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

This page is specific to the COVID-19 pandemic. Our covid protocol still focuses on somewhat older science relating to droplet transmission. We are currently reviewing this. 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. 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.
3. Other infectious disease experts, such as George Fu Gao of the Chinese CDC, 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, may also be spread by people who never show symptoms, though the frequency of this is less clear, and that presymptomatic/asymptomatic people carry an equally high viral load. Studies arguing for presymptomatic transmission estimate that these form a significant minority of all cases, not just a few anomalous individual reports.
5. The exact mechanism of presymptomatic/asymptomatic transmission is unproven, but the COVID-19 virus is known to be carried in saliva.
6. Surgical masks reduce the ability of the wearer to transmit other seasonal coronaviruses, both by droplets and by aerosols.
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. However, other measures such as respiratory etiquette and hand hygiene are equally poorly evidenced as a public health response to respiratory infections, 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. 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.
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.
2. In real-life household settings, individuals who use masks as instructed do appear to receive some protective benefit from respiratory viruses, 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.
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 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.

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. 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 and remain in the air for longer 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. The addition of an extra filtration layer, such as lightweight interfacing fabric, may further improve a cloth masks's ability to block particles.
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.
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.
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, 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.