Should this source be trusted or not? If it’s trusted, does that mean that our efforts to mask were basically a waste of time, or not?
2. You should never look at one study, but at all the evidence. It is generally difficult to study the effects of something like mask wearing, as you can't easily do something like an RCT (or if you try it may not tell you what you wanted to learn, i.e. it's even harder to test if others are protected by mask wearing). There are no easy answers, but it's been my impression that experts reviewing all the evidence in the past came to the conclusion that mask wearing likely works quite well.
It'll take a lot of time and hard work to decisively say if there's an effect and to what degree. The various studies and meta-analysis I've read appear to indicate that masks somewhat slowed the transmission rate, but overall didn't greatly reduce final case counts. There's enough uncertainty that I believe we'll need solid RCTs to get better answers. There was on large RCT slated to happen in west African country (Rwanda maybe?) where covid-19 hadn't hit.
Seems legitimate, though that doesn’t mean it’s a particularly valuable analysis.
> If it’s trusted, does that mean that our efforts to mask were basically a waste of time, or not?
It doesn't say.
In a world where everyone wakes up each day, checks the daily cases nearby, and only puts on a mask if it’s over 1000… you'd get near perfect correlation between mask use & covid cases, even if the masks prevent every single transmission.
And that’s not all that out of line with reality.
High rates of Covid absolutely cause increased levels of mask compliance, so seeing a correlation between the two doesn't really tell us much about mask effectiveness.
You could maybe do some time-series analysis to try and get a better idea. The decision to mask today would be influenced by cases in the recent past, and any actual effect of mask wearing would only affect diagnoses 3-5 days later. But I’ve not done that math, and neither has this paper.