This good cluster RCT is in the midst of being done. https://www.nber.org/papers/w28734 https://www.nber.org/papers/w28734 In this paper they demonstrated an

This good cluster RCT is in the midst of being done. https://www.nber.org/papers/w28734 https://www.nber.org/papers/w28734 In this paper they demonstrated an intervention in Bangladesh that increased mask usage by 30 percentage points (from ~10% to ~40%). In the follow-up paper they are doing random antibody testing between the two groups.