Not a science fiction recommendation, but with those interests you should absolutely check out the most recent Hugo Award winner (Robert Jackson Bennett's The Tainted Cup)

It looks like particpants were randomly assigned to either mask or non-mask, though there could still be an element of people self-reporting differently based on having worn a mask (I think it probably makes me more aware of symptoms?)

I think the absolute difference is a bit unhelpful here (since it depends on what the participants were exposed to). If you read it as "masked participants were 27% less likely to catch something" that feels more significant, though still not a silver bullet.