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Comment by drzaiusapelord | Hacker News Reader

You won't often catch covid from stray particles but from the person breathing next to you, which these filters can't help. This study was in a hospital where everyone is wearing serious PPE, so its a super edge case to get rid of covid in the air because the main use case of catching covid has been handle by plain old PPE.

This doesn't have sweeping applicability. It won't stop covid among the general population. Its for maybe hospitals that are on super covid lockdown that want a little more protection for their most vulnerable patients.

We get rid of covid through mass vaccination like any other virus. There's no shortcuts for those who refuse masks and vaccines.

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>You won't often catch covid from...

how do you know? it seems like most cases are attributed to the nebulous "community spread" and nobody is really sure who they're coming from or how we're catching it. maybe we are getting it from the person breathing near us - if masks help, i don't see a reason that improved air filtration wouldn't.

Because understanding how viruses spread is a solved art. None of this is "unknown."

In the same way that people are dumbfounded that natural immunity from having it is superior than the vaccine.

It’s as if we all forgot how viruses work.

In terms of safety, vaccines are much better then getting COVID.

One of the big factors of "better protection" of immunity gained after recovering from the virus is survivor bias. In data about mortality after reinfection there are no people who died after getting it for the 1st time. This leaves only ones who had body strong enough to recover.

There's no surprise that later that group fares better.

All the data suggest that its not someone breathing on you in passing that's going to result in covid infection, but spending long periods with someone indoors in close proximity. Within a half hour in the massive grocery store? I would be shocked if you caught it. Drinking in someones living room, swapping stories within arms length, swapping spit flecks for 5 hours straight? Definitely going to spread everything you have in such a small enclosed space where the viral load is able to reach a much higher concentration over a long period of time.

yes, that's pretty much exactly what i'm saying: all the data we have is that it spreads exactly as you say, sitting with somebody for an extended period of time. because that's the sort of thing that people remember, and tell the contact tracers about.

but at least in my region, the majority of cases still don't have a known source. maybe that's because people are catching it in scenarios where they're in close proximity to somebody for extended periods of time but forget about it when the contact tracers call them, and all cases are spread that way.

or maybe it's spreading in grocery stores and other casual contact scenarios but that's just impossible to measure or count. we keep hearing that there's no evidence of it spreading in schools, and yet as long as students are in school it spreads a lot more among students. is it not spreading there, or is it just really difficult to find the evidence of that?

> but spending long periods with someone indoors in close proximity.

It's not obvious to me that in-room air filters wouldn't reduce the exposure in this case, as the original commenter claimed? If it reduces the virus in the air, as the OP study finds, it seems like it probably would reduce virus exposure and thus infection rate when spending long periods indoors in close proximity, no?

Sure, just because it reduces the virus in the air doesn't necessarily mean it reduces infection -- like hypothetically the virus could commonny be going right from someone's exhale to someone else's inhale without having a chance to be filtered -- which I think is what the original commenter was suggesting -- but that's not the "spending long periods" scenario at all. I think it's true that while you probably can get infected in as little as ~15 minutes (not necessarily in just a couple minutes), but also clear that the longer you are in the same room, the higher risk.

But sure, we don't know exactly everything. All we can do is build knowledge one brick at a time, AND all we can do is act on the bricks we already have (that we don't have the whole wall yet is not a reason to avoid acting on our best picture (mixed metaphor, sue me) form what we do have)... but reducing virus in the air is a pretty huge brick for a respiratory infection. Based on everything we do know, it is hard to believe what the original commenter asks us to, to believe that reducing viral concentration in the air would NOT significantly lower infection rate.

I think it's becoming increasingly clear we should be spending at least as much attention on HEPA air filtration as people have been on masks.

What data are you referring to? The protests and marches last year seem to be hard to ignore example of things that should have had more spread by these standards.

Those were outdoors with plenty of room for viral particulate to disperse. there has been a lot written about this, its called the infectious dose which is the amount of virus particulate needed for someone to become ill with the virus. I recommend pubmed or google scholar if you want a deeper dive.

if you are having trouble visualizing how this works, imagine a perfume dispenser jetting out a fine mist, that's your covid positive person exhaling. In an enclosed small room, the concentration of that particulate mist is much higher than in a large room, and unable to disperse quickly without any ventilation. Over time, you will inhale more and more of this particulate in this room at this concentration, and you will hit the infectious dose level eventually. If you are outside, or in a larger or well ventilated room, concentration of the particulate is much smaller since its either dispersing into a greater volume of air or being removed from the area due to ventilation, and it might take you a long period of time to inhale enough particulate at this comparatively lower airborne concentration in order to become sick. Outside, even in a protest, the volume of air is so large in which to disperse particulate that the risk of getting an infectious dose is negligible even with someone yelling right next to you.

Some of the marches that I went to... were less than airy, to be honest. Easily comparable to any ballgame I have been to.

So, I have seen a lot of these papers, but too much feels like post hoc justification. I'm game for being cautious, so I'm not really saying to do anything different, but I do want to urge way more caution in messaging than I'm seeing. It is fair and ok to talk of the uncertainty.

You are free to make your own decisions, but know that the risk is negligible outdoors.

Unless it is an outdoor concert? Or a sports game? Or are we ok with those nowadays?

As far as I can tell, COVID has two predominant modes of spread:

- Extended time indoors, with high viral loads. It spreads ridiculously well with no protection (even among the vaccinated).

- Isolated superspreader events, where one person infects many, even with distant contact.

Filters would seem to target the latter more than the former.

What if the air is being actively refreshed and filtered. I believe this is what needs to start happening in most indoor environments. This would definitely help with reducing the amount of cases.

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