Although it needs to be snug enough around the edges that you don’t simply have unimpeded airflow all around.
Hmm. I’ve been using a bandana as a mask, but it’s not a bandito-bandana; the way I fold it, it’s six layers thick, and it’s held pretty snugly in place by rubber bands around my ears. It’s not just folded diagonally and tied behind my head, which would be worthless. (I learnt to do that as an improvised surgical mask during my decades-obsolete wilderness-first-responder training.)
I’ve switched to manufactured cotton face masks, even though I believe they’re less effective than what I’ve been doing, because the popular misreading of this paper has led my local leaders to ban bandanas as face coverings, however worn - and I believe that would extend to a mask sewn from the bandana’s material, because of course the ‘bandana-ness’ will propagate. The paisley print destroys the cotton’s effectiveness, you know!
Leakage around the bridge of the nose is a pretty significant thing. For the bandana-masks, I can work around that again with how they’re folded, because there’s enough fabric that bunches against my cheeks that my classes don’t fog when I’m wearing them. Leakage would be more of a problem with a sewn mask from the same material unless I worked out how to put some sort of stiffener across the top edge to hold it tight on either side of my substantial beak.
From what I know of physics, pore size on the fabric is much less important than tortuosity of the air path and hydrophilicity of the material. Basically, you want to expose the air stream to as much surface area as possible of a hydrophilic surface, to give droplets the greatest possible chance to adsorb.
The cotton + poly or cotton + silk thing is interesting. It stands to reason that if you can charge droplets up electrostatically and then expose them to a surface that’s got the opposite charge, you can precipitate them better.
The jury is out on big droplets vs small droplets. Big ones settle out faster, but small ones evaporate faster, and desiccated virus appears not to be terribly infective. Partly evaporated big droplets are likely the worst, because the virus is still wet, and you’ve concentrated it by evaporating away the water.
)
scientific know-how has handed the world an expertly designed vaccine candidate which is probably at worst harmless, maybe someone will have a bad reaction to the peptide snippits, but which quite probably confers some unknown but likely significant degree of protection.
and 
will have this beaten before 
