They knew this after sars. It's not hard to find the evidence from after then.
It's just a nice visual way to make people feel safer against the thing they're told to fear.
So if wearing masks pushes your infection date out to when vaccines are available (and you get vaccinated), it is not unreasonable to assume the inverse (that not wearing masks led to earlier infections and thus worse outcomes, and death)
Unfortunately many parts of this paper are beyond me, but this study does appear to find that community level masking has an impact, but it is a study that used actual communities and tracked compliance (and impact of different interventions on encouraging making)
> We used high-quality surgical masks that had had a filtration efficiency of 95% [standard deviation (SD) = 1%]; this is substantially higher than the filtration efficiency of the cloth masks we designed, which had a filtration efficiency of 37% (SD = 6%). These cloth masks had substantially higher filtration than common commercial 3-ply cotton masks, but lower than hybrid masks that use materials not commonly available for community members in low-resource settings (54).
The key result:
> We find clear evidence that surgical masks lead to a relative reduction in symptomatic seroprevalence of 11.1% (aPR = 0.89 [0.78,1.00]; control prevalence = 0.81%; treatment prevalence = 0.72%). Although the point estimates for cloth masks suggests that they reduce risk, the confidence limits include both an effect size similar to surgical masks and no effect at all. (aPR = 0.94 [0.78,1.10]; control: 0.67%; treatment: 0.61%).
It's a shame that even with such a large study, the phenomenon is so noisy that we can't get a strong signal here. But i think the takeaway is that masking has an effect, but it is rather small.
There's a lot more detail in the paper. Including this gem:
> We find no evidence that any of our village-level or household-level treatments, other than mask color, impacted mask-wearing. For mask-color, we see marginally significant differences, small in magnitude. In surgical mask villages, blue masks were more likely to be observed than green (adjusted percentage point difference = 0.03, [-0.00,0.06]), and in cloth mask villages, red more likely than purple (adjusted percentage point difference = -0.02, [-0.04,-0.00]). Text message reminders, incentives for village-leaders, or explicit commitment signals explain little of the observed increase in mask-wearing. Compared to self-protection messaging alone, altruistic messaging had no greater impact on mask-wearing, and twice-weekly text messages and a verbal commitment had no significant effects.
I think a key target was spreading out the infections so that hospitals weren't overwhelmed. And delaying infections until vaccines were available. Imagine if everyone in the country had gotten covid over the first half of 2020.
Is there solid evidence anywhere that masks are important one way or the other? While it is plausible that a mask could prevent an infection in an encounter, I've not yet seen anyone waving something conclusive around that masks prevent infections over the course of a pandemic.
I've managed to avoid the coronavirus for 2 years now and as far as I can tell the future still has me getting COVID in it. I'm still not clear what role the mask is meant to play in that forecast.