Asymptomatic transmission, Achilles’ heel of current strategies to control Covid | Hacker News Reader

Asymptomatic transmission, Achilles’ heel of current strategies to control Covid

nejm.org

123 pointsmegamike85 comments

This is why everyone should wear a homemade face mask when they are out in public near other people.

>a homemade face mask

and that highlights the main reason of our situation - total lack of organized massive development, production and distribution of masks, tests, etc. Next - a homemade coronavirus test to be self-administered before going "out in public near other people."

The majority of the homemade stuff (until correctly made from material certified for the role) will block the large droplets, yet it wouldn't block much of the micro-droplets, the aerosol, which is what carries the virus for the distance. And once the mask gets wet one can see how it can become a machine for spreading the virus by way of your exhale aerosolizing the virus carrying moisture mass that got accumulated on the mask as result of catching of the large droplets earlier. Ever seen a bubble maker toy?

Homemade masks are quite obviously a stopgap until better materials are in place. It feels pretty clear that the original comment was meant to say people should take responsibility for preventing themselves from becoming a vector.

Thanks for backing me up, but if the goal is to prevent asymptomatic transmission, homemade masks made from common materials like cotton and paper towels can be 99% effective at preventing _egress_ of respiratory droplets.

The goal of wearing masks is to stop people from spreading their germs, effectiveuly bringing down the R0 value of the virus, slowing the spread, flattening the curve.

The real challenge is that we need about 80% of people to wear masks to really put a dent in things. Really, everyone should be advocating for everyone they know to wear a mask.

See this site for research backed info if you are not convinced: https://www.fast.ai/2020/04/20/skeptics-masks/

I wrote that - thanks for sharing! :) And also wrote this one with Professor Trish Greenhalgh CBE: https://www.fast.ai/2020/04/13/masks-summary/ . Our work on this is getting some attention now in the global medical community; e.g. Martin McKee (Prof of European Public Health LSHTM/ Director Research Policy European Observatory. Past President The European Public Health Association) just tweeted:

"an those speaking about face coverings & #COVID19 please read @trishgreenhalgh evidence review. We are talking about CLOTH FACE COVERINGS not surgical masks Coronavirus IS NOT influenza - coverings work better People spread when talking, NOT JUST COUGHING & when PRESYMPTOMATIC"

I'm so happy to see the progress being made on this issue. 11 out of 12 of the largest countries by GDP now recommend public mask wearing (the other is UK, which is about to change their guidance.)

> I wrote that - thanks for sharing!

Question, did you happen to just stumble upon this thread? Or were you using some type of notification service and alerted about it?

There's two types of notification service (hi Jeremy, nice to see your work still getting lots of attention) - one is a technical thing, and the other is friends.

Way back a couple of jobs before I worked with Jeremy at Fastmail we used to talk about "slashdot distance", as in "I don't read Slashdot, I just read summaries". "I don't even read summaries, I just have friends tell me if there's anything interesting in the summaries"... etc.

That site selectively quotes the literature and misrepresents the scientific record. See here https://rssdss.design.blog/2020/04/24/the-effectiveness-of-c...

Thanks but I don't understand why you commented back as if you're disagreeing with me? I agree with all of this.

Sorry I was just clarifying that we don’t need to wait for better materials. Homemade masks from common materials, when enough of the population (>80%) uses them, can get us where we need to be.

You are unlikely to get exposed to aerosols as a regular person, unless working with high speed and/or rotating equipment. Look up a list of aerosol generating procedures.

You're plainly wrong.

https://respiratory-research.biomedcentral.com/articles/10.1...

"Exhaled air is an aerosol containing endogenously generated droplets."

What about coughing and sneezing? I still see people (yesterday) in public either sneezing into their hand (not elbow) or not covering their mouth at all.

That and they should go outside in the heat and humidity as much as possible. The virus can live indoors for 5 days to 2 weeks in air conditioned environments. Outside in high humidity? Minutes. Open the parks, encourage people to go outside and exercise.

[citation needed]

"The Effects of Temperature and Relative Humidity on the Viability of the SARS Coronavirus"

https://www.hindawi.com/journals/av/2011/734690/

"The dried virus on smooth surfaces retained its viability for over 5 days at temperatures of 22–25°C and relative humidity of 40–50%, that is, typical air-conditioned environments."

Just to be clear, that's about the original SARS, not the novel coronavirus (COVID-19 / SARS-CoV-2). I don't know of any studies about the effects of temperature and humidity on how long the current virus survives on surfaces.

There was this one last month, but it was fast and fairly small and I don't think anyone's tried reproducing it: https://news.ycombinator.com/item?id=22611099

Just to be clear, they are both related coronaviruses that are extremely similar on a molecular level. What would lead you to believe that things that are generally true for viruses and especially true for the highly related SARS-COV would not be true for SARS-COV2?

I didn’t say it was necessarily, or even probably, different. I was pointing out (because you chose not to for some reason) that you were citing a study from a decade ago about a different virus.

And after people go to the park we prevent them from returning indoors for 2 weeks while we wait for the virus to die?

I would argue for people opening their doors and windows and shutting off their A/C as we enter early summer. But even where that's impractical, you're not likely to contract the disease outdoors at all. Besides heat, light, and humidity, there's a huge volume of air for particles to diffuse in. This limits your exposure by default, especially if you pair that with physical distancing.

I for one doubt that home-made reusable masks have much effect especially considering that it is unlikely that people use them and clean them as required.

The surface of the mask might end being the a source of infection that keeps exposing people to the disease. I recall reading a research paper stating the especially the outer side of the mask had the heaviest viral loads.

While I understand the sentiment, I can't help but wonder whether these homegrown mask solutions only end up doing more harm than good when used at such a wide scale and so haphazardly.

I also understand that people really really wish they worked - thus any doubt expressed here is met with scorn.

Masks to work (and to be worthwhile) don't particularly have to stop you from getting infected what the homemade versions are good for is stopping you from infecting others by confining, redirecting and slowing your breath as you breath out. It limits the volume that the breath spreads into and the first surface any droplets will hit will be the mask potentially capturing it.

This is basically wrong. This has been studied, they do work. They don't work nearly as well as proper PPE for personal protection, which tends to trip people up (especially, it must be noted, the libertarians here) when reasoning about them.

But controlling outbreaks is not about minimizing individual risk. By far the biggest effect of wearing a simple mask or face shield, and it's a very significant effect, is that it prevents contagious individuals from spewing droplets into the air around them.

It's not for you, basically. It's for everyone else. Wear a mask.

I have been wearing a homemade mask when I go out. I understand the studies say they work... But man I am pretty sure my breath is just going out around all the edges. My sunglasses fog up as I'm walking into the store. And when I take them off, my breath is just going up around my face at a higher velocity. I guess it's not going towards other people though... the more I read, the less I know about this shit.

Does your mask have a piece of wire around the nose? I find that really helps with the glasses.

All evidence out there indicates that continuous exposure in closed environments is what spreads the infection. Would droplets hang in the air for that long? Most likely not.

All the mask evidence is purely speculative of the style: "it has to work" - something there in front of your face, ergo the effect has to be more than zero.

Yet I will maintain that until a proper public health study it might just as well be harmful. For all we know they may cause asthma, allergies and so on. Breathing through a material cannot be good for the human body.

This appears to be the most common misunderstanding of how public use of masks helps. They don’t help the wearer - they prevent the wearer from unintentionally being a source of infection.

My mask protects you - your mask protects me.

If you’ll permit a tangent:

Don't wear masks in your car alone. Or outside alone. Or outside or in your car with people you live with.

Not only does it not do any good, it actually increases your exposure to any pathogens your mask acquired while in public.

Take your mask off when you're not around other people you don't live with. And take it off appropriately (not touching the mouth or nose area with your hands).

Another stupid thing you shouldn't do: wear a mask in public only to repeatedly pull it off to speak.

OTOH when you are just walking in a middle of nowhere and someone passes by on a bike, if the wind flows in the right direction you can easily get their droplets in your face.

>Take your mask off when you're not around other people you don't live with.

Do you mean that you will automatically get infected if someone you live with does, so it's pointless? Or something else?

Practicing quarantine _inside_ a home is terribly hard. The air exchange is very limited and there are too many shared surfaces. Even if you're paranoid about cleanliness and bleach everything twice a day, in the long run you will get exposed to the microbiome of the other people in the household.

Another factor is that masks get wet after some time. This makes breathing harder, which is not a good thing with any kind of physical activity. Also, the humid feeling makes people want to touch their face more.

Edit: people infecting you is not that easy to defend against. You'd neet N95s or hazmat equipment for that. Situational awareness is key to defend against "drive-by infections".

In the long run though, I assume we are all likely to be exposed to the virus. Let's do everything in our power to avoid it where possible to ensure the dosis is low enough that the infection stays asymptomatic. In the best case, it might grant us some immunity.

Honestly, getting everybody to wear masks so they don't transmit the disease beats getting everybody to wear them right so they don't get it.

Even when doing everything right, the efficacy of the masks to stop people from getting the virus is too iffy to rely upon.

I could see it being important to get the message out that people shouldn't remove their mask to talk to someone. That might be an easy mistake for the very medically-unsavvy.

> If everyone will permit a rant:

> Don't wear masks in your car alone. Or outside alone. Or outside or in your car with people you live with.

That's not useful. Outside alone and then put on if someone approaches? How does wearing it in your car alone increase exposure?

If you've been outside, you want to reduce the amount of times you're touching and putting on/taking off the mask as those are the moments when you're increasingly possibility of transmission (getting on hands, other surfaces). If I see someone wearing it in the car, I'm assuming it's the best practice: reducing the on/off instances.

> How does wearing it in your car alone increase exposure?

Because any pathogens you were exposed to in public are behind held up against your mouth for an extended period of time.

But the point of everyone wearing masks in public is to keep pathogens inside of the masks of those who would be asymptomatic transmitters.

The keep-pathogens-out effect is supposed to be this secondary thing, and I’m not under the impression that it’s even that reliable.

the point of the public masks is to limit the wearer spreading droplets.

I could swear that most people use them to protect themselves, always look out for number 1.

In Asian countries this might be different though.

They might think they're wearing them for that reason, and I'll certainly take it. That said, it's not terribly effective for that purpose..

I have a box of N95s, bought a year ago for unrelated reasons.

Does the same advice still applies for these masks? I fear that handling them increases my risk more than keeping them until I'm back home.

N95s, properly fitted, can actually significantly reduce your exposure risk to outside pathogens, so don't futz with those. Cloth/homemade masks do not meaningfully stop incoming viral particles, but will prevent your saliva from going ballistic whenever you speak; those are meant to protect others from you, in particular if you're asymptomatic and thus don't even know it.

A properly fitted N95 shouldn't be removed unless you're ready to take it off completely and throw away. Seen a lot of people basically ruin those by repeteadly putting on/off, touching the filter, placing them under the chin, etc.

What's more, deciding to wear an N95 should be done if you're sure you're going to be on a high exposure area, and/or wear said mask for extended periods of time (think in hours, 4 to 8 hours).

If you're not a healthcare, or essential worker in a high risk situation, you should consider donating your N95s.

Edit: last paragraph, read as "in general", not particularly directed to P, but partly to GP.

Do we have any idea how much of a difference it would make on the propagation of the epidemics if everyone would wear a (possibly homemade) mask?

It seems like a significant difference -- see how Hong Kong is doing. It needs to be combined with other interventions including social distancing, contact tracing, testing, quarantining. These measures are all additive to each other.

See this thread: https://twitter.com/slowblogger/status/1249712749728284672

So... no, we don't.

Of course not, it's a novel virus, with novel transmission characteristics, there is no evidence for any of the interventions against it until someone tries them, and even then the evidence is usually inconclusive. Such is life.

And look to canada, with infection rates less than half that of the US. I'm there now (BC, west coast, very very low infection rate). We are already talking about reducing measures because our hospitals are well below capacity. With all the cancelled procedures, they are less busy than before this crisis. But masks are nowhere near universal. In my area masks are more social signaling, a fashion object only worn in very particular circumstances if at all. Masks may have a use but they are certainly not a total fix. There are lower-hanging fruit.

Compare the United States. There is a BBC article online now about how 40,000 households a year are having their WATER cut off. That sort of behavior is far more dangerous than not using cloth masks. Forget universal healthcare debate, first agree that people have a right to water.

https://www.bbc.com/news/world-us-canada-52384622

I think Hong Kong is mostly lucky given that people here still go to hotpot dinners, completely ignore the "no groups of more than 4 people" ban and think that getting on a crowded bus in a mask protects them from catching the virus.

Thinking of being protected by face masks is a lazy mental shortcut for "I will limit the spreading of my microbiome, if all others do the same, infection risk goes down"

https://www.theatlantic.com/health/archive/2020/04/dont-wear...

There are a couple countries like the Czech Republic where face masks are mandatory in public and you can be fined if you don't have one covering your nose and mouth, so data about this should be available eventually.

We have a real-world experiment going on right now - East Asia vs. the West. The result is clear despite other variables like population density being very much in the west's favor.

> We have a real-world experiment going on right now - East Asia vs. the West.

I agree this is a very good argument.

I just wonder if it's the end of the story and if the differences we observe can only be attributed to mask wearing. For instance I don't think all Asian countries have the same policies regarding masks though. Also, there may be other factors at play, e.g. cultural and geographical elements.

Not gonna happen, it’s only going to happen if the government insists, i.e. you get fined/jailed for not doing so.

And then you might as well live in North Korea, this is not what free countries should do IMO.

> might as well live in North Korea

... or Taiwan or South Korea or Singapore

... this is not what a "free country" with the largest prison population on Earth would do

I wasn't specifically talking about the US, but agree that there are way too many incarcerated there. It's got to be one of the worst countries to be black in: https://en.wikipedia.org/wiki/File:US_Prisoner_Demographics..... Not even Obama could break the trend :/

Pretty lame non-comment.

In Austria it is mandatory to cover your mouth and nose when inside supermarkets and other buildings accessible to the public. In the Czech republic it is mandatory outside a home as well. Fun fact: it also has one of the most liberal gun laws in Europe.

This is not wrong, but maybe fairly obvious, and why the unwinding strategies require contract tracing to test for pre-symptomatic infection.

The article asserts that additional testing capacity is required for prophylactic testing, but that statement is both true and false. We will probably need ~20x testing capacity for just contact tracing to get R0<1. At that point, prophylactic testing (say every 3-5 days) of health/elderly care workers would be a marginal increase.

> R0<1

A nit: you can’t change R0 without changing the virus — it’s the reproduction constant without intervention.

We want to get R < 1!

I see many posts referring to this virus being contagious during while being asymptotic but infected. Is there any other proper source of this? Our government/health agency is denying asymptotic transmission happens.

Here's a good starting point: https://www.npr.org/sections/goatsandsoda/2020/04/13/8318835...

May I ask which country?

If the CDC, WHO, and PhD's of Public Health are all saying the same thing, it's probably true.

I assume Sweden, the public health agency here has been denying asymptomatic spread, at the same time they wonder how the virus got into the retirement homes which have been hit very hard. They say something along the lines of "we have to find out who didnt follow the guidelines" However they don't consider the obvious, maybe it spread via asymptomatic carriers

If the CDC, WHO, and PhD's of Public Health are all saying the same thing, it's probably true.

The people I trust on this have been quite reserved in their statements as is the article you linked to. It still looks like we don't know for sure.

I was talking about the government of the Netherlands.

I think anyone paying attention knows this already?

It should have been, but strangely even public health officials were very recently advocating strongly against masks unless you had symptoms. No one has ever able to explain to me why symptoms were relevant in any way given that many or most carriers are asymptomatic.

Even the WHO continues to advise against wearing masks for "healthy" people (i.e., presumed asymptomatic carriers).

To me, completely untrained in medicine, their guidance seems much more nuanced than you suggest.

https://apps.who.int/iris/rest/bitstreams/1274280/retrieve (pdf)

It doesn't seem very nuanced to me:

> However, there is currently no evidence that wearing a mask (whether medical or other types) by healthy persons in the wider community setting, including universal community masking, can prevent them from infection with respiratory viruses, including COVID-19.

There is plenty of evidence that masks help. It could be argued that some is contradictory but it's not "no evidence".

I guess you could argue they are correct in the asinine point that it doesn't "prevent them from infection" in every case. Ignoring that I'm not sure that makes sense grammatically, are they just trying to say that no masks provide complete protection? It's true that no single method or equipment provides complete protection. It's a layered network of protections that we need to use to lower the transmission rate. Masks are a very important layer in that system, along with hand washing, quarantines, goggles and contact tracing. Hand washing doesn't help you either if an infected person coughs into your open mouth.

The argument to this point in the thread was about preventing transmission from the mask-wearer, which is where the more nuanced take is.

The asymptomatic point goes to both directions. I don't know who I need protection from as an uninfected person and I don't know when I need to wear a mask to protect others as a potential asymptomatic carrier.

Which more nuanced text are you referring to?

In addition to these factors, potential advantages of the use of mask by healthy people in the community setting include reducing potential exposure risk from infected person during the “pre-symptomatic” period and stigmatization of individuals wearing mask for source control. However, the following potential risks should be carefully taken into account in any decision-making process:

• self-contamination that can occur by touching and reusing contaminated mask

• depending on type of mask used, potential breathing difficulties

• false sense of security, leading to potentially less adherence to other preventive measures such as physical distancing and hand hygiene

• diversion of mask supplies and consequent shortage of mask for health care workers

• diversion of resources from effective public health measures, such as hand hygiene

Whatever approach is taken, it is important to develop a strong communication strategy to explain to the population the circumstances, criteria, and reasons for decisions. The population should receive clear instructions on what masks to wear, when and how (see mask management section), and on the importance of continuing to strictly follow all other IPC measures (e.g., hand hygiene, physical distancing, and others).

That's at least the fourth version, look for the archived recommendations in January, February and March, the April one is the first one to touch upon all the aspects directly and more honestly.

Ok, but the key word in the parent was “still”. That pdf dates back to early April.

(Sorry, “continues”.)

Inertia.

I recently changed my position to "surgical masks for everyone, N95 respirators for people at risk."

I'm immunosuppressed. Transplant when I was a kid. For 30+ years, I had it pounded into my head that surgical masks will not protect me. Especially if I share a space with others for longer than 30 minutes. Because exhaled water vapor saturates the mask and then air comes around the sides.

This is still true.

What's new is understanding that surgical masks protects everyone else from me. I think it's called "source prevention". IIRC, infected people wearing masks cuts risk of transmission by 70%.

And because people can be asymptomatic for up to 14 days, that means everyone should be wearing masks. Just in case.

WHO, CDC, others were not "wrong". Just out of date. It takes longer than instantaneous transmission for best available science to percolate out. Especially in a noisy environment.

Sure, we westerners could have been more proactive about adopting practices from societies who'd already adapted. But the hard data is still very fresh.

Further, none of the advice, messaging was making a distinction between masks and respirators AND many people hoarded respirators AND healthcare workers haven't gotten the N95 PPE they need.

Of all the things which went sideways, the suboptimal advice on masks, now since revised, isn't even in the top 10. Dwelling on it while we're still not taking the steps to end social distancing seems premature. There will be plenty of time to apportion blame later.

>Transplant when I was a kid. For 30+ years, I had it pounded into my head that surgical masks will not protect me. Especially if I share a space with others for longer than 30 minutes. Because exhaled water vapor saturates the mask and then air comes around the sides.

This is not exactly right. It is correct that no mask (not even an N95) provides complete protection. But surgical masks do provide significant protection. As an immunocompromised individual, I want the best protection money can buy of course. But this is different than saying that surgical masks don't protect you, which is incorrect.

>WHO, CDC, others were not "wrong". Just out of date. It takes longer than instantaneous transmission for best available science to percolate out. Especially in a noisy environment.

Well, they were wrong. Masks protect in both directions, obviously. I can point you to the science on this if you're in doubt. But a simple way to think about it is why do doctors wear surgical masks in the Covid ward when N95's are unavailable? The answer is simple some filtration is better than no filtration.

>But the hard data is still very fresh.

It's not. Most of what we know about masks came from research in the early 2000's after SARS.

>Further, none of the advice, messaging was making a distinction between masks and respirators AND many people hoarded respirators AND healthcare workers haven't gotten the N95 PPE they need.

Another common misconception. Hoarding had basically nil effect on this. I understand that governments have tried to blame hoarders for their own failure to stockpile before this happened, but it's not actually based in reality. Average people started buy masks at a higher pace in January and February which pretty much instantly cleared out any store inventory. By the way, this store inventory has basically nil impact on the availability of respirators for the millions of healthcare workers in the country. Before regular stores could be refreshed, hospitals and governments commandeered the supply chains for all quality respirators. A normal person buying a surgical mask these days has zero effect on the ability of hospital staff to get them.

>Of all the things which went sideways, the suboptimal advice on masks, now since revised, isn't even in the top 10. Dwelling on it while we're still not taking the steps to end social distancing seems premature. There will be plenty of time to apportion blame later.

What else is in your top 10. The lies about masks are very close to the top for me. The only thing worse would be the WHO loudly proclaiming there was no evidence of human to human spread when we all knew (including the WHO) that human to human spread was likely happening by then. The WHO had already briefed world leaders on that. Why didn't they tell the public?

"I can point you to the science on this if you're in doubt. ... Most of what we know about masks came from research in the early 2000's after SARS."

Yes please. I'll be pretty grumpy if my care providers were misinformed or worse.

"The answer is simple some filtration is better than no filtration."

For 30 minutes. The concern has always been that laypersons wouldn't understand the limitations and risks.

"What else is in your top 10."

You just wrote that govt didn't stockpile enough masks.

"The WHO had already briefed world leaders on that. Why didn't they tell the public?"

Well, ya. If WHO, CDC, White House, or whomever, flat out lied, even by omission, then pile on.

>Yes please. I'll be pretty grumpy if my care providers were misinformed or worse.

The evidence is plain. Masks work and they save lives. This has been an unfortunate misinformation campaign that they don't. The intention was to save masks for healthcare workers but it failed at that for a number of reasons.

https://stacks.cdc.gov/view/cdc/44313

Also, keep in mind that whether they work at preventing the spread of corona is a very different question than whether a doctor would recommend them to an immunocompromised person in X situation. So it's hard for me to say if your doctor was wrong without knowing precisely what she said.

>For 30 minutes. The concern has always been that laypersons wouldn't understand the limitations and risks.

Masks don't stop working after 30 minutes. Laypersons don't understand the full risks inherent in using condoms, but we don't tell them not to use condoms. We do educate them.

Masks do not provide perfect protection for any length of time. But we're not trying to obtain perfect protection. We're trying to do 1% better than the alternative course of action. (Even procedure masks, by the way, do much more than 1%, which is why it's important for everyone to wear them).

>You just wrote that govt didn't stockpile enough masks.

I meant I'm curious to see which other 10 things are more important than a massive misinformation campaign about masks that will cost thousands of lives. What are the other 10?

>Well, ya. If WHO, CDC, White House, or whomever, flat out lied, even by omission, then pile on.

They did, and I am.

Followup...

"Hoarding had basically nil effect on this."

Thanks.

Being ever skeptical, I had to read up. I've since learned that you are correct. The feds didn't replenish their stockpile. And no one else built their own stockpile up. (Of course, there's always more to the story.)

I'm contrite that I fell for the official line. Again.

A lot of the early don't wear masks was a lot more about preserving supply for people like hospital and other critical personnel that needed to continue working and had no way to avoid or distance from infected people. There's no real way I can think of to say 'yes masks are effective but you shouldn't get them because we need them for doctors' that doesn't wind up with more people buying masks and deeper shortages early.

Problem with this idea was twofold:

1. It completely misunderstands the supply chain. By the time this message was out there, masks were already mostly sold out in retail places. Even a slight increase in demand will cause that because of JIT manufacturing. Anyway, by that time the healthcare system and the government had already taken over the supply chain for all new masks. And obviously hospitals don't buy masks from Home Depot

2. In fighting any emergent public health crisis, the number one thing you need is the public's trust. Why destroy it for essentially no gain.

i mean there is a guy running for congress that doesn't believe that it's real.

Stay at home as much as you can beats any amount of masks.

Go look at how New Zealand has done.

But you understand that New Zealanders aren't going to stay at home for the rest of their lives, yes? So they'll still need an answer to the question of how to safely resume going out.

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