Population-wide use of facemasks keeps the coronavirus reproduction number under 1.0

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Population-wide use of facemasks keeps the coronavirus ‘reproduction number’ under 1.0, and prevents further waves of the virus when combined with lockdowns, a modelling study from the universities of Cambridge and Greenwich suggests.

The research suggests that lockdowns alone will not stop the resurgence of SARS-CoV-2, and that even homemade masks with limited effectiveness can dramatically reduce transmission rates if worn by enough people, regardless of whether they show symptoms.

The researchers call for information campaigns across wealthy and developing nations alike that appeal to our altruistic side: “my facemask protects you, your facemask protects me”. The findings are published in the Proceedings of the Royal Society A.

“Our analyses support the immediate and universal adoption of facemasks by the public,” said lead author, Dr. Richard Stutt, part of a team that usually models the spread of crop diseases at Cambridge’s Department of Plant Sciences.

“If widespread facemask use by the public is combined with physical distancing and some lockdown, it may offer an acceptable way of managing the pandemic and re-opening economic activity long before there is a working vaccine.”

Dr. Renata Retkute, coauthor and Cambridge team member, said: “The UK government can help by issuing clear instructions on how to make and safely use homemade masks.”

“We have little to lose from the widespread adoption of facemasks, but the gains could be significant.”

The new coronavirus is transmitted through airborne droplets loaded with SARS-CoV-2 particles that get exhaled by infectious people, particularly when talking, coughing or sneezing.

For the latest study, Cambridge researchers worked to link the dynamics of spread between individuals with population-level models, to assess different scenarios of facemask adoption combined with periods of lockdown.

The modelling included stages of infection and transmission via surfaces as well as air. Researchers also considered negative aspects of mask use, such as increased face touching.

The reproduction or ‘R’ number – the number of people an infected individual passes the virus onto – needs to stay below 1.0 for the pandemic to slow.

The study found that if people wear masks whenever they are in public it is twice as effective at reducing ‘R’ than if masks are only worn after symptoms appear.

In all modelling scenarios, routine facemask use by 50% or more of the population reduced COVID-19 spread to an R less than 1.0, flattening future disease waves and allowing less-stringent lockdowns.

Viral spread reduced further as more people adopted masks when in public. 100% mask adoption combined with on/off lockdowns prevented any further disease resurgence for the 18 months required for a possible vaccine.

The models suggest that – while the sooner the better – a policy of total facemask adoption can still prevent a second wave even if it isn’t instigated until 120 days after an epidemic begins (defined as the first 100 cases).

The team investigated the varying effectiveness of facemasks. Previous research shows that even homemade masks made from cotton t-shirts or dishcloths can prove 90% effective at preventing transmission.

The study suggests that an entire population wearing masks of just 75% effectiveness can bring a very high ‘R’ number of 4.0 – the UK was close to this before lockdown – all the way down to under 1.0, even without aid of lockdowns.

In fact, masks that only capture a mere 50% of exhaled droplets would still provide a “population-level benefit”, even if they quadrupled the wearer’s own contamination risk through frequent face touching and mask adjustment (a highly unlikely scenario).

The researchers point out that crude homemade masks primarily reduce disease spread by catching the wearer’s own virus particles, breathed directly into fabric, whereas inhaled air is often sucked in around the exposed sides of the mask.

“There is a common perception that wearing a facemask means you consider others a danger,” said Professor John Colvin, coauthor from the University of Greenwich. “In fact, by wearing a mask you are primarily protecting others from yourself.”

“Cultural and even political issues may stop people wearing facemasks, so the message needs to be clear: my mask protects you, your mask protects me.”

“In the UK, the approach to facemasks should go further than just public transport. The most effective way to restart daily life is to encourage everyone to wear some kind of mask whenever they are in public,” Colvin said.

Prof Chris Gilligan, coauthor from Cambridge’s Epidemiology and Modelling Group in the Department of Plant Sciences, added: “These messages will be vital if the disease takes hold in the developing world, where large numbers of people are resource poor, but homemade masks are a cheap and effective technology.”


ackground: During respiratory viral infection, face masks are thought to prevent transmission (1). Whether face masks worn by patients with coronavirus disease 2019 (COVID-19) prevent contamination of the environment is uncertain (2, 3). A previous study reported that surgical masks and N95 masks were equally effective in preventing the dissemination of influenza virus (4), so surgical masks might help prevent transmission of severe acute respiratory syndrome–coronavirus 2 (SARS–CoV-2). However, the SARS–CoV-2 pandemic has contributed to shortages of both N95 and surgical masks, and cotton masks have gained interest as a substitute.

Objective: To evaluate the effectiveness of surgical and cotton masks in filtering SARS–CoV-2.

Methods and Findings: The institutional review boards of 2 hospitals in Seoul, South Korea, approved the protocol, and we invited patients with COVID-19 to participate. After providing informed consent, patients were admitted to negative pressure isolation rooms. We compared disposable surgical masks (180 mm × 90 mm, 3 layers [inner surface mixed with polypropylene and polyethylene, polypropylene filter, and polypropylene outer surface], pleated, bulk packaged in cardboard; KM Dental Mask, KM Healthcare Corp) with reusable 100% cotton masks (160 mm × 135 mm, 2 layers, individually packaged in plastic; Seoulsa).

A petri dish (90 mm × 15 mm) containing 1 mL of viral transport media (sterile phosphate-buffered saline with bovine serum albumin, 0.1%; penicillin, 10 000 U/mL; streptomycin, 10 mg; and amphotericin B, 25 µg) was placed approximately 20 cm from the patients’ mouths. Patients were instructed to cough 5 times each onto a petri dish while wearing the following sequence of masks: no mask, surgical mask, cotton mask, and again with no mask. A separate petri dish was used for each of the 5 coughing episodes. Mask surfaces were swabbed with aseptic Dacron swabs in the following sequence: outer surface of surgical mask, inner surface of surgical mask, outer surface of cotton mask, and inner surface of cotton mask.

The median viral loads of nasopharyngeal and saliva samples from the 4 participants were 5.66 log copies/mL and 4.00 log copies/mL, respectively. The median viral loads after coughs without a mask, with a surgical mask, and with a cotton mask were 2.56 log copies/mL, 2.42 log copies/mL, and 1.85 log copies/mL, respectively. All swabs from the outer mask surfaces of the masks were positive for SARS–CoV-2, whereas most swabs from the inner mask surfaces were negative (Table).

Table. SARS–CoV-2 Viral Load in Patient Samples, Petri Dishes, and Mask Surfaces

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Discussion: Neither surgical nor cotton masks effectively filtered SARS–CoV-2 during coughs by infected patients. Prior evidence that surgical masks effectively filtered influenza virus (1) informed recommendations that patients with confirmed or suspected COVID-19 should wear face masks to prevent transmission (2). However, the size and concentrations of SARS–CoV-2 in aerosols generated during coughing are unknown. Oberg and Brousseau (3) demonstrated that surgical masks did not exhibit adequate filter performance against aerosols measuring 0.9, 2.0, and 3.1 μm in diameter. Lee and colleagues (4) showed that particles 0.04 to 0.2 μm can penetrate surgical masks. The size of the SARS–CoV particle from the 2002–2004 outbreak was estimated as 0.08 to 0.14 μm (5); assuming that SARS-CoV-2 has a similar size, surgical masks are unlikely to effectively filter this virus.

Of note, we found greater contamination on the outer than the inner mask surfaces. Although it is possible that virus particles may cross from the inner to the outer surface because of the physical pressure of swabbing, we swabbed the outer surface before the inner surface. The consistent finding of virus on the outer mask surface is unlikely to have been caused by experimental error or artifact. The mask’s aerodynamic features may explain this finding. A turbulent jet due to air leakage around the mask edge could contaminate the outer surface. Alternatively, the small aerosols of SARS–CoV-2 generated during a high-velocity cough might penetrate the masks. However, this hypothesis may only be valid if the coughing patients did not exhale any large-sized particles, which would be expected to be deposited on the inner surface despite high velocity. These observations support the importance of hand hygiene after touching the outer surface of masks.

This experiment did not include N95 masks and does not reflect the actual transmission of infection from patients with COVID-19 wearing different types of masks. We do not know whether masks shorten the travel distance of droplets during coughing. Further study is needed to recommend whether face masks decrease transmission of virus from asymptomatic individuals or those with suspected COVID-19 who are not coughing.

In conclusion, both surgical and cotton masks seem to be ineffective in preventing the dissemination of SARS–CoV-2 from the coughs of patients with COVID-19 to the environment and external mask surface.

References

1. Johnson DF, Druce JD, Birch C, et al. A quantitative assessment of the efficacy of surgical and N95 masks to filter influenza virus in patients with acute influenza infection. Clin Infect Dis. 2009;49:275-277. [PMID: 19522650] doi:10.1086/600041. [PubMed]

2. Feng S, Shen C, Xia N, et al. Rational use of face masks in the COVID-19 pandemic. Lancet Respir Med. 20 March 2020. [Epub ahead of print]. [PMID: 32203710] doi:10.1016/S2213-2600(20)30134-X. [PMC free article] [PubMed]

3. Oberg T, Brosseau LM. Surgical mask filter and fit performance. Am J Infect Control. 2008;36:276-282. [PMID: 18455048] doi:10.1016/j.ajic.2007.07.008. [PMC free article] [PubMed]

4. Lee SA, Grinshpun SA, Reponen T. Respiratory performance offered by N95 respirators and surgical masks: human subject evaluation with NaCl aerosol representing bacterial and viral particle size range. Ann Occup Hyg. 2008;52:177-185. [PMID: 18326870] doi:10.1093/annhyg/men005. [PubMed]

5. Ksiazek TG, Erdman D, Goldsmith CS, et al; SARS Working Group. A novel coronavirus associated with severe acute respiratory syndrome. N Engl J Med. 2003;348:1953-1966. [PMID: 12690092] [PubMed]


More information: DOI: 10.1098/rspa.2020.0376 A modelling framework to assess the likely effectiveness of facemasks in combination with ‘lock-down’ in managing the COVID-19 pandemic, Proceedings of the Royal Society Aroyalsocietypublishing.org/doi … .1098/rspa.2020.0376

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