There are actually two questions: how much do masks prevent borderline contacts from being infected, and how broad a range of contact do they work over. As far

There are actually two questions: how much do masks prevent borderline contacts from being infected, and how broad a range of contact do they work over. As far as I can tell the two things are at odds with each other: the shallower the slope of reduced infection vs viral load, the broader the range over which masks can work. (Though of course either way there's still the possibility that most transmission happens outside the range where this has any effect worth mentioning.) The flu-based study some mask proponents were pushing had an order-of-magnitude decrease in viral amount giving maybe a 30% reduction in chance of being infected over the range where this was effective, and obviously very little outside that. That's... not great. Definitely much worse than current vaccines. Though of course Covid isn't flu and we don't have the same level of studies covering it yet.