid Expert Consultation on the Possibility of Bioaerosol Spread of SARS-CoV-2 for the COVID-19 Pandemic (April 1, 2020). Washington, DC: The National Academies Press.

Key Findings:

  • SARS-CoV-2 could be spread via bioaerosols generated directly by patients’ exhalation.
  • While the current SARS-CoV-2 specific research is limited, the results of available studies are consistent with aerosolization of virus from normal breathing.

Zhang, R., Li, Y., Zhang, A., Wang, Y., & Molina, M. (June 2020). “Identifying airborne transmission as the dominant route for the spread of COVID-19.”  Proceedings of the National Academy of Sciences of the United States of America.

Key findings:

  • COVID-19 transmits mostly via airborne respiratory droplets (someone inhales the virus when the infected person exhales)
  • COVID-19 has a higher likelihood of transmitting indoors, although COVID-19 can also transmit in the outdoors under certain conditions.
  • It’s best to wear a mask and keep 6 feet from others.
  • Masks may have reduced the number of infections by over 78,000 in Italy from April 6 to May 9 and by over 66,000 in NYC from April 17 to May 9

Chu, D.K., Akl, E., Duda, S., Solo, K., Yaacoub, S., & Schunemann, H. (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.

Key findings:

  • Physical distancing results in a large reduction of infection; 2 meters is more effective than 1 meter.
  • Wearing face masks protects both healthcare workers and the general public from infection. Eye protection can also help.

Royal Society DELVE Initiative. (May 2020). “Face masks for general public”

Key findings:

  • As many as 40-80% of infections occur from individuals without symptoms.
  • Respiratory droplets from coughing, sneezing, talking and breathing from infected individuals are a major mode of transmission. Viral loads are highest in the presymptomatic and early symptomatic phase, decreasing later in the illness.
  • Face masks reduce droplet dispersal. Cloth-based face masks reduce emission of particles by variable amounts, but all masks studied provide at least some filtering of particles.
  • Of individuals who do become symptomatic, viral loads are the highest in the presymptomatic and early symptomatic phase, decreasing thereafter.

Oran, D., & Topol, E. (June 2020). “Prevalence of asymptomatic SARS-COV-2 infection: a narrative review.” Annals of Internal Medicine.

Key findings:

  • Asymptomatic persons seem to account for approximately 40% to 45% of SARS-CoV-2 infections.
  • Asymptomatic persons  can transmit the virus to others for an extended period, perhaps longer than 14 days.

Kronbichler, A., Kresse, D., Yoon, S. et al. (June 2020). “Asymptomatic patients as a source of COVID-19 infections: a systematic review and meta-analysis.” International Journal of Infectious Diseases.

Key findings:

  • Asymptomatic patients with COVID-19 tend to be younger and may be more socially active. Younger patients who were asymptomatic tended to show normal radiology results but abnormalities were noted in 62.2% of asymptomatic patients.

Anfinrud P, Stadnytskyi V, Bax CE, Bax A. (May 2020). Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering. New England Journl of Medicine J Med. 2020.

Key Findings:

  • The act of speaking generates oral fluid droplets that vary widely in size; these droplets can harbor infectious virus particles.
  • Large droplets fall quickly to the ground, while small droplets can linger in the air, where they behave like an aerosol and travel farther.
  • A damp washcloth over the speaker’s mouth showed a decrease in the number of forward-moving droplets.

Konda A, Prakash A, Moss GA, Schmoldt M, Grant GD, Guha S. (April 2020). Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. ACS Nano.
Key findings:

  • Fabrics with tight weaves and low porosity, such as high thread cotton sheets, are preferable. Fabrics that are porous should be avoided for masks.
  • Some materials, such as silk and flannel, can likely provide good electrostatic filtering of particles.
  • Combining layers to provide hybrid masks may be an effective approach.
  • Mask leakage from improper fit can degrade efficiencies by 50% or more.

Aydin O, Emon B, Saif MTA. (April 2020). Performance of fabrics for home-made masks against spread of respiratory infection through droplets: a quantitative mechanistic study. MedRxiv. 2020.
Key findings:

  • The study examined the performance of ten different fabrics, ranging from cotton to silk, in blocking high velocity droplets, using a 3-layered commercial medical mask as a benchmark material.
  • The study also assessed their breathability and ability to soak water. We reason that the materials should be as breathable as possible, without compromising blocking efficiency, to reduce air flow through the sides of the mask since such flow would defeat the purpose of the mask. The study found that most home fabrics substantially blocked droplets, even as a single layer. With two layers, blocking performance can reach that of surgical mask without significantly compromising breathability.

Ma QX, Shan H, Zhang HL, Li GM, Yang RM, Chen JM. (March 2020). Potential utilities of mask-wearing and instant hand hygiene for fighting SARS-CoV-2. J Med Virol. 2020.

Key findings:

  • 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.
  • Mask‐wearing plus instant hand hygiene (MIH) to slow the exponential spread of the virus.

Other related studies and resources

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