Effective use of face masks for COVID-19 (15/36)

Talk by Ben Cowling

Co-Director of WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, The University of Hong Kong, Hong Kong


  • Face masks are part of essential personal protective equipment for healthcare workers
  • Community use of face masks has been more controversial, many health authorities initially recommending against face mask use by the general public, perhaps mainly because of supply shortages
  • Mask used should do something, but limited evidence base on how much masks could reduce transmission in the community
  • Some speculation (without any supporting evidence) that face mask use might increase risk of infection, or lead to false sense of security

We conducted a meta-analysics review of 10 randomized controlled trials on influenza. There was no evidence that face masks are effective in reducing transmission of laboratory confirmed influenza. There was some evidence of a limted benefit of hand hygiene and face masks for confirmed influenza with point estimate 10-20% reduction from universal masking and enhanced hygiene.

Based on this evidence, WHO was advised to update guidelines to recommend universal masking in a pandemic. There is also mechanistic data supporting face masks and there is data indicating even a 10-20% discount is worth having. Without face masks, we need to look at other measures that can reduce transmission even more and the other measures would probably be more disruptive to society. [Our comment: sounds like he will be thrilled when he hears about the Reflow Labs Airsafe!]

A review in Lancet on face masks against SARS and MERS showed strong effects of face masks in health care settings, but data for their use in the community is limited. This review also showed that eye protection could reduce the risk of SARS/MERS/COVID by 75%, which is a phenomenal effect.

A study was conducted in Hong Kong collected exhalations with Don Milton's machine to evaluate the efficacy of masks for source control.

Don Milton's machine evaluates the aerosols from exhalations

246 patients with various respiratory illnesses had their exhaled breath collected for 30 minutes. The particles are split into coarse (>5 microns) and fine (<5 microns). They were randomly allocated to wear or not wear a surgical mask, with some patients agreeing to stay for an hour and do both. Since the machine actively pulls air in, the portion of the exhalation that leaks around the sides of the masks can also be captured.

The presence of influenza virus was detected in nasal and throat swabs. There was a significant difference when wearing a mask in coarse particles, but not fine aerosols. However, a larger study by Milton was able to find significant differences with fine aerosols as well.


When the same study was conducted with a small number of patients with coronaviruses, there was a marginal reduction (p=0.07) in the large droplets and a significant difference in the aerosols (p=0.02).

There is also evidence for masks protecting the wearer.

  • One study showed 50% protection with homemade cloth masks
  • 60% protection with tea cloth masks
  • 76% protection with a surgical mask and 99% FFP2 (for total ambient particles >0.5 microns)

Key challenges with masks: filtering efficiency, minimizing leakage, leakage increase from prolonged use, and the inability to wear masks 24/7.

In Hong Kong where mask compliance in the community exceeds 99% there were still two epidemics of COVID-19, however, these occured in places that masks are not worn (e.g. bars, restaurants, gyms, elderly homes, or worker dormitories). In these instances social distancing was needed and additional public health measures are needed. The outcomes would have most likely been worse without masks.

There is a trial underway now in Denmark with 6000 adults randomized to wear or not wear masks in the community that is powered to identify a 50% reduction in risk of COVID-19 (from 2% to 1%), but such a strong effect is unlikely based on previous literature. We need these sorts of studies, but a 10-20% reduction is more likely. This study may be problematic since a failure to achieve 50% reduction may lead people to the erroneous conclusion that this implies that masks don't work.


  • Limited evidence base for the effectiveness of face masks in the community for influenza epidemics and pandemics, but data are consistent with a 10% to 20% reduction in transmission.
  • Mechanistic evidence that face masks can provide source control of virus-laden droplets and aerosols
  • Mechanistic evidence that face masks can provide protection for the wearer
  • Fallacious to argue "masks don't have 100% effect in stopping transmission therefore masks are useless". A 10% reduction in transmission would be valuable!
  • However, widespread use of face masks in Hong Kong has been insufficient to stop two community epidemics. Both epidemics were controlled after the implementation of moderate social distancing measures.
  • We need to bring R0 from a level of 2-3 to below through a combination of different public health measures.

Previous: Impact of environmental conditions on the infectivity of SARS-CoV-2 in aerosols (14/36)

Next: Which size particles are infectious and for how long? CQ2 Panel (16/36)

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See the key FAQ pages provided by a team of scientists on COVID-19 transmission and preventive approaches:

Benjamin John Cowling is currently Professor and head of the Division of Epidemiology and Biostatistics in the School of Public Health at the University of Hong Kong, and co-Director of the WHO Collaborating Centre for Infectious Disease Epidemiology and Control. He is Editorin-Chief of Influenza and Other Respiratory Viruses, and an Associate Editor of the journal Emerging Infectious Diseases. He conducts research into the epidemiology of influenza and other respiratory viruses. His research team has characterized how easily seasonal and pandemic influenza viruses can spread in households, and the effectiveness of measures to reduce the risk of infection and transmission including vaccination and non-pharmaceutical interventions. His recent research has focused on vaccination strategies in older adults, and immune responses to repeated influenza vaccination. Since early 2020 he has conducted research on the epidemiology and control of COVID-19 including a series of highly cited publications in NEJM, Science and Nature Medicine. He has authored more than 450 peer-reviewed journal publications to date. His work is supported by grants from a number of funding agencies, including a large co-operative agreement with the US CDC on “Research on the Epidemiology, Vaccine Effectiveness and Treatment of Influenza and Other Respiratory Viruses in Southeast Asia and the Western Pacific” (US$10m, 2016-21), and a Theme-Based Research Scheme grant from the University Grants Committee in Hong Kong entitled “Control of Influenza: Individual and Population Immunity” (US$6m, 2020-25). After graduating with a PhD in statistics from the University of Warwick, Dr. Cowling spent a year at Imperial College London before moving to the University of Hong Kong in 2004.

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