I guess it's a choice of analogy. If virus particles are like bullets, protection can be viewed as proportional. But if it's viewed more like a liquid, seeping

I guess it's a choice of analogy. If virus particles are like bullets, protection can be viewed as proportional. But if it's viewed more like a liquid, seeping through any hole, then only full watertight protection works. I'd argue that the reduced transmissibility we see from outdoor exposure supports the first of those as a more apt analogy.