31 August 2020 by Jennifer
Some things I’ve learnt from following doctors and researchers as covid science has evolved, and some of the most informative articles they’ve recommended.
Here’s a simple summary of different transmission risk factors to consider. They interact, of course, and we’ll get to that in a minute.
|quick visit||there a long time|
|breeze bringing fresh air||windows shut, still air|
|mouths & noses covered||some people bare-faced|
|breathing gently||breathing deeply, exercise, coughs, sneezes|
|quiet or talking quietly||talking loudly, singing, shouting|
|keeping distance||close together|
Let’s look at how it works, and what’s worth worrying about.
- The virus is transmitted through the air: one person breathes it out, another person breathes it in. Think of seeing people’s breath in the air on a cold day. The virus is smaller than those little breath-particles, and it travels inside them.
- Early advice from the World Health Organisation assumed that the virus wouldn’t float through the air, so you’ll see a lot of guidelines which don’t take that into account. For example, 6 foot was based on droplets that fall to the floor, not floating ones. (Staying further apart does help with the floating particles too – it’s like backing away from a smoker if you don’t like smoke.)
- Breathing in just a quick whiff of virus isn’t likely to actually infect you. It makes a difference how much virus there is in the air, and how much time you spend breathing it in. (This is known as the “viral dose“.)
- If you get infected, the virus builds up gradually in your body. Then you start breathing out virus in your breath. Then you get symptoms. (Or maybe you don’t get noticeable symptoms, if you have the virus “lightly”.) In other words, you can be infectious to other people before you even realise you’ve got it.
- Shouting, singing or talking loudly help to send virus out into the air. (It could be due to a combination of deeper breaths and what your vocal cords are doing.)
- If you’re outside in a breeze, virus particles will tend to drift away on the breeze and be spread out into all the airspace around you. So if there’s any virus still around at all, it’s likely to be just a few particles, not enough to infect anyone. This is why meeting people outside is a lot safer than inside.
- If an infected person’s in the same place indoors for a while, their tiny breath particles gradually float all over the room (similar to cigarette smoke). In that situation, the “6 foot” distance isn’t necessarily enough to keep other people safe. Depending on the ventilation, the total amount of virus in the room can build up, like smoke would.
- Indoors, having windows open can help – as long as the breeze is actually carrying that room’s air outside, rather than carrying germs to spread out through the rest of the building.
- Originally, people were worrying a lot about picking up the virus by touching something, but we know now that’s not very likely compared to breathing it in.
- The virus can survive for several hours floating in the air indoors, maybe longer. One study suggested 16 hours. It prefers dry, cold conditions. Sunlight helps to destroy it.
- If the infected person’s wearing a fabric face-covering (a.k.a. mask) over their mouth & nose, it slows down how much virus they’re putting into the air. A well-fitted double-layer mask could reduce it a lot, though probably not down to zero.
- Face-coverings are better at catching your germs than at protecting you from someone else’s germs. Ideally, people in an indoor space would all be wearing masks (in case one person is infectious without knowing), not only the people who are personally most cautious. However, they do help to reduce virus amounts to the person wearing them, as well.
- The best home-made masks are two or three layers of fabric, shaped to fit. To make it fit properly round the nose, you can have something like a bit of wire from a paperclip stitched in across the nose shape.
- Quick ways to roughly assess a mask: (a) when you wear it, how much of the air is going through it, versus round the edges? (b), if you try looking through it, are you seeing through tiny holes in the fabric weave? But even a mediocre mask will have some effect on air-flow.
- The main problems I’ve heard of with masks are (a) they get in the way of lipreading, (b) glasses steaming up, (c) some people feel claustrophobic, or have sensory overwhelm or other unpleasant feelings. I’ll link below to the most helpful material I’ve seen about those difficulties.
- The plastic face-shields that look like a welder’s face protection don’t do a lot by themselves, because the virus can easily float around underneath them where they’re open. However, if your situation means you have to be close to possibly-infected people, then adding one of those as well as a mask could help a bit, by shielding your eyes from other people’s coughs or sneezes. Some types of goggles might be even slightly more protective than a face-shield – no gaps to float through.
- A cough potentially contains millions of virus particles – many times more than an ordinary breath, more even than a shout. And the air from a cough or sneeze can easily travel further than 6 foot. If the person covers their mouth and nose (with hand, elbow, mask), that will slow down the breath sent directly outwards, and catch some of the particles in their breath. But really you don’t want anyone coughing or sneezing anywhere near you.
- Children in good health are very unlikely to die of covid. For most young people, the more relevant questions are (a) will they infect family members or teachers, (b) will they acquire a long-term health condition.
- Young and previously fit people (including children) can get what’s been nicknamed “long covid“, where they don’t die but they don’t recover – the illness symptoms carry on. We don’t know yet how common that is, or why some people get it and others don’t. For some people, this has already lasted months; if it’s similar to other post-viral syndromes, it could be years, or permanent, but obviously we can’t know that yet.
- Aside from “long covid”, people who survive covid can also be left with heart or lung damage, or may have a stroke as a result of the illness clotting their blood.
- It seems possible that “how badly you get the illness” depends partly on “how much virus you breathed in in the first place“. As far as I know, this pattern isn’t definitely proven for covid in humans – but it has been researched for other diseases, and would make sense. If only 1 bit of virus gets into only 1 cell and starts copying itself, rather than 100 or 1000 starting the same day, that gives your body a bit more time before there’s loads of virus going around it. It might give your immune system a head start on making defences. (As mentioned above, a small enough viral dose won’t give you the illness at all.)
- About who’s most infectious: At the moment, it seems as though teenagers’ infectiousness is more or less similar to adults’, whereas little children may put out less virus (perhaps because their bodies are smaller). There’s no proof that little children can’t infect anyone else. (It’s a tricky area to research, because (a) children don’t always have symptoms, so if they were first in a family to catch it, that might not be obvious; (b) you can’t do the kind of experiment that would mean infecting people on purpose.)
- The infectiousness pattern is that infectiousness rises up over a few days, to a peak just before symptoms start, then reduces over the first few days of the illness. After a while, you can be ill and not infectious.
- When there’s a “superspreading” event, it may be that was partly just bad luck of timing – that when someone went to a group, it happened to be exactly their peak time of being infectious. However, it does also seem that some people naturally breathe out more virus than others.
- The Covid Symptom Tracker project estimates how many cases are active around the UK (age 20-69, not in care homes, with symptoms). Their estimates are based on real-time reporting from participants around the country who put any symptoms into the app, plus testing if anyone needs it, plus a mathematical model to allow for all the people who aren’t participating. For example, in Nottingham City today they estimate about 1 in every 590 people has the virus. (= rate of 1,696 per million.) It’s only approximate of course.
- As background: all the risky situations are only risky when the virus is circulating in the community, and might be in the room with you – like in England at the moment! In countries with a well-functioning system for “find, test, trace, isolate, support”, you nearly always know nearly everyone who’s got the virus or might have it, and where they are. So then it’s safe for people to gather more freely in the areas where the virus isn’t circulating.
You can use the following chart to do a rough risk assessment of a particular situation, and then see if there’s a way for you to change something to a less risky variation.
It’s from a recent new paper in the BMJ, Two metres or one: what is the evidence for physical distancing in covid-19?
For example, a school class typically involves people being in the same place for an hour or so, with often at least one person talking. The best school “classroom” would be outdoors. If you have to be indoors, you can reduce the transmission risks from “high” to “medium” by having good ventilation, and by everyone wearing masks who can. And in some situations, depending on space & resources, you can reduce the risk further by having only a few people in the room.
(Where they specify “asymptomatic” at the top of the chart, I think the reason is that if someone’s coughing, everything’s instantly much riskier – because coughs can put so much virus into the air in one go.)
The creators add,
Face covering refers to those for the general population and not high grade respirators. … Other factors [include] viral load of an infected person and people’s susceptibility to infection.
You’ve now got the gist of what I know so far! Thanks for reading!
If you want more info, e.g. where I got all that, then please read on for more resources.
“Aerosol” (sometimes referred to as “airborne”) transmission is similar to droplet transmission, except that the bits of fluid are so small that they can linger in the air for minutes to hours.
[Aerosols] act like smoke: after being expelled, they don’t fall to the ground, but rather disperse throughout the air, getting diluted by air currents, and being inhaled by others present in the same space. Contact tracing shows that, when it comes to COVID-19, being outdoors is 20 times safer than being indoors, which argues that aerosol transmission is much more important than droplets; outdoors, there’s plenty of air in which aerosols can become diluted; not so indoors. In addition, researchers have demonstrated aerosol transmission of this virus in ferrets and hamsters.
– Professor Jose-Luis Jimenez, COVID-19 Is Transmitted Through Aerosols. We Have Enough Evidence, Now It Is Time to Act, Time, August 2020
That whole article is well worth a read. It doesn’t only explain what the science says – it also goes a little bit into the history of why people wrongly thought at first that airborne transmission wasn’t happening.
Here’s a very useful FAQ on everything about aerosol transmission, by Prof Jimenez and colleagues:
Ryan Davis made a little video showing 50-micron particles floating in the air. (Explanation of the particle measurements is in the Twitter thread. If the whole thread doesn’t show at first, click on the date of the tweet. You don’t need a Twitter account.)
Now here’s a couple of places where scientists warn against relying on the 6-foot/2-metre distance (because the virus can move around a lot further than that, depending on the situation). “SARS-CoV-2” is the official name of the covid virus.
Breathing out, singing, coughing, and sneezing generate warm, moist, high momentum gas clouds of exhaled air containing respiratory droplets. This moves the droplets faster than typical background air ventilation flows, keeps them concentrated, and can extend their range up to 7-8 m within a few seconds.
– N R Jones et al, Two metres or one: what is the evidence for physical distancing in covid-19?, BMJ, August 2020
The available information on the SARS-COV-2 spreading supports the hypothesis of airborne diffusion of infected droplets from person to person at a distance greater than two meters (6 feet). The inter-personal distance of 2 m can be reasonably considered as an effective protection only if everybody wears face masks in daily life activities.
– L Setti et al, Airborne Transmission Route of COVID-19: Why 2 Meters/6 Feet of Inter-Personal Distance Could Not Be Enough, International Journal of Environmental Research & Public Health, April 2020
Here’s some research about how long the virus lasts in the air.
Although we performed experiment only once across several laboratories, our findings suggest retained infectivity and virion integrity for up to 16 hours in respirable-sized aerosols.
– A C Fears et al, Persistence of Severe Acute Respiratory Syndrome Coronavirus 2 in Aerosol Suspensions, Emerging Infectious Diseases journal, September 2020
“Retained infectivity and virion integrity” means the virus was still in one piece. I don’t think they proved that it definitely would infect someone, but this suggests it could – it wasn’t just “old left-over bits” which they found in the air after 16 hours.
(Sometimes you can find old bits of virus which aren’t dangerous any more. It proves the virus was there, but nobody’s going to be infected from it. This is why you can still test positive when you’re over the illness.)
Here’s a useful step-by-step explanation of how much virus you might need to breathe in to get ill. Some of the actual numbers in this are only guesswork, because we don’t know yet exactly how much is the real amount. But the general idea would be the same with different numbers.
Remember the formula: Successful Infection = Exposure to Virus x Time
If a person coughs or sneezes, those 200,000,000 viral particles go everywhere. Some virus hangs in the air, some falls into surfaces, most falls to the ground. So if you are face-to-face with a person, having a conversation, and that person sneezes or coughs straight at you, it’s pretty easy to see how it is possible to inhale 1,000 virus particles and become infected.
But even if that cough or sneeze was not directed at you, some infected droplets–the smallest of small–can hang in the air for a few minutes, filling every corner of a modest sized room with infectious viral particles. All you have to do is enter that room within a few minutes of the cough/sneeze and take a few breaths and you have potentially received enough virus to establish an infection.
But with general breathing, 20 viral particles [per] minute into the environment, even if every virus ended up in your lungs (which is very unlikely), you would need 1000 viral particles divided by 20 per minute = 50 minutes.
Speaking increases the release of respiratory droplets about 10 fold; ~200 virus particles per minute. Again, assuming every virus is inhaled, it would take ~5 minutes of speaking face-to-face to receive the required dose.
The exposure to virus x time formula is the basis of contact tracing. Anyone you spend greater than 10 minutes with in a face-to-face situation is potentially infected. Anyone who shares a space with you (say an office) for an extended period is potentially infected.
This is also why it is critical for people who are symptomatic to stay home. Your sneezes and your coughs expel so much virus that you can infect a whole room of people.
– Dr Erin Bromage, The Risks – Know Them – Avoid Them
I found that whole article really useful in getting my head round how this works. I recommend it.
This fascinating study goes back to the 1918 ‘flu epidemic and does some mathematical modelling. It comes out plausible (though not proven) that you were more likely to get the ‘flu badly, and die, the higher number of infectious people you’d encountered just before you got ill. This could be similar to doctors and healthcare workers who died in the first wave of covid, who had encountered a lot of sick people before they had good protective gear.
A lot of people have blithely made reassurances (or dismissals) along the lines of “Young people are very unlikely to die!” This is true, but dying isn’t the only risk.
One risk affecting young, fit, healthy people is what’s been nicknamed “Long Covid“:
Most have never been admitted to an ICU or gone on a ventilator, so their cases technically count as “mild.” But their lives have nonetheless been flattened by relentless and rolling waves of symptoms that make it hard to concentrate, exercise, or perform simple physical tasks. Most are young. Most were previously fit and healthy. “It is mild relative to dying in a hospital, but this virus has ruined my life,” LeClerc said. “Even reading a book is challenging and exhausting. What small joys other people are experiencing in lockdown—yoga, bread baking—are beyond the realms of possibility for me.”
– COVID-19 Can Last for Several Months – Ed Yong in The Atlantic, June 2020
Many long-haulers start feeling better in their fourth or fifth month, but recovery is tentative, variable, and not guaranteed. Hannah Davis, an artist in New York City, still has fever, facial numbness, brain fog, and rapid heartbeats whenever she stands up, but she’s sleeping better, at least; at the end of July, she had her first relatively normal day since mid-March. … Hannah Wei, a product designer based in Ottawa who is a Body Politic researcher, has recovered from her neurological symptoms but not the scars the coronavirus left on her lungs. “Will I be living with this lasting damage, or will it eventually go away?” she says. “I don’t have the answers, and no one can tell me.”
– Long-Haulers Are Redefining COVID-19 – Ed Yong in The Atlantic, August 2020
Doctors have also raised concerns about how long the heart, kidney & lung damage might last. (Cardiologist = heart specialist, cardiac = heart-related.)
“These are two studies that both suggest that being infected with Covid-19 carries a high likelihood of having some involvement of the heart. If not answering questions, [they] prompt important questions about what the cardiac aftermath is,” said Matthew Tomey, a cardiologist and assistant professor of medicine …
“The question now is how long these changes persist,” he added. “Are these going to become chronic effects upon the heart or are these — we hope — temporary effects on cardiac function that will gradually improve over time?”
… “Patients come to my office saying, ‘Hey, I’m a 31-year-old who used to run and be completely unlimited in my exercise, and now I get palpitations walking across the street. Or I get out of breath climbing up to my second-floor apartment,’” he said.
– Elizabeth Cooney interviewing doctors for a news story, Covid-19 infections leave an impact on the heart, raising concerns about lasting damage, July 2020
As Dr Nisreen Alwan puts it,
Death is not the only thing to count in this pandemic, we must count lives changed.
– What exactly is mild covid-19?, Dr Nisreen A Alwan, BMJ Opinion, July 2020
About children infecting other people, here’s an overview by Dr Zoë Hyde: Children might play a bigger role in COVID transmission than first thought. Schools must prepare, August 2020.
Several studies show children and adults have similar amounts of viral RNA in their nose and throat. This suggests children and adults are equally infectious, although it’s possible children transmit the virus slightly less often than adults in practice. Because children are physically smaller and generally have more mild symptoms, they might release less of the virus.
Dr Hyde notes that as of 6 August 2020, the German Society for Virology has recommended all children at school wear masks.
Here’s a thread discussing recent research with families:
1/3 of infected kids had infected siblings but no infected adult contact suggesting child-to-child transmission w/ infector being median 12 (IQR 8-16) yrs old.
– A Marm Kilpatrick, @DiseaseEcology on Twitter – August 2020
(If you can’t see the whole thread at first, click on the date of the first tweet.)
Please always remember that you don’t necessarily know someone’s struggles with mask-wearing. On the other hand, some people who found it difficult initially have managed to work up to it – some of these ideas might help.
Ask Metafilter: Help me wear a mask has lots of suggestions in response to someone who feels panicky: “When I put a mask on, I get this panicky feeling like I can’t breathe and am going to suffocate, even though I know I logically that is not the case.”
Ask Metafilter: Sensory issues and coronavirus masks, how to cope? is more about the physical side: “the heat, the constriction, the inner dampness of a mask, and the reduced airflow hit all of my bad-sensation sensitivities.” One tip is simply that different shapes of mask can feel very different to wear.
If part of what you don’t like is the way the mask fabric “sucks” onto your face, look out for the little frames which can be put inside the mask, known as “face mask brackets“, or “mask frames”. (These also help with not ruining lipstick.) You can find them on Etsy or eBay – which are also good places to look for masks, if you don’t fancy making your own.
Feeling Anxious About Wearing A Mask? Here Are 5 Ways To Overcome It, by Dr Jessica Gold (with tips from several other people), aside from a compassionate and practical discussion of anxiety, also talks about helping a young child to get used to reading emotions when the other person is wearing a mask.
What about fog on glasses? How To Stop Face Masks From Fogging Up Your Glasses has three tips: folding and adding a tissue to the top, using soapy water to leave an anti-fog film, or simply getting a better-fitting mask. Glasses positioning might make a difference too: it may work better if the glasses sit on the outside, helping to squash the mask into place.
What about lip-reading? The National Deaf Children’s Society has a page on DIY face masks with clear panels. Some sellers on Etsy also make these.
Be aware, though, that these part-plastic masks won’t work for everyone. For example, I read where one lipreader was saying they cover too much of the jaw and chin. Also, they may not be as comfortable for the wearer as a fully-fabric mask, as there’s less breathing area.
Thanks to all the scientists whose work I’ve tried to explain here.
The background of the title pic is as follows:
Transmission electron micrograph of a SARS-CoV-2 virus particle, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID.
That’s it for today! Hope it’s useful.