Principles and assumptions for using masks during the COVID-19 pandemic

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This page is specific to the COVID-19 pandemic. It will be updated rapidly.

Principles for masks in general

  1. There is currently a severe shortage of masks in the UK, and thus a need to prioritise these for (formal or informal) healthcare workers
  2. This pragmatic question of availability is distinct from the question of whether the wider public use of masks would be valuable.
  3. There is no evidence available to support claims that masks may be actively harmful (for example, by leading to increased face-touching).

Masks to prevent the wearer from transmitting COVID-19

  1. At the time of publishing, WHO guidance recognises the benefit of mask-wearing during home care of a person with symptoms of COVID-19, both to prevent the infected person from spreading disease to others, and to protect the person caring for them.[1] However, it does not endorse mass mask-wearing in other circumstances.
  2. This position appears to stem from the fact that they do not currently recognise presymptomatic or asymptomatic transmission of COVID-19 as a significant factor, but characterise the evidence for this as a 'small number of reports'. By contrast, the WHO does cautiously endorse mass public mask-wearing as a strategy for severe influenza pandemics, where presymptomatic and asymptomatic transmission is recognised as a concern.[2]
  3. Other infectious disease experts, such as George Fu Gao of the Chinese CDC[3], have long taken the position that masks should be worn by asymptomatic people in public/close-contact situations during the COVID-19 epidemic.
  4. There is evidence that the virus can be spread before symptoms develop[4][5], may also be spread by people who never show symptoms, though the frequency of this is less clear[6], and that presymptomatic/asymptomatic people carry an equally high viral load[7][8]. Studies arguing for presymptomatic transmission estimate that these form a significant minority of all cases, not just a few anomalous individual reports.[9]
  5. The exact mechanism of presymptomatic/asymptomatic transmission is unproven, but the COVID-19 virus is known to be carried in saliva.[10]
  6. Surgical masks reduce the ability of the wearer to transmit other seasonal coronaviruses, both by droplets and by aerosols.[11]
  7. At the time of writing, Public Health England currently rejects public mask-wearing, even when caring for people who are infected, on grounds that there is ‘very little evidence' of masks providing any benefit outside of clinical settings[12]. However, other measures such as respiratory etiquette and hand hygiene are equally poorly evidenced as a public health response to respiratory infections[13], but are recommended because there is mechanistic plausibility for their effectiveness.
  8. Some studies do show that compliance with mask-wearing is often low, making it ineffective as an intervention. However, these studies focus on ordinary seasonal influenza cases, and participants were required to wear masks in their own households.[14] They may not reflect people's willingness to wear masks for shorter periods of time while in crowded public spaces during a lethal pandemic.
  9. Any individual infection-control measure can create a false sense of security if not presented as part of a package, and masks are no different in this respect.
  10. Concerns about inappropriate mask usage could be addressed through a public education campaign, as has been done with other interventions such as handwashing.[15]
  11. Masks are not an alternative to social distancing, but they do reduce the ability of the wearer to transmit infection in situations where distancing cannot be maintained. They may therefore have value as source control when worn by asymptomatic people in public or shared spaces, if these spaces cannot be avoided.
  12. They should also be worn by people who do have symptoms of COVID-19, during unavoidable contact with others, unless this causes breathing difficulties.

Masks to protect the wearer from COVID-19

  1. Unlike respirator masks, surgical masks are not designed to protect the wearer, but laboratory tests suggest they may also have some benefit to partially reduce the risk of infection from respiratory viruses in situations where close contact with an infected person cannot be avoided.[16]
  2. In real-life household settings, individuals who use masks as instructed do appear to receive some protective benefit from respiratory viruses[17], though this is difficult to study in isolation from other factors (e.g. people who comply with appropriate mask usage may also be more likely to use appropriate hand hygiene).
  3. The protective benefit of surgical masks may be increased by modifications to improve the fit.[18]
  4. This does not provide complete protection and must never be seen as a reason to disregard social distancing measures.
  5. However, masks may have some value to protect the wearer while caring for someone who is infected, or for use by people who are at high risk but cannot avoid situations where they may be exposed to infection.

Principles specific to cloth masks

  1. Some public health authorities have claimed that cloth masks may increase the risk of infection [19] but evidence cited for this comes from one study which does not, in fact, provide any evidence for whether appropriately-used cloth masks are beneficial or harmful if there is no better alternative.[20]

Masks to prevent the wearer from transmitting COVID-19

  1. Researchers concluded that a tightly-woven cotton mask was better than nothing as a means of preventing the wearer from emitting infectious droplets, although surgical masks are preferable.[21] This study aimed to replicate the conditions of community cloth mask usage, by asking volunteers to construct their own masks.
  2. In order to preserve the limited supplies of surgical masks, cloth could therefore have value for general community use by asymptomatic people as source control, to reduce the spread of infection to others.
  3. While people without symptoms can still transmit COVID-19, symptomatic people are likely to produce a larger number of infectious droplets (e.g. while coughing), and these droplets may travel longer distances[22] and remain in the air for longer[23] when expelled by coughs or sneezes.
  4. Since surgical masks are more effective than cloth, they should therefore be prioritised for use by people with symptoms of the virus.

Masks to protect the wearer from COVID-19

  1. Tightly woven cotton cloth does have some ability to block even fine aerosol particles, though only about 50% of these[21]. The addition of an extra filtration layer, such as lightweight interfacing fabric, may further improve a cloth masks's ability to block particles.[24]
  2. The fit of a mask will significantly reduce its filtration abilities, but another study suggested that a cotton teatowel mask (with no added filtration layer) would still provide a better-than-nothing level of protection to its wearer, though only half as good as a surgical mask.[25]
  3. Another recent study (not yet peer-reviewed) suggests that the protective abilities of cloth masks may be significantly improved by overlaying them with a band of nylon stocking fabric, thus holding them closer to the face.[24]
  4. Surgical masks, similarly adapted, are still superior. They should be used whenever available by people caring for infected individuals, or people at high risk who need to protect themselves from infection.
  5. However, cloth masks appear to be better than nothing if the surgical masks are unavailable.
  6. There are some suggestions that a higher level of exposure to the virus may result in a more severe illness[26], though at the time of writing this is still uncertain
  7. Even when cloth masks are unable to fully protect against infection, it’s therefore still possible that they may have some value by reducing the initial infectious dose.
  1. World Health Organisation 'When and how to use masks'; April 2020
  2. WHO. Non-pharmaceutical public health measures for mitigating the risk and impact of epidemic and pandemic influenza. October 2019
  3. Jon Cohen. Not wearing masks to protect against coronavirus is a ‘big mistake,’ top Chinese scientist says. Science Mag, March 2020
  4. Du et al, 'Serial Interval of COVID-19 among Publicly Reported Confirmed Cases'. Emerging Infectious Diseases, June 2020 (early release, accessed 2020-04-27)
  5. He et al, 'Temporal dynamics in viral shedding and transmissibility of COVID-19', Nature Medicine April 2020
  6. Bai et al, 'Presumed Asymptomatic Carrier Transmission of COVID-19', JAMA February 2020
  7. Kimbal et al. Asymptomatic and Presymptomatic SARS-CoV-2 Infections in Residents of a Long-Term Care Skilled Nursing Facility — King County, Washington, March 2020. MMWR Morbidity Mortal Weekly Report, 2020.
  8. Zou et al. SARS-CoV-2 Viral Load in Upper Respiratory Specimens of Infected Patients. New England Journal of Medicine, 2020.
  9. He et al[1] point out that the exact percentage will vary according to each locality's public health measures - if strict steps are taken to isolate symptomatic individuals, the total of cases will be lower, but the percentage transmitted presymptomatically will be greater
  10. To et al, 'Consistent Detection of 2019 Novel Coronavirus in Saliva'; Clinical Infectious Diseases, February 2020
  11. Leung et al, 'Respiratory virus shedding in exhaled breath and efficacy of face masks', Nature Medicine April 2020
  12. Public Health England. Stay at home: guidance for households with possible coronavirus (COVID-19) infection, April 2020.
  13. WHO. Non-pharmaceutical public health measures for mitigating the risk and impact of epidemic and pandemic influenza. 2019.
  14. MacIntyre et al, 'Facemasks for the prevention of infection in healthcare and community settings', BMJ April 2009
  15. Chan et al, 'To wear or not to wear? WHO's confusing guidance on masks in the covid-19 pandemic'; March 2020
  16. Booth et al, 'Effectiveness of surgical masks against influenza bioaerosols' Journal of Hospital Infection May 2013
  17. Mueller et al, 'Assessment of Fabric Masks as Alternatives to Standard Surgical Masks in Terms of Particle Filtration Efficiency'; (preprint, April 2020)
  18. ECDC. Cloth masks and mask sterilisation as options in case of shortage of surgical masks and respirators. March 2020
  19. MacIntyre et al[2]showed that cloth masks were inferior to surgical masks at protecting the wearer from rhinoviruses - they did not test the masks' ability to prevent the wearing from infecting others - but did not establish whether cloth masks were better or worse than nothing. The study contained a brief speculation by researchers that cloth masks might increase the risk of infection by retaining moisture, but did not test whether this was in fact the case. Additionally, healthcare workers in this study wore their masks throughout an 8 hour shift; even if cloth masks ultimately become detrimental as moisture spreads from the mouth, this does not mean that they have no benefit when worn for much shorter periords.
  20. 21.0 21.1 Davies et al, 'Testing the Efficacy of Homemade Masks', Disaster Medicine and Public Health Preparedness, August 2013
  21. Xie et al, 'How far droplets can move in indoor environments – revisiting the Wells evaporation–falling curve', International Journal of Indoor Environment and Health, May 2007
  22. Bourouiba et al, 'Violent expiratory events: on coughing and sneezing', Journal of Fluid Mechanics April 2014
  23. 24.0 24.1 Mueller et al 'Assessment of Fabric Masks as Alternatives to Standard Surgical Masks in Terms of Particle Filtration Efficiency'; Accessed: 2020-04-27
  24. van der Sande et al, 'Professional and Home-Made Face Masks Reduce Exposure to Respiratory Infections among the General Population'. PLoS One, 2008.
  25. Carl Heneghan, Jon Brassey, Tom Jefferson, 'SARS-CoV-2 viral load and the severity of COVID-19'. CEBM, 2020.