The meta study does not "[show] medical masks had no effect", it shows that we have limited evidence for it: "Most studies were underpowered because of limited

The meta study does not "[show] medical masks had no effect", it shows that we have limited evidence for it: "Most studies were underpowered because of limited sample size, and some studies also reported suboptimal adherence in the face mask group" https://wwwnc.cdc.gov/eid/article/26/5/19-0994_article https://wwwnc.cdc.gov/eid/article/26/5/19-0994_article And indeed you can sympathise with them that it would be hard to perform such an experiment. But the whole point of science is that it allows us to make predictions based on theories. And the current scientific consensus is that wearing masks is beneficial.