Thread by @DellAnnaLuca

FOR A SAFER REOPENING

I took the freedom to translate the quoted thread, as it is very important. HT @XFaure for having shared it earlier.

It has great ideas, see tweets #9 and #18 below.

Translation adapted for brevity 👇👇👇 in the thread, 1/N

https://twitter.com/Pr_Logos/status/1372604733899300871

2/ Can we estimate a measure of the infections risks, so that we can free from the lockdowns all activities that are low-risk, while keeping away from the few high-risk ones?

The answer is yes.

3/ Let's begin with the basics. The probability of infection depends on the viral load we receive (i.e., the cumulative number of virus particles breathed in).

The denser the virus in the air, the higher the load.

Similarly, the longer we inhale, the higher the load.

4/ The average dose at which we have a significant chance of being infected is called an "infection quantum".

It differs person by person, and depends on his or her immune system, age, comorbidities, etc.

5/ We can approximate that the probability of infection is proportional to the dose at low dose. It's not necessarily nor fully true, but it enables us make important considerations.

6/ First consideration on the face masks. If you use a cotton mask, you divide the inhaled load by 3 but also exhale 3 times less load. This means that you reduce risk by a factor of 9.

[…] Surgical masks are even more effective. medrxiv.org/content/10.110…

7/ Luca's note: don't focus on the exact numbers, which I couldn't verify. Instead, focus on the reasoning. Because it makes sense even if the coefficients were lower.

8/ A second consideration that determines the viral loads inhaled: how much ventilation there is per person nearby you.

In other words, the replacement of stale air with fresh one.

The more ventilation, the less viral load to inhale.

9/ We can estimate ventilation with CO2 detectors. CO2 and virus dilute at the same time.

The more the CO2 detected indoors, the higher the risk of infection.

10/ Luca's note: all other things equal. Face masks don't block CO2, so you can have two rooms, same CO2, one safer because people wear face masks.

So, CO2 is a proxy for bad ventilation, thus a proxy for relative risk of infection, not absolute one. Still, useful.

11/ Third consideration. Viral loads depend on the activity. Same room, same ventilation, if people sing there will be higher loads than if they talk.

12/ Let's see some numbers. An average infected person emits 40 quantums per hour. ncbi.nlm.nih.gov/pmc/articles/P…

13/ If you spend 30 mins with two of them, no mask, in your living room with closed windows, you have a 67% probability of getting infected.

If you use surgical masks and ventilate twice for 3 minutes, the probability plummets to 0.2%

14/ Luca's note. Again, don't focus on the exact numbers. They might or might not be exact. But the influence of the conditions is probably in the right order of magnitude, and that's what matters to understand what's going on and what we can do to re-open.

15/ Opening universities? It can perhaps be done, BUT. It can't be a simple "let's go back to normal".

We must ventilate, use CO2 detectors, distribute good masks, install masks decontaminators. It will cost some money, but not that much.

https://twitter.com/Pr_Logos/status/1372665865641066501?s=20

16/ The original thread continues with aerodynamics considerations, I won't translate. But I think that the important point has been made.

17/ Chances of infection are not binary, "if I'm next to an infected I catch it."

Masks and ventilation matter.

Therefore, it shouldn't just be "can we reopen?" but "can we reopen HOW?"

18/ I LOVE the idea of using CO2 detectors to estimate ventilation.

What if, when we’ll re-open restaurants & schools in low-risk areas, we do so WITH THE REQUIREMENT of installing CO2 detectors & keeping CO2 levels below a given threshold to guarantee ventilation & distancing?