Living with COVID and keeping your choir viable while you’re at it. As has been the case throughout this pandemic, your choir must abide by the public health regulations in force at any time. The challenge arising for WA choirs is that we haven’t lived with COVID at all as yet and the fear of doing so, or the reality of doing so, may impact on your choir members in ways that adversely impact the viability of your choir in this phase of opening up to the rest of the country and the world.
Our goal is to assist you and the broader singing community to prepare for this phase, as best you can, while we are still COVID-free and have some time to work through some potentially complex issues in a considered way.
The published public health restrictions planned for WA when we reach the point on 90% full vaccination of the population over the age of 12 are detailed in the “Fact to Share” section below. These will have almost no impact on your ability to rehearse and perform as you are currently doing. The issue that choirs and individual singers have to grapple with is what measures your particular choir feel is appropriate, if any, over and above these restrictions, to address the risk of choir members contracting COVID in the choir setting once COVID is circulating in the WA community.
Every person will perceive the risk that COVID represents to them differently and will have different risk thresholds. Ideally you want to work with your members to arrive at an agreed approach such that your members are comfortable that coming to choir represents an equivalent or lower risk to their wellbeing than undertaking other activities that they are also prepared to continue doing once COVID arrives. You are not striving for or promising zero risk of infection. That’s a promise you can’t keep. You are assisting your members to develop an awareness of the risks they face in every aspect of their lives and to mentally prepare to live with COVID.
The peak bodies for choirs in WA can not and will not mandate specific approaches that every choir should take. This is a matter of individual choice and the collective approach of the specific group of individuals that currently make up your singing group and new members you will attract in the future. However we will provide you with a “shopping list” of measures that your choir should consider and adopt, or not, as your members consider appropriate for their perceived vulnerability and appetite for risk. We will also provide you with another list of scenarios that your members can consider to determine what your group agree to be the most appropriate response should such situations arise for your members in the future. These could form the basis for a series of discussions which will lead to guidelines which you can share with all your current and future members.
Obviously you will be striving to arrive at solutions that suit the needs of the vast majority of your existing members. The willingness of people to be open to the needs of others and accept compromise will assist this and will require some skilful dialogue and, potentially, the sharing of some facts as a basis for these discussions. We have prepared such a fact sheet that we believe represents a reasonable summary of the currently accepted facts as published by reputable health authorities which you may choose to share with your choir members if you see fit.
Having done all this there is still a chance that you will discover that a significant minority of your members have concerns that are unable to be addressed by consensus. In the opinion of the peak community singing bodies in WA, it is most important that choirs continue to survive and prosper through these challenging phases. If your choir discovers that it is going to be challenged through a potential loss of membership, a need to invest in new technology, a need for a new rehearsal venue etc, etc, this is important information for your choir to grapple with sooner than later so that mitigation plans can be put in place and assistance sought.
Your peak singing bodies, Voice Moves WA, SongFest Inc and ANCA (WA) will seek to share information, provide support and make representation as appropriate in support of choirs that identify specific concerns. Please take the opportunity to prepare your choir now and get the conversations started. COVID is coming and we in WA are uniquely fortunate in the world to have some time to prepare. Please use that opportunity.
How Exactly Do You Catch Covid-19? There Is a Growing Consensus
Surface contamination and fleeting encounters are less of a worry than close-up, person-to-person interactions for extended periods
Six months into the coronavirus crisis, there’s a growing consensus about a central question: How do people become infected?
It’s not common to contract Covid-19 from a contaminated surface, scientists say. And fleeting encounters with people outdoors are unlikely to spread the coronavirus.
Instead, the major culprit is close-up, person-to-person interactions for extended periods. Crowded events, poorly ventilated areas and places where people are talking loudly—or singing, in one famous case—maximize the risk.
These emerging findings are helping businesses and governments devise reopening strategies to protect public health while getting economies going again. That includes tactics like installing plexiglass barriers, requiring people to wear masks in stores and other venues, using good ventilation systems and keeping windows open when possible.
Two recent large studies showed that wide-scale lockdowns—stay-at-home orders, bans on large gatherings and business closures—prevented millions of infections and deaths around the world. Now, with more knowledge in hand, cities and states can deploy targeted interventions to keep the virus from taking off again, scientists and public-health experts said.
“We should not be thinking of a lockdown, but of ways to increase physical distance,” said Tom Frieden, chief executive of Resolve to Save Lives, a nonprofit public-health initiative. “This can include allowing outside activities, allowing walking or cycling to an office with people all physically distant, curbside pickup from stores, and other innovative methods that can facilitate resumption of economic activity without a rekindling of the outbreak.”
The group’s reopening recommendations include widespread testing, contact tracing and isolation of people who are infected or exposed.
A Recipe for Infection
Getting the Covid-19 virus involves three steps.
1 Coughing, talking and breathing creates virus-carrying droplets of various sizes.
2 Enough virus has to make itself over to you or build up around you over time to trigger an infection.
3 The virus has to make its way into your respiratory tract and use the ACE-2 receptors there to enter cells and replicate.
One important factor in transmission is that seemingly benign activities like speaking and breathing produce respiratory bits of varying sizes that can disperse along air currents and potentially infect people nearby.
Health agencies have so far identified respiratory-droplet contact as the major mode of Covid-19 transmission. These large fluid droplets can transfer virus from one person to another if they land on the eyes, nose or mouth. But they tend to fall to the ground or on other surfaces pretty quickly.
Some researchers say the new coronavirus can also be transmitted through aerosols, or minuscule droplets that float in the air longer than large droplets. These aerosols can be directly inhaled.
That’s what may have happened at a restaurant in Guangzhou, China, where an infected diner who was not yet ill transmitted the virus to five others sitting at adjacent tables. Ventilation in the space was poor, with exhaust fans turned off, according to one study looking at conditions in the restaurant.
Aerosolized virus from the patient’s breathing or speaking could have built up in the air over time and strong airflow from an air-conditioning unit on the wall may have helped recirculate the particles in the air, according to authors of the study, which hasn’t yet been peer-reviewed.
Sufficient ventilation in the places people visit and work is very important, said Yuguo Li, one of the authors and an engineering professor at the University of Hong Kong. Proper ventilation—such as forcing air toward the ceiling and pumping it outside, or bringing fresh air into a room—dilutes the amount of virus in a space, lowering the risk of infection.
A gym in Chino Hills, Calif., on June 12. PHOTO: WILL LESTER/ORANGE COUNTY REGISTER/ZUMA PRESS
Another factor is prolonged exposure. That’s generally defined as 15 minutes or more of unprotected contact with someone less than 6 feet away, said John Brooks, the Centers for Disease Control and Prevention’s chief medical officer for the Covid-19 response. But that is only a rule of thumb, he cautioned. It could take much less time with a sneeze in the face or other intimate contact where a lot of respiratory droplets are emitted, he said.
Superspreaders
At a March 10 church choir practice in Washington state, 87% of attendees were infected, said Lea Hamner, an epidemiologist with the Skagit County public-health department and lead author of a study on an investigation that warned about the potential for “superspreader” events, in which one or a small number of people infect many others.
Members of the choir changed places four times during the 2½-hour practice, were tightly packed in a confined space and were mostly older and therefore more vulnerable to illness, she said. All told, 53 of 61 attendees at the practice were infected, including at least one person who had symptoms. Two died.
Several factors conspired, Ms. Hamner said. When singing, people can emit many large and small respiratory particles. Singers also breathe deeply, increasing the chance they will inhale infectious particles.
Similar transmission dynamics could be at play in other settings where heavy breathing and loud talking are common over extended periods, like gyms, musical or theater performances, conferences, weddings and birthday parties. Of 61 clusters of cases in Japan between Jan. 15 and April 4, many involved heavy breathing in close proximity, such as karaoke parties, cheering at clubs, talking in bars and exercising in gyms, according to a recent study in the journal Emerging Infectious Diseases.
The so-called attack rate—the percentage of people who were infected in a specific place or time—can be very high in crowded events, homes and other spaces where lots of people are in close, prolonged contact.
An estimated 10% of people with Covid-19 are responsible for about 80% of transmissions, according to a study published recently in Wellcome Open Research. Some people with the virus may have a higher viral load, or produce more droplets when they breathe or speak, or be in a confined space with many people and bad ventilation when they’re at their most infectious point in their illness, said Jamie Lloyd-Smith, a University of California, Los Angeles professor who studies the ecology of infectious diseases.
But overall, “the risk of a given infected person transmitting to people is pretty low,” said Scott Dowell, a deputy director overseeing the Bill & Melinda Gates Foundation’s Covid-19 response. “For every superspreading event you have a lot of times when nobody gets infected.”
The attack rate for Covid-19 in households ranges between 4.6% and 19.3%, according to several studies. It was higher for spouses, at 27.8%, than for other household members, at 17.3%, in one study in China.
Rosanna Diaz with her son Tomas. PHOTO: ROSANNA DIAZ
Rosanna Diaz lives in a three-bedroom apartment in New York City with five other family members. The 37-year-old stay-at-home mother was hospitalized with a stroke on April 18 that her doctors attributed to Covid-19, and was still coughing when she went home two days later.
She pushed to get home quickly, she said, because her 4-year-old son has autism and needed her. She kept her distance from family members, covered her mouth when coughing and washed her hands frequently. No one else in the apartment has fallen ill, she said. “Nobody went near me when I was sick,” she said.
Being outside is generally safer, experts say, because viral particles dilute more quickly. But small and large droplets pose a risk even outdoors, when people are in close, prolonged contact, said Linsey Marr, a Virginia Tech environmental engineering professor who studies airborne transmission of viruses.
No one knows for sure how much virus it takes for someone to become infected, but recent studies offer some clues. In one small study published recently in the journal Nature, researchers were unable to culture live coronavirus if a patient’s throat swab or milliliter of sputum contained less than one million copies of viral RNA.
What to Expect When Flying Now (and in the Future)
Air travel is full of opportunities for coronavirus transmission. Touchless check-in, plexiglass shields, temperature checks, back-to-front boarding and planes with empty middle seats are all now part of the flying experience, and the future may bring even more changes. Illustration: Alex Kuzoian
“Based on our experiment, I would assume that something above that number would be required for infectivity,” said Clemens Wendtner, one of the study’s lead authors and head of the department of infectious diseases and tropical medicine at München Klinik Schwabing, a teaching hospital at the Ludwig Maximilian University of Munich.
He and his colleagues found samples from contagious patients with virus levels up to 1,000 times that, which could help explain why the virus is so infectious in the right conditions: It may take much lower levels of virus than what’s found in a sick patient to infect someone else.
Changing policies
Based on this emerging picture of contagion, some policies are changing. The standard procedure for someone who tests positive is to quarantine at home. Some cities are providing free temporary housing and social services where people who are infected can stay on a voluntary basis, to avoid transmitting the virus to family members.
The CDC recently urged Americans to keep wearing masks and maintaining a distance from others as states reopen. “The more closely you interact with others, the longer the interaction lasts, the greater the number of people involved in the interaction, the higher the risk of Covid-19 spread,” said Jay Butler, the CDC’s Covid-19 response incident manager.
If the number of Covid-19 cases starts to rise dramatically as states reopen, “more extensive mitigation efforts such as what were implemented back in March may be needed again,” a decision that would be made locally, he said.
CDC guidelines for employers whose workers are returning include requiring masks, limiting use of public transit and elevators to reduce exposure, and prohibiting hugs, handshakes and fist-bumps. The agency also suggested replacing communal snacks, water coolers and coffee pots with prepacked, single-serve items, and erecting plastic partitions between desks closer than 6 feet apart.
A crowd gathered at a bar in Columbus, Ohio, on May 15. PHOTO: DORAL CHENOWETH/THE COLUMBUS DISPATCH/ASSOCIATED PRESS
Current CDC workplace guidelines don’t talk about distribution of aerosols, or small particles, in a room, said Lisa Brosseau, a respiratory-protection consultant for the University of Minnesota’s Center for Infectious Disease Research and Policy.
“Aerosol transmission is a scary thing,” she said. “That’s an exposure that’s hard to manage and it’s invisible.” Ensuring infected individuals stay home is important, she said, but that can be difficult due to testing constraints. So additional protocols to interrupt spread, like social distancing in workspaces and providing N95 respirators or other personal protective equipment, might be necessary as well, she said.
Some scientists say while aerosol transmission does occur, it doesn’t explain most infections. In addition, the virus doesn’t appear to spread widely through the air.
“If this were transmitted mainly like measles or tuberculosis, where infectious virus lingered in the airspace for a long time, or spread across large airspaces or through air-handling systems, I think you would be seeing a lot more people infected,” said the CDC’s Dr. Brooks.
Sampling the air in high-traffic areas regularly could help employers figure out who needs to get tested, said Donald Milton, professor of environmental and occupational health at the University of Maryland School of Public Health.
“Let’s say you detect the virus during lunchtime on Monday in a dining hall,” he said. “You could then reach out to people who were there during that time telling them that they need to get tested.”
Erin Bromage, a University of Massachusetts Dartmouth associate professor of biology, has been fielding questions from businesses, court systems and even therapists after a blog post he wrote titled “The Risks—Know Them—Avoid Them” went viral.
Courts are trying to figure out how to reconvene safely given that juries normally sit close together, with attorneys speaking to them up close, Dr. Bromage said. Therapists want to be able to hold in-person counseling sessions again. And businesses are trying to figure out what types of cleaning and disease-prevention methods in which to invest most heavily.
He advises that while wiping down surfaces and putting in hand-sanitizer stations in workplaces is good, the bigger risks are close-range face-to-face interactions, and having lots of people in an enclosed space for long periods. High-touch surfaces like doorknobs are a risk, but the virus degrades quickly so other surfaces like cardboard boxes are less worrisome, he said. “Surfaces and cleaning are important, but we shouldn’t be spending half of our budget on it when they may be having only a smaller effect,” he said.
Plexiglass dividers at the Bellagio Hotel and Casino in Las Vegas on June 4. PHOTO: JOE BUGLEWICZ/BLOOMBERG NEWS
Drugmaker Eli Lilly & Co. has a medical advisory panel that’s reading the latest literature on viral transmission, which it is using to develop recommendations for bringing back the company’s own workers safely.
To go into production facilities, some of which are in operation now, scientists must don multiple layers of personal protective equipment, including gloves, masks, goggles and coveralls. That’s not abnormal for drug-development settings, said Lilly Chief Scientific Officer Daniel Skovronsky. “The air is extensively filtered. There’s lots of protection,” he said.
The places he worries about are the break rooms, locker rooms and security checkpoints, where people interact. Those are spaces where the company has instituted social-distancing measures by staggering the times they are open and how many people can be there at once. Only a few cafeterias are open, and those that are have socially distanced seating. In bathrooms, only half the stalls are available to cut down on the number of people.
“We’ll never be more open than state guidelines,” Dr. Skovronsky said, but “we’re often finding ourselves being more restrictive because we’re following the numbers.”
Globally, health-care authorities are searching for effective measures to prevent community transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although data on factors related to this transmission are scarce, the spread of SARS-CoV-2 is thought to mostly be via the transmission of respiratory droplets coming from infected individuals.1 Small droplets, from submicron to approximately 10 μm diameter, produced during speech and coughing, have been shown to contain viral particles,2 which can remain viable and infectious in aerosols for 3 h.3 The droplets can be transmitted either directly by entering the airway through the air (aerosols),4 or indirectly by contact transfer via contaminated hands. The mode of transmission could affect whether an infection starts in the upper or lower respiratory tract, which is thought to affect the severity of the disease progression.5 Notably, the dose–response relationship of SARS-CoV-2 infection is still unclear, especially with respect to aerosol transmission of the virus. However, aerosols containing a small concentration of virus in poorly ventilated spaces, combined with low humidity and high temperature,6 might result in an infectious dose over time.
To better understand the spreading of respiratory droplets and possible preventive measures, we analysed droplet production due to coughs and speech by measuring the droplet size distribution, travel distance and velocity, and the airborne time in relation to the level of air ventilation.
We did a laser diffraction measurement using a spray droplet measurement system (Malvern Spraytec, Malvern, UK) to determine the size distribution of respiratory droplets in a single cough and during speech. In a cough from a healthy volunteer, we found two distinct types of drops, large droplets (100–1000 μm in diameter) and small droplets (1–10 μm), with the small droplets being much more prevalent (appendix p 1). During speech, only the small droplets were found (appendix p 1). Although large droplets have been specifically related to coughs,4 here we observe that both sizes of droplet are produced by coughing.
appendix p 1
Next, we used a SprayScan (Spraying Systems, Glendale Heights, IL, USA) laser sheet to track droplets by filming the scattering of laser light by droplets from a cough7 to determine the speed of the droplets and their trajectory. Large droplets were observed to fall onto the ground rapidly (appendix p 2). We found that although the speed of the drops ranged 2–7 m/s at the start of the cough, the visible large drops (typically 500 μm in diameter) do not travel far before their trajectory bends down due to gravity to rapidly fall onto the ground within 1 s. This observation can be explained by balancing the forces of gravity (F=mg; where F is force, m is mass, and g is acceleration) and air drag (F=6πηRU, in which η is the air viscosity, R is the radius of the droplet, and U is the falling velocity), from which it also follows that the small droplets of typical radius of 5 μm will take 9 min to reach the ground when produced at a height of 160 cm (ie, average speaking or coughing height). These small droplets are of specific interest because they have been associated with aerosol transmission of the SARS-CoV-2.8 We also investigated droplets coming from the nasal cavity, and found that with normal breathing no droplets are detected above the background noise level (2·3 [SD 1·5] droplets, and 2·6 [1·7] droplets for nasal breathing). From a sneeze, we found mostly very large drops, originating from both the buccal and nasal cavities, that are not persistent.
appendix p 2
The same laser sheet was used to investigate how long small droplets from a cough will float through the air. We used a specially designed spray nozzle from Medspray (Enschede, The Netherlands) to disperse a controlled quantity of small droplets into the air, reproducing the effect of coughing. The droplets have an average diameter of 5 μm and are dispersed homogeneously by the spray nozzle. We analysed the number of droplets passing through the stationary laser sheet suspended in the centre of the experimental chamber using an algorithm that detects the illuminations caused by the droplets. We repeated this experiment in three rooms with different levels of ventilation: no ventilation, mechanical ventilation only, and mechanical ventilation supported by the opening of an entrance door and a small window (appendix p 3). In the best ventilated room, after 30 s the number of droplets had halved, whereas with no ventilation this took about 5 min, in agreement with the air drag calculation that shows that 5 μm drops from the average cough or speech height take 9 min to reach the ground. In a poorly ventilated room, the number of droplets was halved in 1·4 min.
appendix p 3
Although we only studied healthy volunteers and did not study patients with COVID-19 or virus-laden aerosol droplets directly, our data on droplet size distribution and persistence does have implications on requirements to use face masks to prevent virus transmission. Transmission by aerosols of the small droplets studied here can only be prevented by use of high-performance face masks; a conventional surgical mask only stops 30% of the small aerosol droplets studied here for inhaled breath;9 for exhaled breath the efficacy is much better.10
Additionally, the long airborne time of aerosols we found here affects the reliability of temporal and spatial contact data between individuals as monitored by proximity tracing via smartphone apps. These findings need to be considered in the development and implementation of these apps.
This study shows that better ventilation of spaces substantially reduces the airborne time of respiratory droplets. This finding is relevant because typically poorly ventilated and populated spaces, like public transport and nursing homes, have been reported as sites of viral transmission despite preventive physical distancing. The persistence of small respiratory droplets in such poorly ventilated spaces could contribute to the spread of SARS-CoV-2. Our findings confirm that improving ventilation of public spaces will dilute and clear out potentially infectious aerosols. To suppress the spread of SARS-CoV-2 we believe health-care authorities should consider the recommendation to avoid poorly ventilated public spaces as much as possible. The implications are also important for hospital settings where aerosolisation by coughing and medical treatments and close contact with COVID-19 patients is very common.
The most relevant and comprehensive report we have seen so far comes from German researchers who are explicitly seeking to develop risk mitigation approaches for singing and instrument players of all types. While it is clear that key elements of the puzzle are still unknown this report gives a lot of considered proposals which may resonate with your choir members.
Generally, large numbers of people in small, indoor spaces are correlated with significant infection risk. These authors strongly favour outdoor musical activity. Provided the system is utilising 100% fresh air these authors consider ducted air conditioning systems to be the next best option.
Research into a cluster of cases in South Korea relating to dance aerobic exercise sessions generates some interesting data that may be relevant to higher aerobic activities compared to talking. Researchers concluded that factors that may have contributed to the high levels of transmission between the infected instructors and their class members was relatively large numbers of people (classes of 20 had much higher infection rates while classes of 5 had none) in a relatively small room (60m2) undertaking high intensity activity. An infected instructor who taught Pilates and yoga in the same room (with a class size of 8) spread the virus to none of the class. The moist, warm environment of a sports complex coupled with turbulent air flow was also hypothesised as a contributing factor.
The ability of the COVID-19 virus to remain viable in aerosol form may be more significant than its SARS and MERS predecessors. In an, as yet non-peer reviewed comparative study, researchers concluded that:
“The comparison of short-term aerosol efficiencies of three emergent coronaviruses showed SARS-CoV-2 is on par with or exceeding the efficiency estimates of SARS-CoV and MERS-CoV. Some efficiency determinations for SARS-CoV-2 ranged to -5.5log10, a full log difference compared to MERS-CoV. The fact that higher efficiencies trended across independent laboratories strengthens this observation. These data suggest that SARS-CoV-2 generally maintains infectivity when airborne over short distances, in contrast to either comparator betacoronavirus. Results of the aerosol suspension experiments suggest that SARS-CoV-2 is persistent over longer periods of time than would be expected when generated as a highly respirable particle (2 μm MMAD). This is remarkable, as there would be an expected decay and loss in the infectious fraction of airborne virus based on prior susceptibility studies with other relatively environmentally hardy viruses like Monkeypox . A recent study , showing only a slight reduction of infectivity in aerosol suspensions with approximately similar particle sizes, were suggestive of the minimal effects on SARS-CoV-2 infectivity observed in these results.
Collectively, this preliminary dataset on the aerosol efficiency and persistence of SARS- CoV-2 suggest that this virus is remarkably resilient in aerosol form, even when aged for over 12 hours, and reinforces the conclusions reached in earlier studies of aerosol fitness by others . Aerosol transmission of SARS-CoV-2, whether through direct respiratory droplet transfer or fomite generation, may in fact be a more important exposure transmission pathway than previously considered . Our approach of quantitative measurement of infectivity of viral airborne efficiency complemented by qualitative assessment of virion morphology leads us to conclude that SARS-CoV-2 is viable as an airborne pathogen. Humans produce aerosols continuously through normal respiration . Production of aerosols increases during respiratory illnesses,and even during louder-than-normal oration . A fraction of naturally-generated aerosols fall within the size distribution used in our experimental studies (<5 μm), thus leading us to the conclusion that individuals infected with SARS-CoV-2 have the capacity to produce viral bioaerosols that may remain infectious over long periods of time after production via human shedding and airborne transport. Accordingly, our study results provide a basis for a broader recognition of the unique aerobiology of SARS-CoV-2, which may ultimately lead to tractable solutions and prevention interventions in the ongoing pandemic.”
The use of PPE to interrupt transmission of the virus is also an area with a lot of information but little clear data. The full report must be subscribed to but the abstract on a Cambridge University study into the effectiveness of face masks made from non-medical fabric (t shirt fabric for the most part) can be seen here.
They are found to be about a third as effective as medical grade masks but a considerable improvement on nothing at all.
Other research found that some readily available fabrics that were more effective than surgical masks but not as effective as N95 masks. An extract from that source summarises results as follows:
“To test various masks and fabrics, the team pumped air through both types of face coverings.
“Our instruments could read down to 0.3 microns, which is about the size of a big virus,” Segal said.
Regular surgical masks filtered out 62 percent to 65 percent of particles. For comparison, N95 masks filter 95 percent of those particles.
But the fabrics led to a variety of results. One piece of cloth filtered just 1 percent of particles, rendering it virtually useless, while others were found to perform even better than the surgical masks.
“We had some that performed at 79 percent,” Segal told NBC News.
The best masks were constructed of two layers of heavyweight “quilters cotton” with a thread count of at least 180, and had thicker and tighter weave.”
This paper published in Nature Medicine studied seasonal human coronaviruses, influenza viruses and rhinoviruses in exhaled breath and coughs of children and adults with acute respiratory illness. Tests were done with and without surgical type masks. The paper showed much better results for using masks against coronaviruses compared to influenza viruses. The following is an excerpt from the discussion;
“Our findings indicate that surgical masks can efficaciously reduce the emission of influenza virus particles into the environment in respiratory droplets, but not in aerosols….. We also demonstrated the efficacy of surgical masks to reduce coronavirus detection and viral copies in large respiratory droplets and in aerosols. This has important implications for control of COVID-19, suggesting that surgical face masks could be used by ill people to reduce onward transmission.”
