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Can face masks prevent the spread of COVID-19?

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High-grade face masks are vital for health workers and other care providers. Face masks, if correctly worn, might also prevent the transmission of virus by people with symptoms of COVID-19, and protect people who are especially vulnerable to infection. Whether masks used by the public at large can slow the spread of COVID-19 remains unclear. Mass public use of masks could have adverse consequences if this limits supplies for health workers or if this leads to the neglect of other effective measures such as handwashing.

Because surgical masks are vital for healthcare workers,[1] it is natural for the public to assume that they will be of wider benefit in preventing COVID-19. WHO recommends that a household member who has symptoms of COVID-19 should wear a mask.[1] A person caring for people with symptoms should wear a mask whilst with them, with careful handwashing and disposal. WHO does not recommend the use of masks by healthy people in the community. Health authorities in the US, but not in the UK, are currently recommending that the public wear masks for protection.

Masks to stop coronavirus

COVID-19 is spread by the inhalation of coronavirus on droplets produced by coughs and sneezes, and probably in the fine-mist aerosols generated by breathing. Lightweight aerosols remain in the air for longer than heavier droplets and more easily pass through masks. Viruses are minute – having a width of about one thousandth that of a human hair – so fine filters are needed to exclude them. Virus can also be acquired by touching droplets deposited on surfaces, and masks may prevent hand to mouth infection.

Face masks range from simple cloth types, like a scarf, through masks for surgical use, right up to high-grade N95s which are filtering facepiece respirators (FFRs). Filter efficiency and proper fit (for instance, no gaps around the nose and mouth) are key attributes. FFRs are critical in protecting health workers such as anaesthetists intubating patients (inserting tubes into the airways) who could be exposed to high doses of virus.

Recent experimental trials have shown that surgical face masks prevent droplet and aerosol emission from people who had symptomatic seasonal coronavirus infection; the trials did not study the new coronavirus, cause of COVID-19.[2]

In practice, face masks tend to be used less efficiently than in laboratory experiments. If a mask is incorrectly worn, re-used day after day, or removed to eat or talk or because the wearer finds the mask uncomfortable, it will be less effective at trapping incoming or outgoing viruses.

Face masks are routinely worn by people in Asian countries and may have been effective in interrupting the transmission of influenza and coronavirus during the 2003 SARS outbreak.[3] The routine use of face masks has the potential to stop transmission from people who are infected but show no signs of illness, as well as from people who have symptoms. However, recent reviews of mask-use have found little evidence to suggest that masks should be worn routinely by the public at large.[4][5] It is important to understand, though, that studies which fail to show that masks are effective do not necessarily prove that they are ineffective. The results of some studies are inconclusive because participants did not follow instructions on how to wear face masks.[5]

Surgical masks are likely to prevent virus transmission by a symptomatic patient to another person in a healthcare setting.[4] There is some evidence that they can prevent the transmission of infection to and from people in households. There is evidence, too, that vulnerable people can be protected by masks worn for short periods when temporarily in high risk situations – such as on public transport, visiting shops or in healthcare settings.[5]

Cloth masks, especially those improvised from common household fabrics (T-shirts, sweat-shirts, tea towels), are relatively ineffective both for preventing virus transmission and for the protection of uninfected people.[6][7]

In short, the weight of evidence indicates that filtering facepiece respirators give more protection than surgical masks, and surgical masks give greater protection than home-made masks.[3] Home-made and cloth masks might be better than nothing, but they are inferior to higher-grade masks.

Unintended consequences

The mass public use of masks could have unintended consequences.[1][4] Wearing a mask may give people a false sense of security leading them to believe that they are fully protected and no longer need to observe other effective measures, such as hand washing, social distancing and home isolation. Poorly fitting masks may be adjusted frequently, increasing the likelihood of virus on the hands coming into contact with the face. On top of this, people with symptoms might believe that there is no danger of infecting others if they wear a low-quality mask.

Another concern is that the mass use of face masks would put greater strain on the supply chain for medical masks. Rising demand from the public could increase prices and impact supplies to hospitals where there is clear evidence of their benefit in preventing transmission and protecting healthcare workers.

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References

  1. Health Emergencies Preparedness and Response. Advice on the use of masks in the context of COVID-19. World Health Organization. 2020 Apr:WHO/2019-nCov/IPC_Masks/2020.3.

  2. Leung NHL, Chu DKW, Shiu EYC, et al. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nature Medicine. 2020 Apr. DOI: 10.1038/s41591-020-0843-2.

  3. van der Sande M, Teunis P, Sabel R. Professional and home-made face masks reduce exposure to respiratory infections among the general population. Plos One. 2008 ;3(7):e2618. DOI: 10.1371/journal.pone.0002618.

  4. Brosseau LM, Sietsema M. Masks-for-all for COVID-19 not based on sound data. CIDRAP. 2020 Apr.

  5. Brainard JS, Jones N, Lake I, Hooper L, Hunter P. Facemasks and similar barriers to prevent respiratory illness such as COVID-19: A rapid systematic review. medRxiv. 2020 Apr. DOI: 10.1101/2020.04.01.20049528.

  6. Jung H, Kim J, Lee S, et al. Comparison of Filtration Efficiency and Pressure Drop in Anti-Yellow Sand Masks, Quarantine Masks, Medical Masks, General Masks, and Handkerchiefs. Aerosol and Air Quality Research. 2014;14:991-1002. DOI: 10.4209/aaqr.2013.06.0201.

  7. Rengasamy S, Eimer B, Shaffer RE. Simple respiratory protection--evaluation of the filtration performance of cloth masks and common fabric materials against 20-1000 nm size particles. The Annals of Occupational Hygiene. 2010 Oct;54(7):789-798. DOI: 10.1093/annhyg/meq044.

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