SAGE expert explains science behind wearing a mask to fight Covid-19

By Julia Gregory

“It’s a completely altruistic thing to wear a face covering,” according to a virus expert who serves as an adviser to the government.

Professor Wendy Barclay, who is the head of Imperial College’s department of infectious diseases said the face coverings can protect other people.

She warned that coronavirus is “a pretty robust virus” so care is absolutely crucial to help stop the spread.

She is an expert of respiratory illnesses, particularly flu, which is “much more fragile” and is a member of the Government’s scientific advisory group (SAGE).

Professor Barclay is currently involved in projects trying to understand why children’s immune response to coronavirus  seems to be different from adults and helping the team at Imperial develop vaccines to combat the airborne contagion.

She was involved in the largest study of the transmission of Covid in the UK. The REACT (Real Time Assessment of Community Transmission Study) tested swabs from 594,000 people and found that people doing jobs in high contact with others, living in crowded homes and in cities “are more likely to become infected”.

“The way the virus gets out of one person and into another is through droplets,” she said.

“We emit them all the time. If you think about just walking down the road on a cold day you can see this mist just walking along. That’s the respiratory droplets that you’re emitting all the time. You can see yourself that the mist is close to where you are and further away the mist goes away.”

“The idea is that somebody standing very close to you as you breathe or talk or anything else that gets it out there, chances are they could breathe in some of those droplets themselves and that’s exactly how the virus would get across from one person to the next.”

Wear a Face mask, image Hammersmith & Fulham council, free for use by partners of BBC news wire service

Professor Barclay said people could do an experiment on a cold day and watching the difference in the number of droplets they can see if they wear a face covering compared with those visible without a covering.

“Larger droplets which predominantly contain the virus as you breathe out get trapped in the face covering and stay there then close to you. It doesn’t really matter because you are already infected but the point is they don’t go on and have the capability of infecting somebody else.

“It is a completely altruistic thing to wear a face mask. Your own droplets get caught in the face mask and you reduce how much you pass on to someone else.”

She added that if someone does not wear a mask “what actually happens is they [the droplets] get smaller and smaller as they remain in the air. The water in the droplets evaporates out and the droplets contract down into smaller and smaller drops that you can’t see any more, as aerosols. The virus is still in that and can then float in the air and a mask will not protect terribly well from aerosols that are containing virus that someone else breathed out some time ago.”

Doctors and nurses wear higher grade medical masks to protect them “from aerosol that a patient might have breathed out some while ago which are now being carried in very very small droplets which can get around the cloth masks that most people are wearing,” she explained.

Professor Barclay said more breath comes out with more droplets whilst singing, sneezing or exercising.

But she said wearing a mask whilst exercising is less comfortable.

“The other way to do this is to incorporate all the other measures like socially distancing, and really good ventilation as well so that any droplets you breathe out are removed or diluted in the rest of the air so that the chances of anyone breathing that in is less.”

She said exercising outdoors is “much much safer” so the droplets emitted running down the street are so diluted into the atmosphere by the time someone else breathes compared with working out in a small gym with poor ventilation.

“Being outside is so much safer than being inside  in terms of transmission. There aren’t any recorded  cases in the world of  Sars-Covid-2 being transmitted in an outdoor environment in contrast to indoors, in a house, in an office, in a restaurant, in a bar, in a hospital. We’ve got lots of evidence of chains occurring there.”

And Professor Barclay said the science around  the role of children and the virus “is still unclear.”

They are less susceptible in catching the virus and teachers are more likely to contract it in schools – the opposite of common colds and flu.

However children do carry virus loads.

She said: “I couldn’t exclude that children could be infectious to others but like a lot of things in this virus it’s about weighing up the more likely scenarios and the higher risk things.

“There is a danger like everything else like mental health and children’s development whether or not there is good evidence that children are super spreaders – bearing in mind that in the classroom mixing and not wearing face masks they are singing and shouting and sitting close to each other and not spreading it.”

She said the evidence does not suggest that children “absolutely must wear masks”.

But she said the message is out there for adults – unless exempt for medical reasons “wear a mask to protect others”.

Pictured: Professor Wendy Barclay


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