Big thank you to @EricCarroll for pointing out this new WHO document on SARS-CoV-2 transmission.
This document is pretty complex, in-depth, dense, and I still expect it to evolve as we learn along the way. They have some of the correct people to be working on this, for once. Hello Lidia Morawska signing off on it at the beginning of the forward.
First, a tldr. If you don't care about how it came to be, or the science, and just want to know the outcome, here it is:
https://partnersplatform.who.int/tools/aria
Go to the calculator, enter your data, and come out with a probability of infection in a given situation along with the number of expected secondary infections from that interaction.
Here's the document itself if you want to follow along:
https://iris.who.int/bitstream/handle/10665/376346/9789240090576-eng.pdf?sequence=1&isAllowed=y
Disclaimer - This is evolving science.
I'm going to split this up in a thread, because I took a lot of notes of what stood out to me on a first read, and I hope to come back to it, and use it as a general reference moving forward.
I took the same overall data, but added respirators to everyone. Now it's a 1.3% chance of personal infection. Only with the infectious person being 95th percentile did the chances get much above 1%, to 8%.
If I take the masks back off, and increase the room ventilation from 3 ACH to 6 ACH the estimate is essentially the same as the original 45% for short-range interactions, while the long-range interactions only went down to 11%.
Bumping that up to 12 ACH leaves your short-range still at 45%, but now your long-range is down to 8%.
What did we just learn? Their model shows that the amount of air changes per hour doesn't matter if you are face-to-face with someone who's got COVID. Makes sense, right? But people who only have long-range interactions fare better the more air changes.
So then I tried putting in what would be an average CADR for HEPA filtration in the room. Guess what? Still 45% if you're going to be face-to-face. But, now we're all the way down to 6% for people only having long-range interactions.
If I bump that CADR up to twice what's usually recommended for a room that size, the long-range interactions percent now comes down to just 5%.
Their model clearly shows that masks are effective given the data available. It's, in fact, the only way you're going to seriously reduce your short-range interaction risk.
But, ventilation and filtration absolutely have a fairly dramatic effect on long-range interactions according to this model.
How low can you reasonably go given this room's scenario? Respirators on everyone, mechanical ventilation(no open windows, because there weren't any in this room), and HEPA filtration. Now we're down all the way to less than 1%. Period.
0.03 expected new cases amongst the 12 people in the room all day.
Stay safe out there!
@BE And that is for a 1 hour class with everyone unmasked. With 6-8 classes per day, plus the bus, and the halls, that one infection is going to generate some 50-100 per day, with the finest of air cleaning. Ouch!
@BE "How low can you reasonably go given this room's scenario? Respirators on everyone, mechanical ventilation(no open windows, because there weren't any in this room), and HEPA filtration. Now we're down all the way to less than 1%. Period."
This would be close to, say, a hospital operating theater, albeit the patient likely would be unmasked and on a ventilator. IIRC, COVID infections in these sorts of clinical settings aren't much of a concern with the expected precautions.
@BE This is great! Thank you for posting it! I just started plugging in average values for a school classroom. As you observed, the number of air changes per hour has only the smallest effect on the numbers. 1 goes in sick this delivers 6 new infections all because of the short range interactions….