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expert reaction to review of evidence on face masks and face coverings by the Royal Society DELVE Initiative

The DELVE Initiative, from the Royal Society, has conducted a review of evidence surrounding the use of face masks and coverings.


Dr Ben Killingley, Consultant in Acute Medicine (Clinical Lead) and Infectious Diseases, University College London Hospital, said:

“Regarding the analysis of the evidence base – the report is overly optimistic about the value of face coverings and it is incorrect to conclude that the evidence shows that face covering can reduce viral transmission in the community.  There is in fact no good evidence that face coverings achieve this.  The report focuses largely on the mechanistic data of virus shedding and filtering efficiency.  In actual studies of the effectiveness of face coverings, the data are very under-whelming and show that they have low effectiveness outside of the healthcare setting.  This is likely because other modes of transmission, such as contaminated hands, are more important and because it is difficult to use face coverings correctly, particularly adherence to use.  Admittedly these studies are mostly of protection to the user, rather than protection of others, but the principles are likely the same and the authors have largely ignored this extensive real-world data.

“This is not to say that face coverings might not be helpful.  Face mask studies were not undertaken during a pandemic or in the setting of a novel virus.  Factors such as improved compliance with use and increased risk of pre-symptomatic transmission could conceivably lead to better trial outcomes.

“Nevertheless, the report, in my view, does not accurately represent the evidence on face coverings that currently exists.”


Prof William Keevil, Professor of Environmental Healthcare, University of Southampton, said:

“This new report from the Royal Society shows the difficulty when comparing different publications and adopting common definitions.  For example, one reference cited respiratory droplets being larger than 20 micron diameter and aerosols being less than that while other references describe the respiratory droplets as being greater than 5 or 10 micron diameter with aerosols being correspondingly smaller.  This definition is important because aerosols less than 5 microns in diameter penetrate deep into the alveolar sacs of the lung and can be taken up by alveolar macrophages therein with potential transmission to the lymph nodes.  There is some interesting debate in the literature about the concentration of the ACE2 receptor and other co-factors for COVID-19 virus attachment at different sites in the lung, including type II alveolar pneumocytes and alveolar macrophages, and the implications for virus infection via this route.  Various papers have shown breathing, coughing or sneezing produce a range of particle sizes from small aerosol to large respiratory particles, with emphasis on the latter, yet we know little about whether aerosols are more or less infectious for COVID-19 than respiratory droplets.  The Royal Society report acknowledges that while it is currently believed that droplets are the main route of transmission, supported mainly by indirect evidence, the relative contribution of droplet/aerosol transmission has not been estimated.

“The references comparing face mask use are also difficult to interpret because of confounding factors such as improved personal hygiene and other wearer behaviours affecting the statistical analysis, which partly explains why their use is considered to provide only modest benefit to the wearer.  Moreover, many of the studies investigating filtration efficiency rely on laboratory based static experiments where samples of fabrics are held in a device and air or simulated respiratory droplets and aerosols containing bacteria and viruses are pushed through to be collected and quantified.  A study from a US filtration company compared different fabrics in a static test and concluded that loosely woven woollen scarves provided poor filtration ability while the best materials were close woven cotton fabrics such as canvas, denim and old-fashioned teacloths.  Whether these should be made into home made masks with problems of choosing correct size and shape for the wearer, and getting a good fit, or used more like a scarf surrounding the neck as well as the mouth, is open to debate.

“The concern with facemasks has always been fitting them properly to the face without loose gaps around the edges which lead to leakage.  In a paper published in British Medical Journal in 2004, Granville-Chapman and Dunne described how when a surgeon sneezed while wearing a properly fitting, professional surgical face mask the respiratory droplets and aerosol passed out of the sides but particularly under the chin onto the wearer’s chest and clothing; not surprising since air follows the line of least resistance. Imagine this repeated with poor quality, loose fitting face masks.  Perhaps this simple expedient of wearing any kind of mask is sufficient to prevent long distance travel of respiratory droplet or aerosols and may provide some utility in crowded environments such as train stations, trains and buses where it will be difficult to maintain the recommended 2 metre gap?  Some studies suggest that a combination of wearing a mask, washing hands and maintaining social distancing can provide some protection.

“If wearing a mask becomes common then the wearer must recognise that they must not keep touching the mask (because if it is loose fitting then it moves around or irritates the skin).  Furthermore, professional surgical masks and respirators are single use items and typically not washable for reuse, while people may be tempted to reuse them because of the cost.  If home-made fabric masks are worn they may become damp because of breath moisture, respiratory droplet and aerosol production, and virus in asymptomatic shedders; masks must be carefully removed when entering the home and placed in the washing machine, for a 60oC cycle with detergent, and the hands washed immediately.

“If Governments are going to endorse wearing masks then there must be an immediate education program to show the general public how to choose the best quality masks or fabrics available, fit them correctly, wear them for a recommended length of time in a crowded environment and safely dispose of the mask, followed by washing hands.  Ideally, eye protection should also be considered.  Collateral damage may include making it more difficult for front line staff and emergency responders to obtain the professional masks they really need for their hazardous work.

“People should not become complacent about their hygiene because they or everyone else are wearing a mask or face cover; they will not become invincible to the virus and must continue to practice rigorous, regular hand washing, cleaning their outer garments and keeping the 2-metre gap wherever possible.”


Dr Antonio Lazzarino, Department of Epidemiology and Public Health, UCL, said:

“That is not a piece or research. That is a non-systematic review of anecdotical and non-clinical studies. The evidence we need before we implement public interventions involving billions of people, must come ideally from randomised controlled trials at population level or at least from observational follow-up studies with comparison groups. This will allow us to quantify the positive and negative effects of wearing masks. Based on what we now know about the dynamics of transmission and the pathophysiology of covid-19, the negative effects of wearing masks outweigh the positive.”


Dr Simon Clarke, Associate Professor in Cellular Microbiology, University of Reading, said:

“This report from the Royal Society, while interesting, represents a set of valid opinions rather than any conceptual advance in our understanding of the usefulness of face-masks or coverings.  It falls short of delivering new evidence and too casually dismisses precautionary principle when addressing the possibility that masks and coverings could have negative effects on people’s behaviours.  Until more evidence is delivered in either direction, that’s all advice can be based on – opinions.”


Prof Babak Javid, Principal Investigator, Tsinghua University School of Medicine, Beijing, and Consultant in infectious diseases at Cambridge University Hospitals, said:

“This is a welcome and important report from the Royal Society’s DELVE initiative, comprised of some of the most senior and prestigious scientists in the UK.  It recommends that the use of facemasks “could offer an important tool for contributing to the management of community transmission of Covid19“.  In particular, they highlight the potential contribution of transmission of Covid from people with no symptoms.  Specifically, they note that masks for the general population have a role in “source control”, i.e. preventing onward transmission by those infected (and who may not even know they are infected), and that cloth masks are fairly effective in this context, although they are inferior to medical masks for personal protection.  They also examine a widely held objection to mask use: that it may contribute to risky behaviours in the wearer, and find no evidence supporting this claim.

“Important considerations are that if masks/face coverings are to be mandated, how will access be provided, especially to more economically vulnerable populations.  Furthermore, for masks to be effective in source control, the majority of the population would need to comply.  Countries with no recent history of widespread mask use, such as Italy, have shown widespread uptake of masks is possible.  The group also acknowledges a role for public education campaigns, similar to those currently being employed for handwashing and physical distancing.”


Prof Trish Greenhalgh, Professor of Primary Care Health Sciences, Nuffield Department of Primary Care Health Sciences, University of Oxford, said:

“The publication from DELVE on face masks for the general public is welcome.  It draws together a wealth of evidence including the science of transmission and the efficacy of cloth-based materials both in laboratory studies and in randomised controlled trials, in relation to both source control and protection of the wearer.  The authors flag the important finding that much if not most transmission of Sars-CoV-2 happens from people who do not currently have symptoms.  As would be expected from this distinguished group of scientists, the underlying evidence base is meticulously dissected and presented, including pointing out areas where more research is needed (for example, on the question of whether some people might continue to spread the virus without ever developing symptoms, whether recommendations on mask wearing should vary with the risk of exposure – for example bus drivers versus dog walkers – and how important it is for the mask to be a snug fit).  The report includes very recent research studies which previous, more critical reports did not include.  It lends weight to the argument that face coverings by the lay public could be a key strategy to help the country ease out of lockdown.  One issue not covered by the report is the danger that widespread use of masks by the public could deplete stocks of medical-grade masks for healthcare workers.  Medical-grade masks are not needed outside of healthcare settings.  A cloth mask provides very good protection, and is more comfortable to wear.”


All our previous output on this subject can be seen at this weblink:


Declared interests

Prof Babak Javid: “I am the lead author of one of the papers cited by the authors of the report in support of their recommendation.”

None others received.

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