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expert comments about outdoor transmission of SARS-CoV-2 and use of face masks outdoors

Following Chris Whitty’s interview this morning on BBC Radio 4’s Today programme, we’ve had a few questions from journalists about the use of facemasks outdoors, so here are some general comments from scientists.

 

Dr Julian Tang, Honorary Associate Professor/Clinical Virologist, University of Leicester, said:

“Epidemiologically-based comparisons of the spread of COVID-19 in countries where universal masking has been practiced since the start of the pandemic (like Taiwan, Vietnam, Thailand, Hong Kong) have shown that this contributes to the effective control of this virus:

https://www.cmaj.ca/content/192/19/E511

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7177146/

https://www.pnas.org/content/117/51/32293

“This will apply to the new variant also, as lab-based studies have shown that universal masking reduces overall virus shedding (using influenza virus) from the source as well as exposure to the recipient:

https://www.hse.gov.uk/research/rrhtm/rr619.htm

https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1003205

https://pubmed.ncbi.nlm.nih.gov/23498357/

“It is more difficult to study outdoor virus transmission but this epidemiological analysis of >300 transmission events from China during their earlier COVID-19 pandemic only found one case of outdoor transmission:

https://onlinelibrary.wiley.com/doi/full/10.1111/ina.12766

It says: ‘Our study does not rule out outdoor transmission of the virus. However, among our 7324 identified cases in China with sufficient descriptions, only one outdoor outbreak involving two cases occurred in a village in Shangqiu, Henan. A 27‐year‐old man had a conversation outdoors with an individual who had returned from Wuhan on January 25 and had symptom onset on February 1. This outbreak involved only two cases.’

“So outdoor transmission is rare but now we are looking to reduce all forms of transmission with this new more highly transmissible virus variant.

“Masking outdoors will have some impact on reducing the spread of the this virus – particularly when queueing, e.g. for entry to various shops or buses – where people are gathering in large numbers and not moving.

“We know that this virus mostly spreads through aerosols exchanged during close (< 1 m) conversational contact – especially if masks are not worn or not worn properly (i.e. not covering both nose and mouth at all times). 

“Several studies have now shown the presence of virus in exhaled breath, as well as the production of more aerosols that may be carrying viruses when talking (and more when talking louder) – and this will apply to the new variant also:

https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa1283/5898624

https://www.sciencedirect.com/science/article/abs/pii/S0021850220301786?via%3Dihub%20

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6382806/

https://www.pnas.org/content/117/22/11875

“So wearing masks indoors and outdoors when near (within 2 m) of any other person should reduce transmission risks – even with this new variant.

“If everyone stayed in their houses and never came out, then the virus cannot transmit between people – it cannot pass through bricks and mortar and glass windows. But we still need to go out for essential shopping – so this creates a risk; if we have to do nursery/school runs – this creates another risk.

“Colder outdoor air with less sunlight will help this lipid-enveloped coronavirus  survive in aerosols – yes. So if you are standing in a queue outdoors, even >2 m apart, wearing a mask will reduce the transmission further from any airborne virus that can linger for longer in the air – to be breathed in by you.

“Think of it like this – if you are standing in a queue for the bus, and someone is smoking – even if you are standing more than 2 m away, if you are downstream of that smoke – you can still smell it.

“Aerosolised virus is similar. People breathe out 0.5-1 L of air, 12-16 times a minute (this is your usual breathing ‘tidal’ volume/rate) – so each breath can pump more virus into the air if you are infected. The virus can survive longer in these colder, darker winter days – so it can travel downwind for others to breath in – even further than 2 m away.

“So yes, based on what we know about the physics and physiology of aerosol virus transmission, wearing a mask when outdoors when standing in queues with other people will help to reduce transmission.”

 

Dr Simon Clarke, Associate Professor in Cellular Microbiology, University of Reading, said:

“If wearing masks outdoors was thought to reduce coronavirus transmission, we would probably already have been told to do so.  While they might have an effect in very crowded outdoor environments, these are currently closed, so it’s unlikely that it would have any impact.

“The government would probably have more impact if it re-visited the 2 meter rule, reduced the density of shoppers in supermarket and where appropriate encouraged people to wear masks properly, including changing them regularly and to not touch them; an old or dirty face mask is no different to a used handkerchief tied to someone’s face.  It’s also true that mask wearing on public transport can be particularly patchy, so it may also be more effective to actually bother enforcing their wearing in places where they’ve already been made mandatory.”

 

Professor Lawrence Young, Virologist and Professor of Molecular Oncology, University of Warwick, said:

“We know that the virus survives better at cold temperatures. This was highlighted during the first phase of the pandemic when outbreaks of infection were common in meat processing plants and other factories where refrigeration was necessary. This coupled with the virus variant which is more infectious and produced at higher levels in infected individuals increases the risk of transmission.

“Close contact even outdoors could result in infection and any situation where folk are crowded together (e.g. queues outside shops, public transport) will increase virus transmission. In these situations wearing facemasks and keeping at least 2 metres apart is essential. Most important of all is to stay at home as much as possible and to be very careful of any contact outside of your household!”

 

Prof Paul Hunter, Professor in Medicine, The Norwich School of Medicine, University of East Anglia, said:

“I think Chris Whitty is correct in that general mask wearing out of doors is not appropriate.

“1. We do know that the risk of transmission of out of doors is substantially less than indoors by 18.7 times https://www.medrxiv.org/content/10.1101/2020.02.28.20029272v2 

2. Masks do have value but they are no replacement for proper social distancing and in the vast majority of outdoor settings social distancing will should be much easier.

3. Masks are ineffective if they get wet and need to be preplaced as per WHO technical guidance https://www.who.int/publications/i/item/advice-on-the-use-of-masks-in-the-community-during-home-care-and-in-healthcare-settings-in-the-context-of-the-novel-coronavirus-(2019-ncov)-outbreak. The problem is that if you wear a mask outside and it is raining, your mask gets wet very quickly. If it is cold your mask gets wet from your breath moisture. This may not matter if you are outdoors but if you are then moving indoors with a wet facemask this is likely to be bad news.

“So I would argue strongly against wearing masks outdoors unless you have to be in a crowd and the examples that Chris Whitty gives of huddling around a market stall and being in a large queue are probably the only such situations when this may be justifiable. But it is much better to avoid those situations and go shopping at your outdoor market when it is not busy if you can.”

 

Prof Keith Neal, Emeritus Professor of the Epidemiology of Infectious Diseases, University of Nottingham, said:

“It is unlikely that the new variant has much effect on outdoor transmission given the inherently low risk when outside with distancing.  Air movements and the ability to stay further apart are independent of the virus’s infectivity. 

“Walking or running past someone is such a fleeting contact then the possibility of infection is incredibly low and approaches zero. 

“Standing in queues, such as for supermarkets, for prolonged periods with less than 2 meters distance puts people at a substantially higher risk of spreading or catching the virus than simply passing on the pavement.  In these circumstances wearing masks is prudent but more distancing is required. Better still would be 2m distance markers for the queue as in the supermarket I do my shopping in. There is no need to stand close to people, you will get into the store.”

 

Dr Shaun Fitzgerald FREng, Royal Academy of Engineering Visiting Professor at the University of Cambridge, said:

“The risk outdoors is much lower than indoors. However, if you are outdoors and there are other people around, then if the air is relatively stagnant the risk will be higher especially if you are stationary. If you are queuing at a market stall which is only really open to one side then this is very different from passing someone on a walk by the sea. I absolutely agree with the suggestion of wearing face coverings outdoors in environments where the air is relatively stagnant and where queues form.”

 

 

All our previous output on this subject can be seen at this weblink:

www.sciencemediacentre.org/tag/covid-19

 

 

Declared interests

None received.

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