Well there was the study involving medical professionals wearing N95s, which showed no obvious prophylaxis, probably the best study out there. Your first line is the same old, "they didn't prove the negative" which is never possible.
Regardless, the point being, they are still making people wear masks in LA County medical facilities, well aware there is a absence of substantial medical support. You might as well mandate praying to our Lady of Fatima or spraying yourself with water from Lourdes, as studies also fail to show that praying for medical intervention, does any good -- though there are many people who will swear by it.